ROADVALE SALINITY PROJECT

Salinity processes, management options and planning considerations for upper Purga Creek Catchment

August 2013

Report by Roger Shaw1 and Lauren Eyre2

Final report for:

- Scenic Rim Regional Council

- SEQ Catchments

- The Roadvale-Milbong community including Roadvale State School

- Queensland Department of Natural Resources and Mines

- Bremer Catchment Association

1: Natural Resource Consultant, 36 Sandon Street Graceville 4075

2: Department of Natural Resources and Mines, PO Box 573 Nambour 4560

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Acknowledgments

The willing cooperation by landholders who have offered comments and advice at the four meetings held to date and those who willingly gave of their time to take us around their properties, explain situations and locate and allow us to measure wells, bores and streams is very gratefully appreciated. The breadth and wealth of information has made a major difference to the compilation of this report.

Jean Bray of SEQ Catchments for coordination and links with landholders and SEQ Catchments staff for spatial information, Andrew McLoughlin of Scenic Rim Regional Council, David Costin and Kaye Montgomery and people of Roadvale State School for hosting the meetings, staff of Department of Natural Resources and Mines and in particular Kate Goulding and Michael Francis, for groundwater and surface water data, maps, advice and information. Gary Wenzel and Les Draheim for assistance with water level monitoring and associated activities. Steve Mocker for rainfall records for Roadvale area. Warwick Willmott provided geologic advice on geological maps of the area. Ashley Bleakley and Andrew Biggs for peer review and comments on the draft of this report.

Copyright

This report is copyright © SEQ Catchments and is licenced under the Creative Commons Attribution 3.0 Australia License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/au/.

The report relies heavily on prior published and unpublished information and data from Queensland Government sources. Copyright on some of the data and information has been licenced to SEQ Catchments. All sources of information have been acknowledged in the report.

The source of some of the information in this report as acknowledged is from the Queensland Department of Natural Resources and Mines (NRM) and is licensed under the Creative Commons Attribution 3.0 Australia License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/au/.

Disclaimer: Based on the data provided by the State of Queensland in 2012 and 2013, the State makes no representations or warranties in relation to the supplied data and you agree that to the extent permitted by law, all warranties relating to the data including accuracy, reliability, completeness, currency or suitability for any particular purpose and all liability for any loss, damage or costs (including consequential damage) incurred in any way (including but not limited to that arising from negligence) in connection with any use of or reliance on the supplied data are excluded or limited. Data must not be used for direct marketing or be used in breach of the privacy laws. Some of the copyright material has been modified by the authors as stated in the text of the report.

Authors’ Disclaimer

The material and conclusions presented in this document are the view of the authors and includes information from discussions, interviews and a range of sources. None of the sources are responsible for any interpretation made of the information provided. All reasonable care has been taken in compiling this report but known gaps exist in necessary data. Any additional information that becomes available in the future may change the conclusions and recommendations of this report.

The views and conclusions contained in this document should not be interpreted as necessarily representing the official policies, either expressed or implied, of Scenic Rim Regional Council, SEQ Catchments Ltd, their staff or the State of Queensland.

The report covers a number of technical issues associated with processes and management of salinity in upper Purga Creek catchment. It does not cover all the aspects concerned with salinity.

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The information and recommendations made should not be acted on, or ignored, without independent consideration and advice.

Citation

Shaw, Roger & Eyre, Lauren 2013. Roadvale salinity project. Salinity processes, management options and planning considerations for upper Purga Creek catchment, South East Queensland. Final Report, SEQ Catchments, Brisbane.

Availability

The report is available on www.seqcatchments.com.au/resources-general-reports.html

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Summary

The Roadvale Salinity project resulted from concerns and discussions about the adverse impact of salinity on resources in the Roadvale-Milbong area of the upper Purga Creek sub-catchment. Investigations were conducted during late 2012 and 2013. Field visits to landholder’s properties and several meetings with landholders at Roadvale State School were held.

This report provides an interpretation of the hydrology and salinity processes operating in the Roadvale-Milbong area based on available information and proposes possible salinity management and planning options for consideration. Uncertainties and gaps in available knowledge are outlined.

Salinity has been present in the Roadvale area for the whole of living memory. The high salinity level in the groundwater of the alluvium around Roadvale indicates that salt has been concentrating for centuries. A survey of soil salinity for SEQ Catchments indicates that more than 180 ha are severely affected by salinity in the upper Purga Creek area.

The upper Purga Creek catchment has restricted groundwater outflow due to geological features at One eye waterhole, near Milbong, and a restriction in the width of alluvium upstream near the Ipswich-Boonah Road crossing of Purga Creek. The very low slope of the land surface of the creek alluvium in this area means very low hydraulic head resulting in very low groundwater outflow downstream and thus a rise in watertable levels with increased water inputs.

The analysis of salinity changes over time using aerial photographs and imagery analysis for the period 1944 to 2012 indicated that the extent and severity of salinity at four salted sites within the catchment was the most extensive in 2012 for all four sites.

An analysis was made of the groundwater level response to rainfall, characteristics of the upper Purga Creek catchment which occurs within the Clarence Moreton sub basin of the Great Artesian Basin, the number of bores and wells that are artesian (under upward water pressure) or overflowing at ground level, the water chemistry of surface and groundwater, soil deep drainage of the upland soils and the number of farm dams. This analysis indicated that the area is a local discharge area for groundwater from aquifers within the Walloon Coal Measures and Marburg Subgroup under artesian pressure.

The estimate of the relative contribution of the casual factors for salinity in upper Purga Creek is:

Landscape and geologic features 30%

Local discharge area with the Clarence-Moreton basin 40%

Land clearing 10%

Leaking dams 10%

Current land use 5%

Increased sedimentation on the alluvial flats 5%

A range of salinity management options have been proposed, together with an estimate of their likely impact on controlling or living with salinity. Planning considerations are outlined to minimise salinity impacts. The management options were discussed and evaluated by those present at two landholder meetings in 2013.

Based on the Great Artesian Basin Resource Operations Plan legislation, the groundwater flows in the artesian areas are protected. Salinity management is possible on the alluvial areas of Purga Creek. The option of controlled dewatering of the alluvium is preferred in conjunction with other options. A trial of the feasibility of controlled dewatering of alluvial groundwater to reduce the watertable levels and consequent evaporative concentration of salt is recommended. This has the potential to turn a salinity problem into a resource where groundwater can be used to irrigate salt tolerant pastures.

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Table of Contents

Summary .............................................................................................................................. 3

List of Figures ....................................................................................................................... 5

List of Tables ........................................................................................................................ 7

Abbreviations and glossary of terms ..................................................................................... 8

1 The brief ...................................................................................................................... 10

1.1 This report ............................................................................................................ 10

1.2 Method .................................................................................................................. 10

2 Brief history of Roadvale - Milbong area ...................................................................... 11

3 Geology, soils and landforms ....................................................................................... 12

3.1 Geology ................................................................................................................ 12

3.2 Soils ...................................................................................................................... 17

3.3 Salinity occurrence ................................................................................................ 17

3.4 Landscape features and the occurrence of salinity................................................ 19

4 Hydrogeology ............................................................................................................... 29

5 Salinity in upper Purga Creek catchment ..................................................................... 29

5.1 History of salinity ................................................................................................... 29

5.2 Changes in salinity over time from aerial imagery ................................................. 30

5.3 Response to rainfall .............................................................................................. 35

6. Where is the recharge water coming from? .................................................................. 38

6.1 Water levels .......................................................................................................... 38

6.2 Clarence-Moreton basin ........................................................................................ 38

6.3 Water chemistry .......................................................................................................... 43

6.5 Dams .................................................................................................................... 46

6.6 Deep drainage ...................................................................................................... 47

7 Guess/estimate of relative contributing factors to salinity ............................................. 47

8 Expected salinity trends in the future ............................................................................ 48

9 Uncertainties to be considered and addressed ............................................................ 48

10 Salinity management ................................................................................................... 49

10.1 Water management plans ..................................................................................... 49

10.2 Salinity management options ................................................................................ 50

10.3 Landholder preferences for salinity management options ..................................... 58

11 The benefits and trade-offs in salinity management ..................................................... 59

12 Planning considerations ............................................................................................... 59

13 Next steps .................................................................................................................... 60

14 Conclusions ................................................................................................................. 60

15 References .................................................................................................................. 61

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List of Figures

Figure 1. The upper Purga Creek catchment area. The Roadvale salinity project is concerned with the area from the top of the catchment to around Milbong. The catchment boundary is shown in purple. The figure shows the main streams. Source: Kate Goulding DNRM. .................................................................................. 11

Figure 2. Map of the extent of the Clarence-Moreton water resource management area of the Great Artesian Basin Water Resource Plan area from DNRM (2005). The figure shows the management area shown in yellow with dark brown boundary. The Clarence-Moreton basin does extend south into New South Wales ......................................................................................................................... 13

Figure 3. Geology map of upper Purga Creek from Cranfield et al., (1981). The alluvium of Purga Creek, and Warrill Creek is identified as Qa, the Walloon Coal Measures as Jw and the Marburg Subgroup as Jbm. The formation Jbmh called Heifer Creek sandstone member of quartz sandstones is shown in darker green. The darker yellow formations labelled with T are volcanic intrusions Tis as syenite and Tid as basalt. The basalt intrusion near Milbong is evident and the approximate position of a fault line is marked with the dotted black line. The cross section in Figure 4 is of the section north east from J on the map above. It shows the relationship of the strata in the middle Purga Creek area. The dip of the Marburg beds exposed on the east of Purga Creek is towards the west as shown with degrees adjacent to the small arrows on the Marburg Subgroup. ............ 14

Figure 4. The cross section of strata following the transect line in black from point J on Figure 3 to the north east. Source Cranfield (1981). ................................................... 15

Figure 5. Image of upper Purga Creek from Google Earth showing the approximate location of Jbmh (western edge of Marburg Subgroup) of Figure 3 as the white line. The basalt intrusion near Milbong is identified as an orange circle. The area within the yellow ellipse is a ridge of highly siliceous sandstone somewhat atypical of surrounding country and is discussed in the text. Image date 2006. .......... 15

Figure 6. An example rock outcrop on the ridge of siliceous sandstone within the yellow ellipse shown in Figure 5. ................................................................................ 16

Figure 7. Soils of the Boonah area from Loi et al., (2005). The Purga Creek alluvium is shown as blue on the map with symbols B, Ls (Bremer and Lockrose). The soils formed on the basalt intrusion near Milbong are shown in light brown with the symbol Na (Neara). Soils on the Walloon Coal Measures are identified as Rv (Rockville, medium clays) and those on the Marburg Subgroup as Rv-KI Ka.(Rockville, Kenilworth (hard setting surface) and Kamerigo (hard setting and showing a texture contrast in the profile)). The soils upstream of the probable fault line of Cranfield et al., (1981) are shown as Ly-Wh. These soils Lockyer – Woodhill are on more sandy subsoils and may have orange mottles in the subsoil indicating fluctuating wetness. The Wg Sw (Wyaralong – Swan Creek) soils occur within the yellow ellipse of Figure 5 and are characterised by bleaching and strongly mottled acid subsoils with a sandy texture. ............................ 18

Figure 8. Extent and severity of surface soil salinity in upper Purga Creek. Red areas are severely affected and orange areas (north of Roadvale) are moderately affected, from Eyre and Goulding (2013 in press). ..................................................... 20

Figure 9. Common forms of salinity in Queensland identifying restrictions to groundwater flow and processes operating. Based on Shaw et al., (1987) and Salcon (1997). ........................................................................................................... 21

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Figure 10. Former fence post almost buried near Purga Creek upstream of Hansen Road and the current fence post illustrating the depth of sediment deposited in this area since land clearing and settlement. .............................................................. 22

Figure 11. Tributary to Purga Creek along Ehrich Road where the current fence is the third fence constructed on top of the previous two fences covered by sediment. ....... 23

Figure 12. Map of transects through Purga Creek using LiDAR data illustrating the very narrow valley at one eye waterhole as transect E, the extensive flat area around the 92 to 91 m contour interval as transect B and the very low slope and hence hydraulic gradient down Purga Creek alluvium as transect D. Analysis and transects provided by Kate Goulding DNRM. ............................................................. 26

Figure 13. Land surface elevations from LiDAR data showing one metre contour intervals as metres above Australian Height Datum (AHD). Numbers on the coloured contour lines indicate metres AHD. The narrowing of the alluvium between the 92 m and the 91 m contour interval (marked with a blue arrow) is evident. ...................................................................................................................... 28

