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Australian Geomechanics Vol 39 No 3 September 2004 73

HYDROGEOLOGY OF THE BOTANY BASIN

R.K. Hatley Golder Associates

1 INTRODUCTION This paper seeks to review the current knowledge of the geology and hydrogeology of the Botany Basin, and focuses on how the latter impacts on its geomechanical behaviour. It will consider, briefly, the basin’s encapsulating bedrock foundation rocks, their intersection with the basin fill sediments, the latter’s structure, stratigraphy, lithology, distribution and how these impact on its hydrogeological behaviour. The discussion will then consider the basin’s economic and beneficial value, development within the basin and how this has historically impacted on the basin’s hydrogeology, hydrogeochemistry, water quality and economic value and how development projects need to consider their impact on the basin’s condition and the existing development it supports. Two case studies are presented to illustrate the latter.

At the outset, it is appropriate to define what the term ‘Botany Basin’ constitutes. Rickwood (1998) notes that there “… are those geologists who regard it as a tectonically formed bedrock depression that is the result of post Triassic uplift and warping, and is the smaller part of the larger Sydney Basin” (referencing Roy, 1983), which contrasts with the general view held by hydrogeologists that tend to apply ‘… the name Botany Basin to the topographic depression that is covered by the unconsolidated sediments that form the Botany Sands aquifer’ (referencing Griffin, 1963). Rickwood (1998) further develops the interpretation of the Botany Basin as being “… an easily verifiable bedrock basin … centred on Botany Bay and approximates to the catchment area, but excludes(ing) the extensive drainages of the major rivers entering the basin.” Rickwood then considers the relevance of the Pleistocene basin area and that of the modern basin, settling on the latter as the general basis for outlining the extent of the Botany Basin boundaries.

This paper broadly adopts the general extent of the modern basin as the definition of the Botany Basin, and focuses on the Quaternary sediments contained within that basin and how this aquifer interacts with older bedrock formations which comprise the Pleistocene bedrock paleochannel/paleobasin. The hydrogeological aspects of the bedrock formations, primarily the Hawkesbury Sandstone and Ashfield Shales, are discussed in detail in papers elsewhere in this volume.

2 BACKGROUND AND RECENT HISTORY McNally and Branagan (1998) provide an eloquent summary of the early history of colonial settlement in and around Sydney, as it interacted with and exploited the water resources of the Botany Basin. Their material was sourced Thorpe (1935), Henry (1939), Aird (1961), Griffin (1963), Lee et al. (1971), Dale and Burgess (1988), Branagan (1996) and Dale et al. (1997). The following provides the reader with a synopsis of their narrative.

From the very early days of European settlement in the Sydney Basin, the groundwater resources of the Botany Basin have been exploited for drinking water and agricultural use and, later, for industrial uses. With the First Fleet’s arrival and Captain Arthur Phillip’s choice of Port Jackson as the preferred location for settlement came the need for a potable water supply, and the development of the Tank Stream. It did not take long before this supply became fouled and the search for a substitute began. It was with the arrival of John Busby in Sydney in 1824, that this search began in earnest, when he was charged with the job of planning the city’s water supply. Busby considered the Lachlan Swamp, located in the southern portion of Centennial Park, as the most prospective resource in close proximity to the city. He conceived a plan to construct a 3 km tunnel to convey the water harvested from the swamps to a location on the south-eastern side of Hyde Park, within the northern portion of the Botany Basin.

Tunnelling began in 1827 from the Hyde Park end of the planned route and progressed along a route which follows the present-day Oxford Street alignment to the Victoria Barracks, through the north portion