Figure 14. Photo of old dam near Hanson Road with high EC (17.5 dS/m) and surrounded by bare salted areas. ............................................................................... 30

Figure 15. Location of the four salinity sites used for changes over time from aerial imagery as described in the text. Map from Google Earth, 2012 imagery. .................. 31

Figure 16. Comparison of annual rainfall for Boonah and Roadvale area. Data for Roadvale area from Steve Mocker and for Boonah from Bureau of Meteorology. ...... 32

Figure 17. Rankings of the extent and severity of salinity at 4 sites based on aerial imagery as explained in the text and Table 1. ............................................................ 34

Figure 18. Various annual rainfall indicies for the rainfall stations of Boonah and Warwick over the period of the aerial imagery. Data sourced from the Bureau of Meteorology .............................................................................................................. 36

Figure 19. Monitored DNRM bore water levels in upper Purga Creek . A comparison with Figure 18 rainfall indicies indicates a variable response of water level trends to rainfall. The depth in the bore number legend is the depth of the slotted section of the bore below ground level in metres. The EC in dS/m is the range in values measured over the length of record. Data from DNRM. .................................. 37

Figure 20. Standing water level (SWL) in bores and wells in upper Purga Creek catchment from measurements taken between August and December 2012 on farm visits and from monitored bores by Department of Natural Resources and Mines. Water levels are given in metres below ground eg. -1, above ground eg. 2, and at ground level or overflowing as 0. Those that are artesian or at ground level are shown in red, SWLs shallower than 0.5 m are shown in orange, SWLs 0.51 to 3 m below ground in yellow and SWLs > 3 m in blue. The SWL in the bore without a depth was not able to be determined. ................................................. 39

Figure 21. The area with expected major artesian discharge is outlined with a red line. The key to the figure is described in Figure 20. .......................................................... 40

Figure 22. Elevation above AHD of standing water levels in bores and wells and monitored bores shown in metres. Ground level reference taken from field GPS, DNRM groundwater database and Google Earth. SWLs of red labelled bores are above ground level. Orange labelled bores and wells have SWL at ground level. The elevation in the bore without a value was not able to be determined. .................. 41

Figure 23. The overflow from a well 14.2 m deep with SWL at ground level on the west side of Zillman Flat Road. .......................................................................................... 42

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Figure 24. An expanded view of the boundary between the Surat basin to the left of the Blue dashed line representing the water table divide and the Clarence-Moreton basin to the east illustrating the watertable elevation at the boundary between the two basins. Figure from Geoscience Australia published as Figure 6.19 in Kellett et al. (2012). Figure used with permission. The locations of Peak Crossing, Warrill View and Boonah have been added to the figure. ........................... 43

Figure 25. The proportions of calcium, magnesium and sodium in the measured bores, wells and streams. Coloured ellipses indicate waters of different water compositions as explained in the legend above and in the text. ................................. 45

Figure 26. An overlay of upper Purga Creek bore and well composition data from Figure 25 with Loamside stream gauging data illustrating the similar composition. The samples were taken at a range of stream flow rates. Data from DNRM. ............. 46

Figure 27. Change of state from a normal catchment situation to a degraded and saline catchment with increasing groundwater input/output imbalance until a critical soil salinity level is exceeded where death of vegetation occurs. The degree of reversal required to restore the area to a non saline state requires a return of the groundwater imbalance to a point where evaporation is reduced to below a critical soil salinity level, from Shaw (2008). .................................................. 50

Figure 28. EC of surface water and dams in upper Purga Creek sampled during this investigation. The dark blue line represents a likely flow line with groundwater of EC < 7.5 dS/m. .......................................................................................................... 52

Figure 29. For discussion purposes, the effectiveness of the salinity management options in Table 2 in relation to the cost and effort required over the medium to longer term. ................................................................................................................ 53

List of Tables

Table 1. Ranking of extent and severity of salinity in four salted areas in upper Purga Creek as described in the text. Ranking 1 has vegetation effects but no bare ground, ranking 2 shows bare salted areas and ranking 5 has the most severely affected and extensive area over the series of imagery. Boonah rainfall from Bureau of Meteorology and Roadvale area rainfall courtesy of Steve Mocker. ........... 33

Table 2. A comparison of the possible salinity management options for upper Purga Creek ......................................................................................................................... 54

Table 3. Vision for upper Purga Creek from landholder meeting at Roadvale State School on 19 January 2103. ...................................................................................... 58

Table 4. Preferred salinity management options by landholders at the meeting at Roadvale State School on 19 January 2013. ............................................................. 58

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Abbreviations and glossary of terms

AHD Australian Height Datum is a geodetic datum for altitude measurement equivalent to mean sea level.

Aquifer a permeable formation or group of formations capable of storing and transmitting sufficient quantities of water under normal hydraulic gradients to cause changes in the watertable level in a short period.

Artesian the upward movement of water under pressure in rocks or unconsolidated material beneath the earth's surface.

Catena a sequence of different soil profiles that occur down a slope usually on the same parent material but varying in soil properties largely due to varying hydrology, weathering rates and deposition.

Discharge area an area in the landscape where the net movement of groundwater is out of the catchment. Waterlogging and salting are most likely to occur in this area, as groundwater discharging at the soil surface by seepage or evaporation.

DNRM Queensland Department of Natural Resources and Mines

EC - Electrical Conductivity is the conductance of a solution. The conductance is directly related to the concentration of salts in the water. Rainfall has an EC of 0.03 dS/m and sea water 55.0 dS/m. The general limit for agriculture is 8 dS/m. Units of µS/cm are also used which are 1 000 times larger than dS/m. EC can be converted to mg/L (or parts per million) - EC in dS/m multiplied by 640. ECse is the EC of an extract of saturated soil.

GAB Great Artesian Basin

GPS global positioning system

Groundwater water occurring below the surface of the earth occupying cavities and spaces in soils and rocks. The upper surface of the groundwater is the watertable.

Hydraulic head the height at which water stands in a piezometer tube or bore measured relative to a chosen elevation datum.

Hydrologic the study of the properties, distribution, and effects of water on the earth's surface, in the soil and underlying rocks, and in the atmosphere.

Landform features a specific geomorphic feature on the surface of the earth, ranging from large-scale features such as plains, plateaus, and mountains to minor features such as hills, valleys, and alluvial fans.

LiDAR (Light Detection and Ranging) is an optical remote sensing technology.

Piezometer a tube, open to water flow at a determined depth, sealed along the rest of its length, and open to the atmosphere at the top, in which the hydraulic head or elevation of the watertable can be measured.

RA - Residual Alkali is the excess of carbonate and bicarbonate ions over calcium plus magnesium ions in a water analysis and is mostly associated with sodium.

Rainfall residual mass curve the cumulative deviation of the annual rainfall from the average annual rainfall over the years of record.

Rainfall 5 year moving average the average annual rainfall of 5 year periods incremented one year at a time.

Recharge area an area of the landscape where the net movement of water is downwards into and ‘recharging’ the groundwater.

SAR - Sodium adsorption ratio the relative content of sodium to calcium and magnesium in a soil solution or water that approximates the exchangeable sodium percentage (ESP) of the soil. Soil ESP is a good indicator of soil clay dispersion.

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SEQ South East Queensland

SWL Standing water level – see watertable

Watertable the upper surface of a zone of saturation in an unconfined aquifer. Below the watertable, the aquifer material is permanently saturated; above the watertable, the rock or soil is unsaturated. The ‘depth’ of the watertable is measured relative to the soil surface as standing water level.

Unsaturated zone is the part of the earth between the land surface and the top of the watertable.

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1 The brief

The Roadvale Salinity project was prompted by concerns expressed and discussions held between representatives of Roadvale State School, officers of Scenic Rim Regional Council, SEQ Catchments, landholders in the area and Bremer Catchment Association about the adverse impact of salinity on resources in the Roadvale-Milbong area of the upper Purga Creek sub-catchment.

The project objectives are to:

Identify the salinity processes operating in the upper Purga Creek catchment.

Outline potential salinity management options to control, manage and/or reclaim salt affected areas.

Consult with landholders, the community, Council and SEQ Catchments on preferred priority options for salinity management.

Propose appropriate planning principles and guidelines for future development of the area that will reduce the impact of salinity.

The project is being conducted in stages with decision points at key times during the project to evaluate progress and assess the next steps. This report addresses stage 1 of the project which focuses on salinity processes operating in the upper Purga Creek catchment and identification of preferred management options. Future stages of the project are being developed.

The project is being funded by the Scenic Rim Regional Council and coordinated by SEQ Catchments. Other partners to the project include local property owners, Roadvale State School, the Department of Natural Resources and Mines and the Bremer Catchment Association.

1.1 This report

This report is structured to provide:

a summary of relevant background information.

an interpretation of the hydrology and salinity processes operating in the Roadvale-Milbong area based on available information.

to propose possible salinity management and planning options for consideration by Scenic Rim Regional Council, SEQ Catchments and landholders. Discussion on the report were held at Roadvale State School on 19th January 2013 and 4 May 2013 from which management options were considered and prioritised. A strategy for stage 2 of the project was discussed at the May 2013 workshop.

1.2 Method

The method employed included the collation of historical information, review of previous investigations, comparison of available air photographs and collation of oral history from interviews, publications and spatial and specific data from relevant government databases. On-farm visits to ten landholders in the area were made over the period August 2012 to January 2013. The locations of salt outbreaks, wells, dams and bores were identified and measurements of water levels and electrical conductivity (EC) were taken, with a selection of water samples collected and analysed for chemical composition. Two landholder meetings were held at the Roadvale State School in August and September 2012. Participants included representatives from Scenic Rim Regional Council, SEQ Catchments and Bremer Catchment Association. These workshops focussed on discussing the principles of salinity and preliminary progress on investigations and to seek comment and input. Two further workshops in January and May 2013 focussed on salinity management options based on the salinity investigations and the conclusions outlined in this report.

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The area of interest for the project is the Roadvale – Milbong area of the upper Purga Creek catchment as shown in Figure 1. An analysis of the water quality at the Loamside gauging station on Purga Creek near Ipswich has been included. It is the closest gauging station for Purga Creek. Loamside is some 32 km downstream of Milbong.

2 Brief history of Roadvale - Milbong area

In the 1800s, the ease of obtaining pastoral leases on Crown land and the closeness to the Brisbane settlement encouraged settlement in the area (Fassifern Centenary Committee, 1944). The early resistance of the established grazing community to the introduction of agricultural and cropping was overcome by the late1870s with German settlers moving into the area. Dairying commenced in the late 1870s. Extensive clearing followed and significant maize cropping was established by the 1890s until around 1918. A range of other crops have been grown over many years.

Previously the Roadvale area was under quite dense Brigalow scrub. Milbong (previously called One eye waterhole) was settled in 1869 and was the site of the first Council headquarters (Fassifern Centenary Committee, 1944). Cropping on the upland scrub soils

Figure 1. The upper Purga Creek catchment area. The Roadvale salinity project is concerned with the area from the top of the catchment to around Milbong. The catchment boundary is shown in purple. The figure shows the main streams. Source: Kate Goulding DNRM.

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on Walloon Coal Measures geological formation largely ceased around 1992 because the soils became too difficult to cultivate due to major erosion of surface soils. One of the stories about One eye waterhole, now a water reserve near Milbong, was that horses were sent into the waterhole to stir up the fish so the settlers could catch mullet. Currently the water hole would be too shallow for this to occur.

The rapid land use change towards the end of the 1800s with productive cropping and dairying on the scrub soils (Walloon Coal Measures) resulted in severe erosion, soil degradation and loss of soil fertility of the cropping soils with the sediment being deposited in the alluvial areas of Purga Creek. This is discussed further in section 3. The current dominant land use in the Roadvale – Milbong area is grazing.

Supplementary data which includes the location of sampling sites and groundwater and surface water analyses is available on request.

3 Geology, soils and landforms

The geology, landforms and soils information are necessary to determine the hydrological characteristics of the area. Since salinity is a result of water input, flow paths in the landscape, restrictions to water flow, evaporation concentrating the salt on the soil surface and evapotranspiration, the landscape characteristics are important.

3.1 Geology

The geology has been mapped at the broad 1:250 000 scale by Cranfield (1973) and described in Cranfield et al., (1976) and also at the 1:100 000 scale by Cranfield et al., (1981). The Purga Creek catchment is within the Clarence-Moreton basin, a sub basin of the Great Artesian Basin (GAB). The Clarence-Moreton Basin underlies south-east Queensland and north-eastern New South Wales. The basin contains over 1 000 metres of sediments of Late Triassic to Middle Jurassic geologic age in the deepest parts. The Basin is connected with the Surat Basin of the GAB to the west over the Kumbarilla Ridge (DNRM, 2005). Figure 2 show the water management area of the Clarence-Moreton basin in Queensland.

In summary, the geologic features are eroded basaltic and volcanic areas from intruded volcanic rocks overlying the Walloon Coal Measures formation. The Purga Creek catchment is of smaller scale than the adjacent Warrill Creek catchment and does not have significant volcanic material within the catchment boundary.

There are remnants of lake formed limestone deposits of more recent Tertiary age, which are mostly in the northern parts of the Bremer River catchment and overlie the Walloon Coal Measures. The Walloon Coal Measures consist of interbedded sandstones, siltstones, claystones and coal which form the undulating hilly country to the west of Purga Creek with the soils commonly called scrub soils. To the east of Purga Creek, the Marburg Subgroup which underlies the Walloon Coal Measures is exposed and has formed a more hilly terrain. The Marburg Subgroup comprises inter-bedded sandstone, siltstone and occasional coal. The soils on the Marburg Subgroup are more sandy, less fertile have higher exchangeable sodium content and are commonly referred to as forest soils. Folding and some faulting of the sediments has occurred with a major syncline (depression shaped) in the Warrill Creek area with an anticline (arch shaped) to the east of Purga creek which results in strongly downwards dipping beds towards Purga Creek. The geology map of Cranfield et al., (1981) is shown in Figure 3. The west Ipswich fault is a major fault to the east of area of Figure 3. It is discussed further in section 4.

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Figure 2. Map of the extent of the Clarence-Moreton water resource management area of the Great Artesian Basin Water Resource Plan area from DNRM (2005). The figure shows the management area shown in yellow with dark brown boundary. The Clarence-Moreton basin does extend south into New South Wales

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Roadvale

Milbong

Peak Crossing

Figure 3. Geology map of upper Purga Creek from Cranfield et al., (1981). The alluvium of Purga Creek, and Warrill Creek is identified as Qa, the Walloon Coal Measures as Jw and the Marburg Subgroup as Jbm. The formation Jbmh called Heifer Creek sandstone member of quartz sandstones is shown in darker green. The darker yellow formations labelled with T are volcanic intrusions Tis as syenite and Tid as basalt. The basalt intrusion near Milbong is evident and the approximate position of a fault line is marked with the dotted black line. The cross section in Figure 4 is of the section north east from J on the map above. It shows the relationship of the strata in the middle Purga Creek area. The dip of the Marburg beds exposed on the east of Purga Creek is towards the west as shown with degrees adjacent to the small arrows on the Marburg Subgroup.

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Figure 5. Image of upper Purga Creek from Google Earth showing the approximate location of Jbmh (western edge of Marburg Subgroup) of Figure 3 as the white line. The basalt intrusion near Milbong is identified as an orange circle. The area within the yellow ellipse is a ridge of highly siliceous sandstone somewhat atypical of surrounding country and is discussed in the text. Image date 2006.

Figure 5. Image of upper Purga Creek from Google Earth showing the approximate location of Jbmh (western edge of Marburg Subgroup) of Figure 3 as the white line. The basalt intrusion near Milbong is identified as an orange circle. The area within the yellow ellipse is a ridge of highly siliceous sandstone somewhat atypical of surrounding country and is discussed in the text. Image date 2006.

Figure 4. The cross section of strata following the transect line in black from point J on Figure 3 to the north east. Source Cranfield (1981).

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The basalt intrusion near Milbong has probably resulted in some restriction to down valley flow of Purga Creek. The yellow ellipse in Figure 5 has not been separately identified on geology maps. It appears to have a significant effect on the flow of Purga Creek and its upstream tributary flowing along the western edge of the Marburg escarpment as can be seen on Figure 5 and more clearly in Figure 1. Figure 6 is a photograph of the rock outcrops on the ridge. A siliceous sandstone that appears to be more resistant to weathering than the surrounding area. The vegetation pattern on the ridge is different from adjacent ridges.

The 1:250 000 geology map of Cranfield (1973) locates the fault further upstream just north of Zingleman Road and opposite the basalt intrusion which is also located further to the south on that map. These patterns are more consistent with the observable features and the diversion of Purga Creek at One eye waterhole as shown in Figure 5. The variability in mapping these landscape features is an indication of the complexity in this region. There are some soils differences as identified by Loi et al., (2005) as discussed in section 3.2.

There have been a number of reports on the coal and potential petroleum resources of parts of the Purga Creek catchment. These reports have not been considered apart from Willmott et al., (1979), Cooper at al., (1978) and an overview of the relevant papers in Wells and O’Brien (1994).

The landforms of upper Purga Creek comprises scrub soils on undulating Walloon Coal Measures to the west. The catchment becomes steeper towards the headwaters to the south and to the north east near Milbong. The gullies in the Walloons are generally broad and shallow compared to the much more deeply incised and steeper sided gullies in the Marburg Subgroup to the east. Both tend to show saline seepages in the base of the gullies. The floodplain of Purga Creek is quite extensive towards the north to Milbong and then becomes narrow through One eye waterhole area and beyond for some distance before becoming a wider floodplain again.

Figure 6. An example rock outcrop on the ridge of siliceous sandstone within the yellow ellipse shown in Figure 5.

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In summary the geological features indicate general dominance of Walloon Coal Measures over the main areas, a basalt intrusion near Milbong and some faulting and other features that have probably restricted down valley flow near Milbong.

3.2 Soils

The soils of the Boonah area have been mapped at medium density by Loi et al., (2005) and the soils map for Purga Creek is shown in Figure 7. A reference section on the western edge of the Purga Creek catchment around Kalbar which adjoins the mapping of Loi et al., was mapped by Powell (1979). Johnston (1979) mapped the broad soil groups and the land degradation status of the Bremer catchment. The soil properties in the report of Loi et al., (2005) have been used to estimate deep drainage below the root zone as discussed in section 6.6.

The alluvium as mapped by Loi et al., (2005) is of larger extent than shown on the geological maps. The significance of the Lockyer and Woodhill soils and the Wyaralong and Swan Creek soils with more sodium, sandy mottled subsoils and in Wyaralong with acid subsoils, which occur within the area of the yellow ellipse in Figure 5 confirm different parent material seen in this area.

Each of the above geology and soils reports provides details about earlier investigations.

3.3 Salinity occurrence

There has been a number of salinity investigations in the area over the years. A salinity survey across Queensland by Hughes 1979 found approximately 2 288 ha in South East Queensland (SEQ) with the main areas located in the Bremer and Lockyer Valley catchment. Also in 1979 there was a Land degradation Study for the Bremer Catchment which found four salt outbreaks totalling 269 ha (Johnston, 1979).

In 1991 another Queensland survey was undertaken which found salinity had increased (to 9,665 ha) across SEQ and Wide Bay Burnett. The majority of salt outbreaks were mapped in the Lockyer Valley, Ipswich and Boonah Shires.

Eyre & Goulding (2013 in press) mapped the extent and severity of surface soil salinity in SEQ for SEQ Catchments and found 650 ha of salt affected land in the Bremer catchment. This project involved both desktop and field investigations. Electrical Conductivity of the top 10 cm of the soil was tested and salinity processes and indicators were noted along with the extent and severity of each polygon.

The majority of the salt affected land mapped in the Bremer Catchment was found on the alluvium (465 ha) with the remainder mostly linked with the Walloon Coal Measures (144 ha) and Gatton Sandstone (37 ha) geology which is part of the Marburg Subgroup. Approximately 145 ha of salt affected land was identified in the project area. Most areas mapped in the Upper Purga Creek area were severely affected (>8.0 dS/m ECse). The single largest area mapped in the Bremer Catchment is 140 ha comprising the alluvium between the upstream of Roadvale School and Milbong as shown in Figure 8.

In Upper Purga Creek area, the salt affected area shown in Figure 8 has more than doubled in extent with 48 additional areas identified compared with four in the Bremer land degradation study (Johnston, 1979). This apparent increase may not represent a real increase in salt affected land, but could reflect more intensive field activities which identified previously unrecognised areas. It is also unlikely in this study and in previous studies that all salted sites were identified. Additional areas of salt affected land have been identified (44 ha) during the recent field investigations as part of this project giving a total area affected by salinity of 189 ha.

Salinity investigations were carried out at Roadvale State School in the 1980s but the data has not been located within Departmental files.

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Figure 7. Soils of the Boonah area from Loi et al., (2005). The Purga Creek alluvium is shown as blue on the map with symbols B, Ls (Bremer and Lockrose). The soils formed on the basalt intrusion near Milbong are shown in light brown with the symbol Na (Neara). Soils on the Walloon Coal Measures are identified as Rv (Rockville, medium clays) and those on the Marburg Subgroup as Rv-KI Ka.(Rockville, Kenilworth (hard setting surface) and Kamerigo (hard setting and showing a texture contrast in the profile)). The soils upstream of the probable fault line of Cranfield et al., (1981) are shown as Ly-Wh. These soils Lockyer – Woodhill are on more sandy subsoils and may have orange mottles in the subsoil indicating fluctuating wetness. The Wg Sw (Wyaralong – Swan Creek) soils occur within the yellow ellipse of Figure 5 and are characterised by bleaching and strongly mottled acid subsoils with a sandy texture.

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3.4 Landscape features and the occurrence of salinity

Shaw et al., (1987) and Salcon (1997) identified a range of landscape and geomorphological features of hydrologically sensitive landscapes which are commonly associated with salinity outbreaks in Queensland. These features have proven to be a useful approach to integrate landscape features in relation to groundwater flow restrictions and indicate locations where salinity risk occurs. For the Purga Creek area, there are several landscape forms present that contribute to the occurrence of salinity.

Evidence in the landscape of previous salinity through carbonate deposits or gypsum as well as high salt levels in sub soils and groundwater is very common. Also high soil sodium levels indicate that salt has accumulated in the soils under natural conditions. Pre-clearing vegetation patterns in the catchment would have reflected the general availability of water in the various parts of the landscape and salt and waterlogging tolerant vegetation (such as tea tree) was often found in valley floors where current salinity is present. Thus clearing changed the hydrology of the area by allowing greater deep drainage below the root zone leading to recharge of the groundwater through the wetting up of the soil profile in the unsaturated zone above the saturated region. Episodic high rainfall years such as the 1950s and 1970s for south east Queensland tend to show the first evidence of salinity since the higher rainfall provides sufficient recharge to raise the water tables close enough to the soil surface where evaporation of water occurs and salts are concentrated at the soil surface. Many aspects of salinity are discussed in detail in Salcon (1997) and the second edition (Salcon, 2011) available on the web at http://www.nrm.qld.gov.au/salinity/managing_salinity.html.

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Figure 8. Extent and severity of surface soil salinity in upper Purga Creek. Red areas are severely affected and orange areas (north of Roadvale) are moderately affected, from Eyre and Goulding (2013 in press).

Figure 9 shows the landscape features associated with salinity outbreaks in Queensland. As discussed in earlier meetings, upper Purga Creek has several of these features contributing to the expression of salinity. The main landscape features relevant to Purga Creek are alluvial valley, catchment restriction, stratigraphic, roads, catena (the sequence of soils on the same parent material that differ in properties progressing downslope) and dams. These landscape features are discussed below.

In all these instances, the rate of recharge of the groundwater exceeds the rate at which groundwater can flow out from the system. This results in raised watertable levels with capillary rise to the soil surface and consequent evaporation leaving salt on the soil surface and/or seepage into streams.

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Figure 9. Common forms of salinity in Queensland identifying restrictions to groundwater flow and processes operating. Based on Shaw et al., (1987) and Salcon (1997).

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Alluvial valley

Upper Purga Creek has extensive alluvial flats extending to One eye waterhole near Milbong. Alluvial flats usually develop where there is a restriction to down valley flow resulting in a reduced water flow velocity and spreading out of water which results in deposition of sediment. Flat alluvial valleys have low hydraulic gradient and thus quite limited groundwater flow laterally downstream. If incision by stream flow occurs, drainage of groundwater can occur. However, where additional sediment is deposited over time, these stream channels may be filled and the general elevation of the soil surface raised, further restricting down valley water movement. This is confirmed for similar catchments on the eastern Darling Downs (Andrew Biggs pers comm).

Land clearing on undulating lands generally results in soil erosion and gullying. Considerable work has been carried out on soil conservation practices in the region over the years by Bill Steentsma and others. Landholders recall extensive quantities of sediment being deposited on roads in the catchment following heavy rainfall which had to be cleared for the road to be trafficable. In the northern end upstream of Hansen Road, a former fence post was exposed in a lower lying area indicating the depth of sediment that covered the alluvial flats upstream of Hansen Road, Figure 10.

Figure 10. Former fence post almost buried near Purga Creek upstream of Hansen Road and the current fence post illustrating the depth of sediment deposited in this area since land clearing and settlement.

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An old well adjacent to Zillman Flat Road and upstream of S Muller Road shows evidence of sedimentation around the well with an elevated soil surface considerably above the old well structure. In an eastern tributary to Purga Creek above the Roadvale State School, the third fence has been constructed across the valley on top of the previous two fences as shown in Figure 11.

Deposition of sediment on alluvial flats changes the hydrology of the alluvial areas by filling in stream channels, restricting water flow and raising water levels. Reduction of riparian vegetation results in an increase in erosive power of streams and new channels may appear.

Basalt

There are no large areas of basalt in upper Purga Creek catchment and since salting associated with basalt occurs because the basalt is fractured and porous, usually over more impermeable strata, then this form of salting is not present in Purga Creek. As Pearce et al., (2007) have noted, volcanic intrusions do interrupt flow in the Bremer catchment and this is discussed under Catchment restriction.

Catena

A soil catena is a sequence of different soil profiles that occur down a slope usually on the same parent material but varying in soil properties largely due to varying hydrology, weathering rates and deposition. Salting associated with a catena occurs on the western side of Purga Creek and is closely related to the higher than natural water table levels under the alluvial flats. Water is then restricted in its rate of flow into the alluvial areas and accumulates upslope resulting in shallow water tables and salting upslope. This is a consequence of the reduced rate of groundwater flow. This occurs in upper Purga creek particularly on the western side between S Muller Road and Gray Street (Roadvale Road). It is significant for upper Purga Creek but closely related to the water table level in the alluvium.

Figure 11. Tributary to Purga Creek along Ehrich Road where the current fence is the third fence constructed on top of the previous two fences covered by sediment.

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Dykes

Geologic dykes of near vertical orientation can cause permeability barriers to groundwater flow. There has not been observable evidence of dykes in upper Purga Creek although in some areas associated with volcanic intrusions within the broader Bremer catchment, dykes have been observed (Pearce et al., 2007).

Dams

There are many dams in drainage lines and on the alluvium in Purga Creek, some 186 in 2012 in the upper catchment south of Milbong, and there is evidence from aerial imagery and water levels below dams that leakage is occurring. This will result in recharge to the groundwater and raised water levels in the adjacent area. This water table rise is dissipated into the surrounding area or down the drainage line. Dams will significantly exacerbate salinity in already hydrologically sensitive landscapes or where shallow water tables are already present. It is considered a factor in salinity management in the catchment as discussed in section 6.5.

Confluence of streams

A smaller watercourse enters Purga Creek from the east upstream of Milbong. The stream has probably contributed to the deposition of sediment immediately upstream in Purga Creek restricting groundwater outflow at the junction.

Catchment restriction

The soil mapping of Loi et al., (2005) and the geology maps of Cranfield (1973) and Cranfield et al., (1981) suggest that there are geological features that have restricted down valley groundwater flow just downstream of Zinglemann Road and the One eye waterhole. These were outlined in Figure 5 and discussed in section 3.1. This catchment restriction is considered to be an important factor contributing to the occurrence of salinity in upper Purga Creek.

There is forest vegetation on the eastern and western sides of One eye waterhole where the constriction in geology has resulted in an incised stream compared to the wider flat alluvium upstream. Exploratory drilling and elevations would be necessary if confirmation of the geologic restriction was required.

Figure 12 shows some transects through upper Purga Creek illustrating the significant narrowing of the creek alluvium near Milbong and where the Ipswich-Boonah Road crosses Purga Creek. This illustrates the probable restrictions to down valley flow of groundwater. From the LiDAR (Light Detection and Ranging) remote sensing data that can give accurate ground level contours at 1 m intervals as shown in Figure 13, there appears to be a narrowing of the alluvium between the 92 and 91 m contour intervals which closely aligns with the bare salinity in the area at site N in Figure 15.

Because of the narrowing of the alluvium (although the depth is unknown but if the alluvium is narrow, the depth tends to be shallower than adjacent areas) the height of the groundwater needs to increase to achieve the same volume of flow through the region as would be occurring in a much wider alluvial cross section upstream of the narrow part. If the raised water table is close to the soil surface (say 0.5 to 1 m deep) capillary rise and evaporation will be considerable and salt concentration will occur on the soil surface.

Information supplied by landholders suggest that Brigalow occurred in this area prior to clearing and only a few remnant areas still exist in the area. Below the 91 m contour interval it seems Eucalypts may have dominated in the alluvium suggesting lower watertable levels and salinity in this region in the pre land clearing period. The observable restriction at One eye waterhole from the siliceous sandstone ridge and Basalt intrusion was probably instrumental in forming the extent of the alluvium upstream. However, the extent of the hydrologic change on land development to date appears to have been mostly accommodated by the evaporation of the shallow watertable around the 91 to 92 m contour

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interval across the alluvium as shown in Figure 12. Any increase in water inputs into upper Purga Creek including from above average rainfalls for extended periods may result in shallow watertables and surface salinity between the 91 m contour and One eye waterhole.

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Figure 12. Map of transects through Purga Creek using LiDAR data illustrating the very narrow valley at One eye waterhole as transect E, the extensive flat area around the 92 to 91 m contour interval as transect B and the very low slope and hence hydraulic gradient down Purga Creek alluvium as transect D. Analysis and transects provided by Kate Goulding DNRM.

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Figure 12 continued

0

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0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5

Transect B Elevation Profile

Distance (km)

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Purga Creek

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Transect D Elevation Profile

Distance (km)

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Purga Creek

0

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Transect E Elevation Profile

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Purga Creek

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Figure 13. Land surface elevations from LiDAR data showing one metre contour intervals as metres above Australian Height Datum (AHD). Numbers on the coloured contour lines indicate metres AHD. The narrowing of the alluvium between the 92 m and the 91 m contour interval (marked with a blue arrow) is evident.

Geologic fault

Apart from the fault mentioned in section 3.1 there appears to be no faulting related to the occurrence of salinity. There are some preferential upward flow paths for groundwater in some sections of upper Purga Creek catchment that result in surface seepages and/or salting. These occurrences would be captured under the stratigraphic form. It is interesting that further to the west on the Walloons, landholder comments indicate that groundwater is largely unknown so that the seepages, springs and artesian bores and wells only occur close to Purga Creek for the western side.

Roads

Roads contribute to salinity because in the early days, traffic across historically wet areas in non-formed roads caused compaction in the upper layers and reduced groundwater flow. Evidence of this is present near Roadvale School and also S Muller Road. The Ipswich-Boonah Road may also have had an effect in the past near Hansen Road but the extensive shallow watertable at present around the 91 to 92 m contour interval appears to dominate over any road effect. The occurrence of bare salted areas at sites S and M in Figure 13 are closely associated with roads across the alluvium.

Stratigraphic

The presence of seepages at elevated positions in the catchment such as Roadvale School, near Blantyre road and the number of wells and bores with water table levels above or close to the soil surface suggests that there are many small aquifers through the Walloon Coal Measures that are under hydraulic pressure and reaching the soil surface through preferential flow paths.

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Summary of landscape features and salinity

Considering all the landscape features and their relevance to upper Purga Creek, the major features contributing to salinity are listed below in decreasing order of significance:

1. Catchment restriction to down valley flow near the 91 to 92 m contour intervals associated with a narrowing of the alluvium with a second restriction to down valley flow likely at One eye waterhole

2. The extensive alluvial valley formed upstream of One eye waterhole and the additional sediment deposited since land clearing with reduced groundwater hydraulic gradient and hence flow rate

3. Geologic features together with artesian pressure head in bores and wells indicate that upward leakage of water from underlying strata is significant. The strong probability is that the region is a local discharge area within the Clarence-Moreton basin. This probably dominates the water inputs to a considerably greater extent than recharge from land clearing and land use change

4. Dams and their frequency across the landscape which maintain high water levels in drainage lines through leakage, and

5. The catena form and roads exacerbate the salinity issues largely since groundwater flow is restricted and the water table in the alluvium is elevated.

4 Hydrogeology

Pearce et al., (2007) investigated the hydrogeological features of the Bremer River catchment. Other than this report there is very limited data available on the groundwater in the Bremer catchment and particularly in the Purga Creek catchment. Some 50 bores were drilled into hard rock across the Bremer River catchment between 2004 and 2006 (Pearce et al., 2007). Many of these bores have been monitored for water levels since that time and the ones in upper Purga Creek are discussed in section 5.3.

Hillier (2010) and DNRM (2005) indicate that there is a hydrological connection between the Clarence-Moreton sub basin of the GAB and the Surat basin of the GAB. The inclusion of the Clarence-Moreton management area within the GAB is relatively recent. “The Clarence-Moreton Management Area is a new management area based on the extent of the Jurassic to Upper Triassic sediments in the Clarence-Moreton Basin to the west of the Ipswich Fault. The fault is a hydrological boundary and the aquifers to the east are much more variable in water quality and yield.” (DNRM, 2005). The Clarence-Moreton management area is included within the GAB because the more recent sediments in the Jurassic period are continuous over shallow basement ridges. DNRM (2005) indicate that bores in the Walloon Coal Measures and Marburg formations generally are of poor water quality and water yields are low. In the report the authors identify some artesian bores present in the Marburg Subgroup in the Clarence-Moreton basin but do not give locations. The influence of the Clarence-Moreton basin hydrogeology is discussed further in sections 6.1 and 6.2.

5 Salinity in upper Purga Creek catchment

5.1 History of salinity

Brigalow scrub is universally associated with soils that have root zone salinity and usually high exchangeable sodium in the root zone. This salt accumulation is mostly due to:

the low permeability of the generally heavy clays soils on which Brigalow occurs

the water use capabilities of the vegetation which results in very small quantities of deep drainage to occur below the root zone; and

chemical weathering of the sediments.

Following clearing, extra recharge results in an increase in drainage below the root zone and thus some mobilisation of salts from the root zone and unsaturated zone (below the root zone) in the landscape to the groundwater. This process is generally slow due to the low

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permeability of the soil as reflected in the generally high exchangeable sodium levels at depth which increases soil dispersion and particle packing with consequent reduced soil porosity and hence hydraulic conductivity.

The usual pattern of dryland salinity development is that after clearing has occurred there is an increase of recharge to groundwater resulting in the landscape reaching a new equilibrium. In this stage the usually dry unsaturated zone below the root zone and above the watertable is gradually wet by the increasing recharge. This can take anywhere from a few years to decades. This stage is followed by a somewhat steady state situation with shallow water tables. When the water table rises close to the soil surface, evaporation of water occurs leaving the salt behind. The extent of bare salted area is related to the degree of groundwater imbalance between inputs and outputs. The steady state situation varies with rainfall cycles.

The oral history of early settlement in the Roadvale Milbong area indicates that the current significant salt outbreaks in the Upper Purga Creek area have been present throughout living memory (Roadvale State School area, western side of Zillman Flat Road and the lower reaches upstream of Milbong). A dam constructed upslope from Hanson Road near the Ipswich-Boonah road creek crossing would have been essentially fresh water when constructed around the 1950s but is now very saline with an EC of 17.5 dS/m and surrounded by bare salted areas is shown in Figure 14. There has also been an increase in salinity in recent years upslope of S Muller Road across Purga Creek.

Figure 14. Photo of old dam near Hanson Road with high EC (17.5 dS/m) and surrounded by bare salted areas.

5.2 Changes in salinity over time from aerial imagery

A comparison of old and current aerial imagery provides snapshots of changes over time in the extent of bare and very wet areas. Aerial photographs and satellite imagery are available for 4/1944, 5/1959, 1/1974, 8/1978, 6/1990, 8/1993, 10/1997, 6/2000, 3/2002, 7/2004, 2/2006, 8/2009 and 4/2012 from Department of Natural Resources and Mines and from Google Earth.

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Using the available aerial imagery, and focussing on four salinity areas in the catchment that have shown bare salted areas in recent years, it is possible to evaluate trends over time. The four sites chosen represent two areas where there are artesian bores: site S (Roadvale State School area) and site W (west of Zillman Flat Road), and two sites on Purga Creek floodplain, site M immediately south of S Muller Road and site N on the east side of Purga Creek crossing on the Ipswich-Boonah Road. The locations of the sites are shown in Figure 15.

Figure 15. Location of the four salinity sites used for changes over time from aerial imagery as described in the text. Map from Google Earth, 2012 imagery.

S

W

M

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Each site was rated from 1 to 5 in severity and extent of bare saline ground across the years 1944 to 2012 (5 the most extensive and bare). A score of 1 was given if the site showed some evidence of poor vegetation and a score of 2 if the site had bare salted areas. The scores of three assessors were discussed and where there was no final agreement, the score of the two with the same score was used. The assessment of extent and severity is an approximation and depends on the contrast of the aerial photographs. Rain can wash salt from bare areas and some green cover can be established over a few months. The rainfall for the calendar year was used for periods greater than one year, even though the aerial photographs were taken at varying times in a given year.

The scores together with the average annual rainfall for the previous five years for Boonah are presented in Table 1. The previous five year period was chosen as a period for the groundwater to respond and any impact on bare salted area to be seen. An evaluation of the one year rainfall, being the 12 months to the month of imagery, and a three year period were also evaluated but did not show any related trends. The rainfall residual mass curve was also evaluated with no clear relationship. Given that sites S and W had artesian groundwater and the hydraulic connection with the Surat basin of GAB, then the rainfall for Warwick was also considered. Rainfall for the Roadvale area has been supplied courtesy of Steve Mocker since the original analysis was completed. It shows a higher annual rainfall than Boonah. It is also shown in Table 1. Figure 16 shows the annual rainfalls for Boonah and the Roadvale area since 1995.

Figure 16. Comparison of annual rainfall for Boonah and Roadvale area. Data for Roadvale area from Steve Mocker and for Boonah from Bureau of Meteorology.

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Table 1. Ranking of extent and severity of salinity in four salted areas in upper Purga Creek as described in the text. Ranking 1 has vegetation effects but no bare ground, ranking 2 shows bare salted areas and ranking 5 has the most severely affected and extensive area over the series of imagery. Boonah rainfall from Bureau of Meteorology and Roadvale area rainfall courtesy of Steve Mocker.

Year aerial imagery

site S

School

artesian

site W

West

artesian

site M

Middle

floodplain

site N

North

floodplain

Rainfall average previous 5 years Boonah (mm)

Rainfall average previous 5 years Roadvale#

1944 2 2 1 1 806

1959 4 3 1 1 1001

1974 5 4 3 2 1161

1978 4 2 2 2 1021

1990 3 3 4 3 932

1993 4 4 3 3 954

1997 3 4 4 3 720

2000 3 3 4 3 731 803

2002 4 5 5 5 694 703

2004 -* 4 3 4 673 706

2006 3 4 3 5 716 740

2009 4 3 5 3 773 779

2012 5 5 5 5 839 904

* 2004 imagery not available for this area of the catchment # Data courtesy Steve Mocker

From the second last column in Table 1 it is obvious there is a generally declining rainfall from the late 1970s to 2009. Figure 17 shows the change in salinity ranking across the imagery dates.

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Figure 17. Rankings of the extent and severity of salinity at 4 sites based on aerial imagery as explained in the text and Table 1.

Sites S and W, the artesian sites, seem to have taken until the late 1970s to come to an approximate equilibrium. The floodplain sites M and N have taken until 2002 in the order of another 30 years to reach a ranking of 5. Salinity as bare areas was obvious in the 1944 aerial photographs for sites S and W and an indication of emerging salinity in the other two sites.

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A ranking of 5 does not mean that the salinity will not get worse, it is simply the most extensive and severely salt affected that was observed in the series of aerial imagery.

The region between site N and One eye waterhole does not show bare salted areas on aerial imagery at present. Based on the response of site N over time, the extension of salting into this area seems likely over time unless there is reasonable groundwater outflow or the watertable levels in this region are still below the depth that would cause soil surface salting. Some preferential subsurface flows of lower salinity groundwater may be occurring in this region given the EC at One eye waterhole is only 6.8 dS/m. There is a gap in knowledge of watertable levels in this area and taking a risk management approach there is a concern about the expansion of salinity into this area over time.

5.3 Response to rainfall

Water inputs drive the expression of salinity with excess inputs over the ability of the catchment to export water accounted for by areas of evaporation at the soil surface or evapotranspiration through vegetation. Since evaporation from the soil surface concentrates salts which kill vegetation, bare area is a useful index of the quantity of excess water in the landscape, the imbalance between inputs and outputs. Thus the changes in salted bare area over time and water levels in monitored bores offer a way of evaluating trends in salinity.

Figure 18 shows various indicies of rainfall to correlate with the changes in salted area. In Australia there is often a good correlation between groundwater levels over time and rainfall residual mass curves (the sum of the cumulative difference between annual rainfall and the average rainfall over all of the years of record). A five year moving average is a similar approach which averages some of the short term rainfall variability. The rainfall residual mass curve can be quite lagged following a long series of wetter or drier years. A comparison of Figures 17 and 18 shows that the five year moving average rainfall is better correlated with the changes in salinity ranking with time than other rainfall indicies. The Warwick rainfall peaks and dry periods show a similar timing to Boonah in the five year moving average rainfall index.

Because of the infrequent imagery in the early years prior to 1990, it is difficult to relate the trend in salted area to rainfall directly as the years with data points have been joined to show the trend over time. For sites M and N, there is a longer period of increasing salted area and severity which will mask some of the variation in rainfall. Thus the relationship with rainfall is not consistent across the four salinity sites which suggests other processes besides rainfall are contributing. At site N, the processes increasing the area of salinity over the period to about the year 2000, appear to dominate any influence of the dry periods.

The relationship between monitored water levels in bores and rainfall is a similarly useful measurement of response to inputs. Figure 19 shows the monitored bores in the upper Purga Creek and the response to recent rainfall. These bores have only a short record so far, 2004 to 2012 and the three monthly measurement period is too infrequent to capture the short term response to rainfall. A comparison of Figure 19 with Figure 18 shows that there is not a good relationship with rainfall residual mass and a mixed response to the five year moving average rainfall for Boonah. The variability in the red line bore 14310220 is very high and appears to be due to the difficulty of measuring the artesian head in this bore and the slow response of the water level in this bore.

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Figure 18. Various annual rainfall indicies for the rainfall stations of Boonah and Warwick over the period of the aerial imagery. Data sourced from the Bureau of Meteorology.

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Figure 19. Monitored DNRM bore water levels in upper Purga Creek . A comparison with Figure 18 rainfall indicies indicates a variable response of water level trends to rainfall. The depth in the bore number legend is the depth of the slotted section of the bore below ground level in metres. The EC in dS/m is the range in values measured over the length of record. Data from DNRM.

Summary

The salted bare area assessment from aerial imagery and the bore water level monitoring do not align well with rainfall inputs suggesting other sources of water are affecting groundwater and thus changing watertable levels.

The trends over the last 20 years have been:

Decreasing cultivation on the Walloon Coal Measures since around 1992 which is expected to reduce groundwater recharge on the Walloon areas.

A general decreasing rainfall since the late 1970s until around 2005 with an expected reduced groundwater recharge, but an

Increasing extent and severity of salted area over the same period with the highest salinity extent and severity being recorded at all four sites from 2000 onwards.

Thus when the area of salted land is expected to be decreasing, it has been increasing.

Processes that may be operating are:

1. Rainfall and rainfall recharge are not the only processes contributing to watertable levels and salinity in upper Purga Creek

2. The number and distribution of bores and wells with either artesian head or discharging at the soil surface is quite large and significant water is likely to be contributing to the area from deeper aquifers.

3. The rate of upward flow of water from the deeper aquifers in the artesian area is controlled by the permeability of the intermediate strata and thus the measured water levels in the bores will not necessarily be related to the bare salted areas and only poorly related to rainfall

The sources of recharge water to the groundwater in upper Purga Creek are discussed in the next section

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6. Where is the recharge water coming from?

The source and magnitude of recharge water is an important consideration in selecting appropriate salinity management options. Based on the analysis of the previous section there are three possible sources:

Increased local recharge within the catchment since land development in the 1880s

Artesian water within the Clarence-Moreton basin of the GAB flowing into the area; and

Recharge of groundwater from leaking dams in the catchment.

These options are considered based on evidence from: water levels, characteristics of the Clarence-Moreton basin of the GAB, water chemistry, water isotope chemistry and farm dams.

6.1 Water levels

Figure 20 shows the standing water levels (SWLs) below ground level in bores and wells in the upper catchment as measured in the farm visits from August to December 2012 and from DNRM monitored bores. The number of sites with artesian head shown in red in Figure 20 is very high and combined with SWLs shallower than 0.5 m shown in orange which are effectively very close to ground level and if deeper, may be artesian by tapping into deeper strata. Thus there is a large area responding to upward hydraulic head. The sites marked in yellow if shallower than 1.5 m would be indicative of a marginally saline affected area. Those bores shown in blue at greater than 3 m in depth below ground level are unlikely to have a salinity impact.

Pearce et al., (2007) considered the 50 bores drilled into hard rock in the Bremer catchment and found only three bores that were artesian or with water levels at ground level. One was near Roadvale State School (bore 14310220) and the other two on the west side of Franklin Vale and Western Creeks on the western edge of the Bremer catchment. Generally groundwater levels are at a greater depth below ground level towards the north in all sub-catchments and particularly in Purga Creek north of Milbong where the water levels in the bores are around 10 m below ground. An extensive review of the available bore data from Warrill Creek catchment from DNRM from both Walloon Coal Measures and the Warrill Creek alluvium did not show any artesian bores. Water levels were deeper than 2 m below ground level and often deeper. Figure 21 gives an outline of the likely artesian area contributing water to the catchment based on the bores and wells with artesian head.

6.2 Clarence-Moreton basin

Upper Purga Creek is unusual in having many bores that are artesian. This is characteristic of a discharge area and most probably a discharge area within the Clarence-Moreton basin of the GAB. In general the water levels are sub artesian across the Surat and Clarence-Moreton sub basins (DNRM, 2005). While mapping of springs and discharge areas was undertaken in preparation of the GAB Resource Operations Plan, the Clarence-Moreton sub basin was not included in the survey of springs or discharges (DNRM 2005).

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Figure 20. Standing water level (SWL) in bores and wells in upper Purga Creek catchment from measurements taken between August and December 2012 on farm visits and from monitored bores by Department of Natural Resources and Mines. Water levels are given in metres below ground eg. -1, above ground eg. 2, and at ground level or overflowing as 0. Those that are artesian or at ground level are shown in red, SWLs shallower than 0.5 m are shown in orange, SWLs 0.51 to 3 m below ground in yellow and SWLs > 3 m in blue. The SWL in the bore without a depth was not able to be determined.

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Figure 21. The area with expected major artesian discharge is outlined with a red line. The key to the figure is described in Figure 20.

Figure 22 shows the approximate elevation of the watertable above Australian Height Datum (AHD). The ground elevations were derived from field GPS, the elevations for the Department of Natural Resources and Mines bores from the groundwater database and also Google Earth for specific locations. There is a range of about 4 m in ground level elevation between sources of information for the worst case scenario with usually less than 2 m difference. More precise elevations are possible using LiDAR remote sensing contour data if required. The figure shows that the groundwater elevation generally follows the land surface with watertable elevations decreasing downstream except for bores in the NW and those with red labels that are overflowing at ground level

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Figure 22 shows the approximate elevation of the watertable above Australian Height Datum (AHD). The ground elevations were derived from field GPS, the elevations for the Department of Natural Resources and Mines bores from the groundwater database and also Google Earth for specific locations. There is a range of about 4 m in ground level elevation between sources of information for the worst case scenario with usually less than 2 m difference. More precise elevations are possible using LiDAR remote sensing contour data if required. The figure shows that the groundwater elevation generally follows the land surface with watertable elevations decreasing downstream except for bores in the NW and those with red labels that are overflowing at ground level.

Figure 22. Elevation above AHD of standing water levels in bores and wells and monitored bores shown in metres. Ground level reference taken from field GPS, DNRM groundwater database and Google Earth. SWLs of red labelled bores are above ground level. Orange labelled bores and wells have SWL at ground level. The elevation in the bore without a value was not able to be determined.

Figure 23 shows the overflow from a well 14.2 m deep with standing water level at ground level on the west side of Zillman Flat Road.

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Figure 23. The overflow from a well 14.2 m deep with SWL at ground level on the west side of Zillman Flat Road.

Smerdon et al. (2012) describe the relationship between the Surat basin and the Clarence-Moreton basin. The authors state that “In the eastern portion of the Surat basin, a complex groundwater divide exists with the Clarence-Moreton basin that depends on both deeper geological structures and shallower groundwater conditions”. Figure 24 from Kellett et al. (2012). shows the relationship of Purga Creek to the Surat basin and the watertable divide between the two basins. The water table divide is some distance to the west of the escarpment. There is a sharp decrease in groundwater elevation on the east of the escarpment of the Great Dividing Range which further decreases down the streams towards the north east. The position of the watertable divide and the sharp decrease in watertable levels does suggest the possibility that some groundwater is flowing east from the Surat basin. Time lags in response of the groundwater would be expected because of the groundwater flow paths and the different rainfall patterns to the west of the groundwater divide and thus the response to rainfall will be masked.

The significant region of artesian head in the catchment which may be even larger than that shown in Figure 21 will mean that the normal response of the watertable to rainfall will not occur since there is a source of additional water input from the elevated area to the east of the watertable divide. The magnitude of this contribution is unknown. Also, the only significant change in watertable levels may well occur only when the pressure in the artesian areas is reduced by factors outside the upper Purga Creek catchment boundary. This also explains the lack of response of standing water levels to rainfall.

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Figure 24. An expanded view of the boundary between the Surat basin to the left of the Blue dashed line representing the water table divide and the Clarence-Moreton basin to the east illustrating the watertable elevation at the boundary between the two basins. Figure from Geoscience Australia published as Figure 6.19 in Kellett et al. (2012). Figure used with permission. The locations of Peak Crossing, Warrill View and Boonah have been added to the figure.

6.3 Water chemistry

The salinity of the shallow groundwater in some areas of the catchment is very high, EC between 15 to 20 dS/m, particularly around Roadvale State School and upstream and also around S Muller Road suggesting this discharge process has been occurring for centuries with limited flushing of salts and is now probably made worse by land use change. The EC of Purga Creek water in non-flowing water holes was often > 20 dS/m. Figure 25 shows a portion of a Piper diagram also called a trilinear diagram of the relative proportions of the major cations in a water sample that is, calcium, magnesium and sodium. For the wells, bores and streams sampled between August and December 2012, there are four groups of waters of different compositions:

I. Deep sodium bicarbonate (NaHCO3) dominant - Bores with a very high proportion of sodium, usually associated with bicarbonate shown as brown in Figure 26. These are deeper bores near the Roadvale State School, the northern end of Zillman Flat Road and on the Zingleman Road near Milbong. These bores are deeper than 15 m (in some cases ranging in depth from 30 m to 100 m) and have a different chemistry to other groundwater, it is probable that they are not directly contributing to the salinity present. Otherwise there would be some bores of intermediate chemical composition.

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II. Limestone and dolomite composition - Bores with a very high proportion of calcium and reasonable proportions of magnesium. These wells are illustrated by the blue circle. Two of these are in the corner of the Ipswich-Boonah and Zingleman Roads. This is in proximity to the volcanic intrusion at Milbong and it is likely that there may be some residual limestone in this area that is contributing to the chemistry. The limestone may also be contributing to restricting down valley flow in this area.

III. Upland bores and wells - The group in the green circle are bores and wells in the upland areas on both sides of Purga Creek. They are characterised by a moderate EC mostly around 4 to 6 dS/m and have higher proportions of calcium and magnesium. The composition of this group is typical of Walloon Coal Measures and Marburg sandstone waters. These geologic formations have a wider range of compositions than are shown in Figure 25. The close grouping of water composition suggests a relatively uniform source of water even though the EC varies. Thus whatever groundwater recharge mechanism is operating, the recharge water is flowing through a sufficient length of flow path in the Walloon Coal Measures to reflect the composition of this formation.

IV. Purga Creek floodplain - The group marked in orange are wells, bores and streams in the Purga Creek floodplain. They show lower calcium and slightly reduced magnesium with a higher EC ranging up to EC 19.7 dS/m. As waters concentrate through evaporation, the least soluble salts precipitate out of solution first. These are calcium carbonate and magnesium carbonates. The green and orange groups fit on the normal expected concentration curve for this chemical composition moving from the composition of the green group through the orange group ending up with a seawater composition shown in red.

The red square is the nominal proportion of cations (Magnesium, Sodium and Calcium) in seawater and is often used as the end point of the concentration line for many terrestrial waters, unless they have an atypical chemical composition when they can concentrate to different end points.

Thus Figure 25 suggests that the source of water is the groundwater of the green group. When this water composition is then concentrated in the low lying positions in the landscape through evaporative concentration, the composition of the cations would be represented by the orange group. If the water was then concentrated further, it would end up with a composition close to seawater as represented by the red square. The EC of seawater is around 55 dS/m. There are some areas in the lower alluvium where the EC of the stream water is around 6.5 dS/m. One eye waterhole had an EC of 6.8 dS/m suggesting the presence of some flow paths through the alluvium with waters that are not concentrated by evaporation.

If recharge of the artesian aquifers in the Walloons was coming from adjacent shallow recharge in the Walloon Coal Measures or Marburg Subgroup, then the salt loads would be smaller (high salt generally indicates low movement of water). Thus recharge of the Walloon aquifer contributing most water to the alluvium would appear to be coming from a source that allows time or distance for recharge water to come to equilibrium with the chemistry of the Walloons, yet to be identified.

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Figure 25. The proportions of calcium, magnesium and sodium in the measured bores, wells and streams. Coloured ellipses indicate waters of different water compositions as explained in the legend above and in the text.

The comparison of the water chemistry of upper Purga Creek to that in the stream gauging station at Loamside, near Ipswich some 32 km downstream, is shown in Figure 26. The overlay of the available water analyses from the gauging station taken during different flow events shows a very close similarity to the floodplain water composition from upper Purga Creek. (orange colour in Figure 25). Given that the groundwater levels in lower Purga Creek are generally quite deep around -10 m (Pearce et al., 2007), then upper Purga Creek seems to be an important contributor of salt downstream at Loamside. There is a slightly higher calcium and magnesium content in the samples taken at higher flow rates at Loamside, as expected, which would come from runoff from surface soils in the lower catchment. Also Purga Creek flows through a wetland area before Loamside.

Otherwise the high salinity at Loamside suggests the salinity processes in the Roadvale - Milbong area may have been occurring for centuries and that discharge of the groundwater has been contributing and concentrated by evapotranspiration as the water flows downstream, otherwise the high salinities would not be expected. The flow data for Loamside indicates that high runoff flows are dilution flows of the water in Purga Creek rather than the addition of other water sources. Pearce et al., (2007) also found that Purga Creek at Loamside had the highest salinity of all the sub catchments of the Bremer catchment.

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Figure 26. An overlay of upper Purga Creek bore and well composition data from Figure 25 with Loamside stream gauging data illustrating the similar composition. The samples were taken at a range of stream flow rates. Data from DNRM.

Summary

Based on the water chemistry it appears that upper Purga Creek is an active discharge area within the Clarence-Moreton basin of the GAB with groundwaters in the Walloons and Marburg Subgroup discharging into the valley. This is contributing a significant quantity of salt into the catchment as evidenced by the water chemistry and EC flow relationships at Loamside.

The water chemistry is consistent with a significant water source coming from the Walloon Coal Measures and Marburg Subgroup. The artesian heads found in part of the catchment indicates this water is likely to be contributing to groundwater recharge.

6.5 Dams

Dams are common in upper Purga Creek catchment. From an aerial imagery survey, 186 dams were located down to Milbong. Measuring the area of a sample of the dams from the imagery, the average area was 1 500 m2 giving a catchment are under dams of some 30 hectares. Most dams leak to some extent and Schmidt (2001) surveyed 136 on-farm storages in the cotton industry and found the average leakage was 2.3 mm/day with a median value of 1.2 mm/day. Even if the leakage rate from a dam is small at say 1 mm/day, this will amount to some 360 mm per year. Because the soil and unsaturated zone porosity is only in the order of 5 to 10% between a wet soil and a saturated soil, then 360 mm of water leakage per year will translate to a groundwater level rise of 3.6 to 7.2 m per year

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under the dam. This will dissipate to the surrounding area but result in a general rise in the water table. At 1 mm/day for the 30 ha under dams, is equivalent to 100 ML/year of recharge to the groundwater from dams.

Since a number of dams have been constructed on seepage areas, they will have a good hydraulic connection to the groundwater and will increase the pressure head in the groundwater by the height of the water level in the dam thus enhancing to some extent the artesian pressure in the groundwater aquifers. This is a further reason for the lack of response of water levels to rainfall.

6.6 Deep drainage

The salt balance model of Shaw and Thorburn (1985) and given in ANZECC & ARMCANZ (2000) allows estimates of the quantity of deep drainage (Dd) below the soil root zone (0.9 m) using annual rainfall, soil properties and soil exchangeable sodium percentage. Estimates were made for the major soil groups of Loi et al. (2005) for the Walloon uplands. The Dd ranged 2, 3, 3, 4 and 9 mm/year for the four soil samples representing the Rockville soil type, giving an average of 4 mm/year. This means that some 2 500 ha would give the equivalent of the estimated Dd from 30 ha of leaking dams

Thus land use is contributing some recharge to the groundwater, particularly during extended wet periods when the soil available water storage is saturated. It is contributing to overloading the capacity of the groundwater of upper Purga Creek to discharge groundwater downstream and thus leading to elevated watertable levels.

If land use Dd was a very significant input, compared to artesian groundwater and dams, there would be a noticeable response of water levels to rainfall. Given the response is not readily discernible, then land use is making some contribution but is not a dominant issue. Certainly effective grazing management to maintain actively transpiring vegetation as well as increased tree cover will reduce this component of the water inputs.

7 Guess/estimate of relative contributing factors to salinity

Based on the understanding of processes in upper Purga Creek, this is the guess/estimate of the relative contributions of the causal factors for salinity issues in the catchment;

Factor % contribution to the salinity present

Landscape & geology features 30

Local discharge area within 40

the Clarence-Moreton basin

Land clearing 10

Leaking dams 10

Current land use 5

Increased sedimentation on alluvial flats 5

While these are indicative figures based on expert judgement they give a perspective for decisions about choosing effective salinity management options.

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8 Expected salinity trends in the future

The trends in salted area in Figure 17 with all 4 sites reaching their equal most severe level of salinity on the rankings in 2012, strongly suggests the salinity will continue to increase in the future. Site S, the Roadvale School, is the only site that might have reached equilibrium earlier since it also attained a 5 ranking in 1974.

For a “do nothing” salinity management, it is expected that salinity will get worse in the upper Purga Creek area down to Milbong. As the watertable levels in the lower floodplain area increase, water levels upstream will also rise as there is less down valley flow and tributary streams will become more affected.

If the whole catchment becomes more saturated with shallow water tables, flood events are likely to have more severe impacts for several reasons. Saturated soils with shallow watertables have no soil water storage capacity during wet weather leading to faster and more frequent flooding. Saturated saline soils, which will tend to have high soil exchangeable sodium percentages from evaporation and concentration of salts will become increasingly unstable due to limited wet strength and increased soil dispersion so that increased erosion and incised stream channels are likely. Thus the evidence indicates that the salinity issue in upper Purga Creek will expand over time.

If some contribution to the artesian water is in fact coming from the Surat basin of the GAB, and Coal Seam Gas dewatering does have some impact, then it may reduce the hydraulic pressure in these aquifers in upper Purga Creek and a reduction in salinity may occur. This is very uncertain and there would be an unknown lag time in response as well.

9 Uncertainties to be considered and addressed

The source of groundwater recharge is uncertain. Adding data loggers to a selection of bores will allow small short term changes in water levels to be tracked in real time with rainfall. Depending on the outcomes, other chemical tracing approaches can be considered including tritium and carbon14 but may not confirm the sources. An option is to apply adaptive management principles and choose the most effective salinity management options with good monitoring and defined review periods when the strategy can be changed based on new evidence available.

The relative contribution of dams to recharge and the groundwater issue is uncertain. Techniques to measure dam leakage are time consuming and variability is high. Insertion of piezometers below dams (or use existing wells where present) is possible. Alternatively landholders who are prepared to remove a dam could have monitoring piezometers installed before dam removal so that the effect can be monitored over time. An attempt to look at the increase in the numbers of dams over the period covered by aerial imagery is possible but does not allow an estimate of the quantity of recharge.

The apparent lower water table levels in the alluvium between upstream of Hansen and Zingleman Roads (between the 91 m contour interval and One eye waterhole) requires some monitoring of trends to anticipate emerging issues.

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10 Salinity management

10.1 Water management plans

Water resource management plans

Two water resource plans cover the upper Purga Creek and outline available water resource management options.

Great Artesian Basin Resource Operations Plan

The Purga Creek Catchment is part of the Great Artesian Basin Resource Operations Plan, (Department of Natural Resources and Mines, 2012). It fits within the Clarence-Moreton section as shown in Figure 2.

The plan has implications for water management. The GAB Resource Operations Plan seeks to ensure water is to be allocated and managed in a way that seeks to achieve a balance in the following outcomes-

(a) to protect the flow of water to springs and baseflow to watercourses that support significant cultural and environmental values

(b) to provide for the continued use of all water entitlements

(c) to reserve water in storage in aquifers for future generations

(d) to ensure a reliable supply of water in the plan area

(e) to make water available for new users

No increased take of artesian water is permitted. Stock and domestic use is permitted on established bores and irrigation is limited to an area less than 0.25 ha with produce for domestic use only. These restrictions make it very difficult to manage watertable levels and salinity within the Clarence Moreton Basin management area.

Water Resource (Moreton) Plan 2007

The Water Resource (Moreton) Plan 2007 (Office of the Queensland Parliamentary Counsel, 2011) covers the water resource management of Purga Creek catchment. The purposes of the plan are:

(a) to define the availability of water in the plan area;

(b) to provide a framework for sustainably managing water and the taking of water;

(c) to identify priorities and mechanisms for dealing with future water requirements;

(d) to provide a framework for reversing, where practicable, degradation that has occurred in natural ecosystems;

(e) to provide a framework for—

(i) establishing water allocations to take surface water; and

(ii) granting and amending water entitlements for groundwater; and

(iii) granting water entitlements for overland flow water.

Section 76 of the Water Resource (Moreton) Plan 2007 Limitation on taking of groundwater –Act, s(20(6) states

“A person may not take groundwater in the Warrill-Bremer alluvial groundwater management area (the management area) other than –

(a) for stock or domestic purposes; or

(b) under a water entitlement or water permit; or

(c) to allow monitoring or salinity control; or

(d) under an authorisation under section 78.

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Water can be used from bores or wells without a licence if the depth of the bore or well is less than 6 m deep. A licence is required for deeper depth bores. A discharge licence is also required if water is extracted and discharged directly into a watercourse.

The implications of both of these plans are that the flow of water from springs and baseflow in watercourses are to be protected if they support significant cultural and environmental values, but with an option to reverse degradation in natural ecosystems through water management.

10.2 Salinity management options

The aim of salinity management is to minimise salinity impacts on natural resources and built infrastructure. Expectations of reclamation of salt affected areas are commonly not fulfilled. This can be attributed to the long lead times for measurable change to occur, large accumulation of salt over historic time periods and change of state once salinity has developed. A return to stable non saline state often requires a much greater reversal usually to near pre existing conditions even earlier than when salinity first appeared as illustrated in Figure 27. As the soil root zone and unsaturated zone above the water table gradually becomes saturated from recharge after land clearing in hydrologically sensitive landscapes, the degree of groundwater imbalance between the increased inputs and restricted outputs increases.

Once the water table rises close to the soil surface, evaporation commences and the salt is left behind on the soil surface killing vegetation. The water level has to be reduced considerably before the area will be suitable for vegetation establishment.

Figure 27. Change of state from a normal catchment situation to a degraded and saline catchment with increasing groundwater input/output imbalance until a critical soil salinity level is exceeded where death of vegetation occurs. The degree of reversal required to restore the area to a non saline state requires a return of the groundwater imbalance to a point where evaporation is reduced to below a critical soil salinity level, from Shaw (2008).

Three changes are required concurrently to restore a saline catchment based on Figure 28:

reduce soil salinity levels in the root zone to less than the critical soil salinity value so vegetation can survive. Salt can be stored in the unsaturated zone between the root zone of the vegetation and the water table if the water table level is lowered.

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reduce the degree of groundwater imbalance (the difference between water inputs into a catchment and water outputs through surface, and groundwater flows) to lower the watertable levels since shallow watertables and evaporation drive the system, and

increase the resilience of the catchment to be able to withstand some variation in hydrology from variable rainfall cycles without changing back to a saline state. This often means reducing the groundwater inflow/outflow imbalance to achieve a greater watertable depth buffer than would be required for an average rainfall situation. The depth to the watertable in an affected area should be > 2 metres below ground level and preferably deeper where possible.

The following options cover ways in which watertable salinity areas can be managed. Often a single option is not sufficient on its own and combinations of options are required. Often management strategies need to occur at the sub catchment scale rather than property scale in order to effect change. There are four broad options:

do nothing

reduce water inputs,

increase water outputs or

live with the salinity through optimising salinity management.

These options are outlined below and details given in Table 2.

1. Do nothing means no significant change to current practice. It is similar to living with salinity (option 4) but living with salinity involves a proactive input to minimise adverse effects.

2. Reduce groundwater inputs in recharge areas

2.1. Revegetation of recharge areas. This option involves planting trees usually at a density similar to what occurred naturally prior to clearing. Trees not only use soil water and dry out the soil profile, they also intercept rainfall on their leaves but only for small falls of rain

2.2. Pasture management on recharge areas. A high level of ground cover will minimise evaporation for marginally affected areas and will transpire more water than a heavily grazed pasture.

2.3. Efficient water management on-farm. This will mean considering water requirements, use and efficiency in water storages

3. Increase groundwater outputs (in a region upstream of the discharge area or if necessary in the discharge area)

3.1. Sub-surface drainage has been used for many years for waterlogged areas but is less applicable for saline areas since the drained discharge has impacts downstream.

3.2. Controlled dewatering by intercepting groundwater and using or discharging it. The salt is stored in the unsaturated soil zone between the root zone and the watertable. This approach targets better quality groundwater and seeks to lower the watertable to minimise evaporation of water on the soil surface and salt concentration. It is fast at removing large quantities of water, if adequate aquifer flows are attainable and can be tailored to only pump what is necessary to maintain the water table below the critical depth. A discharge licence is required and an experimental period can be tried to assess its effectiveness.

4. Live with salinity by not seeking to control salinity but to be proactive in minimising its impact, some degree of improvement of productivity of salted lands and waters is possible. This approach is directed towards managing the symptoms of salinity.

5. Other options or combinations of the above options

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To prioritise the preferred salinity management options, there is a need to consider the desired outcome for salinity management.

Controlled dewatering

Since the major area of salinity problems in upper Purga Creek occur on the Purga creek alluvium (Figure 8), an approach to lower the water table levels in the most salt affected parts of the alluvium is proposed. The aim would be to control the water table level to a depth greater than 1 m below ground level in the worst affected bare salted areas and preferably 2m by using better quality water in adjacent less salt affected areas. Pumping of groundwater in the alluvium is permissible under the Water Resource (Moreton) Plan 2007.

By pumping the water from the better water quality areas (Figure 28) and using it for irrigation of salt tolerant pastures on adjacent areas would result in turning a salinity problem into a resource to be used. A guide would be to select areas with a groundwater EC of < 7 dS/m and check the pumping flow rates by digging a pit to 6 m depth. A pit allows a much larger surface area for interception of groundwater and hence higher flow rates that make is feasible to use the water for irrigation. The water would be reticulated to the salt tolerant pasture areas such as Rhodes grass. The sodium adsorption ratio (SAR) of the water would need to be less than the value that would cause soil degradation.

Figure 28. EC of surface water and dams in upper Purga Creek sampled during this investigation. The dark blue line represents a likely flow line with groundwater of EC < 7.5 dS/m.

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Figure 29 is a graphical representation of the relative effectiveness of the management options for salinity reclamation and control versus the cost and input required. The cost also includes the long term costs. This is a judgement only.

Figure 29. For discussion purposes, the effectiveness of the salinity management options in Table 2 in relation to the cost and effort required over the medium to longer term.

Table 2 summarises relevant information for the options listed above for upper Purga Creek catchment. The information for each option is not complete but provides a guide to the effectiveness of each option in the Table. The last line of Table 2 gives the authors’ rankings (out of 5) for the salinity management options according to the specific criteria for each line of the table

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Table 2. A comparison of the possible salinity management options for upper Purga Creek

Do nothing Revegetation

Pasture management

Water manage-ment on-farm

Drainage Controlled dewatering

Living with salt

Description Expect a slow increase in water table levels, salt affected area & expansion of salinity in lower areas & upslope of roads. The floodplain will become increasingly unstable from a shallow water table & erosion in flood events.

On the basis that land clearing has increased groundwater recharge, reducing groundwater recharge is beneficial in hydrologically sensitive landscapes.

Maintaining good ground cover will increase evapotranspiration of soil water during growth periods, allow increased rainfall storage in the root zone & decrease groundwater recharge.

Since groundwater levels are at or above ground level, there is no soil water storage buffer for wet years. Leaking water storages will recharge groundwater and drainage lines. Increased soil water storage is required.

Open drains in badly salted areas lower the water table and reduce evaporative concentration of salt allowing vegetation to be established.

Bores which are artesian indicate upward hydraulic pressure and can cause surface soil wetness & salting.

Vegetation is not able to handle the quantities of water nor the increasing salt levels in the longer term.

Stabilise salt affected areas to reduce erosion and evaporative concentration of salt on soil surface.

Enhance salt tolerant vegetation.

What is required

Nothing Revegetate upland cleared & floodplain areas including riparian areas with trees.

Fallow cropping is high risk – opportunity cropping has recharge advantages.

Reduced grazing pressure to maintain a Leaf Area Index (area of leaf to area of ground covered) of >3.5 to optimise water use when water is available.

Removal or sealing of leaking dams, less dams. Shallow dams evaporate around 1 500 mm/yr and dams need to be deeper if required in dry periods.

Expand Roadvale water scheme to drought proof high drought risk areas.

Significant earthworks and outfall into Purga Creek downstream (or on site storage but unlikely to be effective).

Licence to dispose and permission from downstream neighbouring landholders required.

The GAB Resource Operation Plan preserves water in GAB aquifers and springs. Alluvial flats can be dewatered by pumps in higher flow aquifers. A licence to dispose of water is required unless quality is satisfactory for irrigation of pasture.

Restrict grazing in fringing regions of salted areas.

Plant salt tolerant vegetation on salted area margins, raise soil level in salted areas with banks, irrigate with groundwater if it is EC <8 dS/m.

Expected benefits

No cost for most landholders. Landholders with salinity already will have high maintenance costs of roads, infrastructure

Considerable benefits to ecosystem health from revegetation & stream bank stability. Significantly reduced erosion of uplands,

Improved pasture health, reduced erosion and greater water use.

Less recharge to groundwater, less erosion on saturated drainage lines during flood flows.

Productive vegetation re-established on previously saline discharge areas.

Control of watertable to sustainable levels >1 m below ground.

Stabilisation of saline or waterlogged areas, reduced waterlogging

Stability of salted area, reduced erosion. Salt tolerant vegetation may need to be re-established after

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Do nothing Revegetation

Pasture management

Water manage-ment on-farm

Drainage Controlled dewatering

Living with salt

& erosion. flood plains & stream banks.

and salinity in catena and tributary areas.

Salt stored in soil above watertable.

wetter periods due to extended shallow watertable levels.

Contribution to salinity control

<5%

except on badly salinity affected areas

<10%

since catchment shows evidence of salt accumulation prior to clearing, most of catchment would need to be revegetated to make any contribution

<5%

depends on soil permeability but can generate a soil water deficit to temporarily store and use rainfall.

10 to 15%

probably

10 to 15% overall but 80% for the drained area

> 80%

Engineering solutions have been a most successful method for Murray Darling Basin to manage river salinity*. An experimental trial period is suggested.

Symptom management 30%.

Strategies won’t control salinity.

Time for a response

Gradual increase in salinity and then a very gradual improvement from natural erosion of salted areas and lowering of water table – maybe 100 years.

In the order of 50 years. Until the soil water storage is dried, and a buffer to wet periods established, no improvement. May not respond if GAB discharge is primary water source.

Within 12 months. A short term effect within months followed by longer term improvement as downstream areas dry out over a few years.

Less than 2 years.

Within 18 months.

pumping would only be used when water table was < 1 m below ground level.

Less than 3 years for grasses.

Salt tolerant trees unlikely to be sustainable on bare salted areas.

Uncertainty in prediction

10%

will depend on extent of any climate change and frequency of extreme climate events

15%

would need to target higher recharge areas first. Soil profiles suggest limited benefit to groundwater recharge but major advantages to ecosystem

< 5%

20%

depends on the source of recharge water.

10%

Soil permeability and outfall will determine drainage design.

20%

Suitability of aquifer for reasonable pumping > 3 L/sec = 0.25 ML/day. Availability of sites and adjacent irrigation areas need to be considered.

<5%

Extra investigation– cost &

None required - no improvement in uncertainty.

Water balance modelling based on field measured soil

This is well documented and pasture

Could measure leakage in selected dams but that is

Drainage design required and licence to

Selected drilling in areas upstream of badly affected shallow water

Based on established methods and

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Do nothing Revegetation

Pasture management

Water manage-ment on-farm

Drainage Controlled dewatering

Living with salt

benefit in improving uncertainty

properties.

Improvement in uncertainty around 20%, improvement in prediction about 10%. An option to use expert advice or salt profile modelling.

Management guidelines are available.

Very variable and costly. Install piezometers downslope of suspect dams and monitor short term. Only useful if there is a commitment to do something about dams. Around $1000 /dam for piezos

discharge, if granted.

table areas. Trial of dewatering in a key area.

Rough estimate of cost around

$35 000 for field study

improvement in uncertainty 50%

plant species.

Consequen-ces of actions

More degradation, erosion and loss of property values.

Major benefits for ecosystem health, reduced economic productivity from the land unless forestry for carbon credits.

Possible reduced income from lower stocking rates but the general consensus is farm income is increased.

Less surface water available, but much of storage is not used. Shallow dams will dry too soon for long term drought water supply.

Transfers the salt problem downstream since groundwater in shallow drains is usually quite saline.

Control of water table levels allowing productive agriculture. Will reduce water levels in nearby streams & wells. Downstream greater flow with slightly elevated salinity levels.

Salted areas and fringing areas need to be fenced for very limited & controlled grazing.

Costs – up-front and ongoing

None up front.

Moderate to high for most affected properties for remedial works.

Very high tree establishment costs. Possible economic benefit longer term from forest harvesting but still requires high vegetative cover to manage salinity.

Cost neutral. Cost for dam removal or dam sealing.

Significant up-front cost and ongoing drain management cost.

Infrastructure cost to be estimated. Operational cost $41/ML pumped (MDBA)**. Would need around 200 ML to be pumped from whole upper catchment in first 2 years and then as watertable level rises to shallower than about 1 m below ground.

Establishment including of any banks & any irrigation system installed.

Who benefits & who pays

Up slope landholders benefit, downstream catchment landholders pay increasingly.

All contributing will pay. The limited salinity benefit mostly for those in low catchment positions. Other ecosystem benefits equally

All benefit from reduced recharge although the benefit is small. Individual landholder pays and benefits.

All benefit from reduced recharge to groundwater. Those on lower slopes benefit more. Those removing dams

Individual landholder benefits and pays. Some small benefit to whole catchment landholders.

Benefits all who have salinity affected areas except more elevated areas. Stream bank stability in lower areas improved.

Local landholder affected by the salinity.

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Do nothing Revegetation

Pasture management

Water manage-ment on-farm

Drainage Controlled dewatering

Living with salt

shared between participants. Major costs for those requiring economic return from their properties and through riparian stability.

lose short term water storage.

Downstream landholders disadvantaged by high salinity stream flow.

Downstream user neutral with slightly elevated salinity but increased flow mostly around wet season.

How capital costs are funded will determine who pays.

Overall rating for effectiveness of each option for the following criteria. 0 has no effect and 5 is maximum effectiveness. These are author’s opinions only.

Salinity control

0 2 2 3 0 4 1

Ecosystem health & sustainability

0 5 3 3 1 4 3

Long term economic benefit

2 2 4 3 2 4 2

Notes to the table

* Engineering works have been estimated to have contributed 70% to the reduction in River Murray salinity at Morgan (near the Victorian SA border), (Wicks et al., 2013)

Victoria is moving from recharge area management to discharge area management for salinity. Over the years the uptake for recharge area management has been very small and the discharge areas have still required management. (Wicks et al., 2013)

** Calculated cost for pumping from 10 m depth based on the records kept by the Murray-Darling Basin Authority. (Phil Pfeiffer pers comm.).

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10.3 Landholder preferences for salinity management options

Landholder meetings were held at Roadvale on 19 January 2013 and on 4 May 2013. At the first meeting landholders contributed to a vision for the catchment and also prioritised their preferred salinity management options as outlined below in Table 3. At the May 2013 meeting, a strategic plan was outlined for future actions.

Two group voting sessions were held on the 19th January following discussion and presentation of updated salinity investigations. The vision and desired outcomes are shown in Table 3. Each participant had five votes to allocate across the items raised at the meeting as they wished.

Table 3. Vision for upper Purga Creek from landholder meeting at Roadvale State School on 19 January 2103.

No. Vision Votes Rank

1 Landscape health 17 3

2 Control or eliminate salt 8 5

3 Landcover, productive use of salted areas 25 1

4 Maintaining land values 23 2

5 Capacity building and knowledge/awareness 9 4

Following the presentation on salinity management options for upper Purga Creek, each participant had 7 votes to allocate across the 8 options discussed. The results are given in Table 4. The do nothing option is not an option from a salinity impact perspective and was not considered an option from the landholder’s perspective either.

Table 4. Preferred salinity management options by landholders at the meeting at Roadvale State School on 19 January 2013.

No. Management option Votes Rank

1 Do nothing 0

2 Reduce groundwater recharge -

2.1 Revegetation in recharge areas 20 3

2.2 Pasture management 21 2

2.3 Efficient water use on farm 15 5

3 Increase groundwater outputs -

3.1 Subsurface drainage 19 4

3.2 Controlled dewatering 23 1

4 Live with salinity 15 5

5 Other options 2 6

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11 The benefits and trade-offs in salinity management

For all management options there are trade-offs between different aspects and between landholders in different parts of the catchment. Cost and input is taken over the longer term and can include opportunities that might be lost. Some of these trade-offs that need to be considered are:

Cost and long term sustainability. Sometimes benefits may be for the next generation

Productivity of larger recharge areas of the catchment versus the improvement in productivity for smaller salt affected areas. When does an area become sacrificial for the greater productivity

The benefits of turning a problem into a resource that can be used which includes use of groundwater causing the problem and productive use of salted lands

Control of the water table to a lower level to reduce evaporation and salting will probably mean a lowering of the water levels in bores and wells and may impact in the very dry periods

Who benefits and who pays needs to have some equity and fairness. How this is achieved or negotiated needs to be considered. Community versus private benefit is part of this negotiation as well as how co-investment options may be decided.

The principle of the adaptive management cycle which is “Plan, Do, Check Review” means ongoing monitoring at a greater intensity and formal reviews to evaluate the evidence and decide the most appropriate way forward. There is a time and input requirement over the longer term.

12 Planning considerations

Upper Purga Creek is a hydrologically sensitive catchment that is currently largely saturated with water and appears to be acting as a local discharge for groundwater within the Walloon Coal Measures and Marburg formations. The following are future planning options for consideration. Reducing groundwater discharge to the alluvial flats and lowering the water table levels are priority considerations.

1. No non-sewered subdivisions be allowed within the catchment since the addition of reticulated water, and a lack of sewerage, adds additional localised recharge from waste water disposal as well as additional recharge from reticulated water, dams and rainwater tanks used for hobby farm activities.

2. Farm dams appear to be a significant salinity risk through leakage to groundwater. A policy on dam usage that provides for ongoing use according to need and considers depth of the dam. Deep dams increase usefulness in prolonged dry periods. Shallow dams recharge greatest in the wet period when they are full and since the water has evaporated by the dry period their effectiveness is minimal. If high risk areas are to be considered, a policy of dam lining may be required based on further work.

3. An alternative to dams maybe to extend the Roadvale Water scheme to additional areas on the basis that the number of farm dams is reduced with active monitoring to determine effectiveness.

4. Incentives for riparian re-vegetation to stabilise stream banks and seepage areas commencing on the more upland flood plain to minimise erosion and return the area to a more ecologically sustainable condition. Fencing and stock control will be required.

5. Salted areas need controlled grazing and fencing if there is to be any effort towards living with salinity. Guidelines can be drawn up.

6. Review roads and culvert infrastructure to see if the restrictions caused to groundwater flow can be eased or reversed.

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13 Next steps

Further investigations and trials can be undertaken to reduce the uncertainty in predictions about the causes and the effectiveness of the suggested options depending on funding options. These are:

Estimate probable sources of groundwater contributing to the discharge using low cost approaches such as loggers in selected bores in Purga Creek and Warrill Creek catchments and analysis of water chemistry of the areas.

Install piezometers in the alluvium between Hansen and Zingleman Roads to monitor trends in watertables in this area that is not yet showing bare salted areas.

Install piezometers at selected leaking dams or those that might be removed to assess potential leakage.

Trial controlled dewatering on a few sites on the alluvium to minimise the area of salted and very erodible soils.

14 Conclusions

Salinity has been present in the Roadvale area for the whole of living memory. The high salinity level in the groundwater of the alluvium around Roadvale confirms that salt has been concentrating for centuries. A survey of soil salinity for SEQ Catchments indicates that more than 180 ha are severely affected by salinity in the upper Purga Creek area.

It is expected that the area affected by salinity will get worse in the area upstream of Milbong through more extensive shallow watertables. This will cause increased salinity upstream and in the tributaries to Purga Creek.

The analysis of salinity changes over time using aerial photographs and imagery analysis for the period 1944 to 2012 indicated:

The 1944 and later aerial photographs show bare salted areas and the progressive reduction in cultivated areas adjacent to the salted areas.

The analysis of the extent and severity of four salted sites within the upper Purga Creek catchment shows 2012 to have the most extensive saline areas for all four sites. Two of the sites located in the Purga Creek alluvium have taken close to 30 years longer to reach the more severe salinity ratings compared to the two areas with artesian bores and wells.

As more of the catchment area becomes affected by shallow water tables, the areas will become saturated and dispersive from the sodium concentration caused by evaporation and will be unstable during flood flows resulting in increased erosion and gullying. Flood impacts are increased because of shallow watertables.

Upper Purga Creek catchment is a hydrologically sensitive catchment which means that additional water inputs will have a considerable impact because of the restriction to groundwater outflow in the Milbong area. There are observable geologic, landform and soils features that confirm a hydrologic restriction to groundwater outflow.

The number of bores and wells that are artesian (under upward water pressure) or overflowing at ground level, or just below ground level in the Roadvale School to S Muller Road area is much higher than normal and indicates the area is a local discharge area for groundwater within the Clarence-Moreton basin, a sub basin of the Great Artesian Basin.

There is a source of recharge water to the area in addition to rainfall. The water chemistry indicates aquifers within the Walloon Coal Measures and Marburg Subgroup under artesian pressure are a major source of groundwater flow in the area. Other sources of recharge are likely to be from the large number of farm dams in the upper catchment and also a contribution from land use in the area.

The correlation of rainfall patterns with the change in extent and severity of salted areas shows a reasonable relationship to 5 year moving average rainfall. The correlation of rainfall

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with the water levels in monitored bores in the upper catchment is not clear. It appears that recharge from other sources besides rainfall is likely. The chemistry of the groundwater indicates the main source water to be typical of Walloon Coal Measures and Marburg subgroup with an EC of around 4 to 6 dS/m.

The guess/estimate of the relative contribution of the casual factors for salinity in upper Purga Creek are:

Landscape and geologic features 30%

Local discharge area with the Clarence-Moreton basin 40%

Land clearing 10%

Leaking dams 10%

Current land use 5%

Increased sedimentation on the alluvial flats 5%

The causes of salinity directly influence the choice and implementation of appropriate salinity management options.

A range of salinity management options have been proposed, together with an estimate of their likely impact on controlling or living with salinity in the future. Planning considerations are included to minimise salinity impacts. The management options were openly discussed and evaluated by those present at the meeting on 19 January 2013 and the option of controlled dewatering on the alluvium preferred in conjunction with other options. Those who may be impacted by the adoption of salinity management strategies need to be consulted. However, the “do nothing” management option is not a sustainable option for upper Purga Creek.

15 References

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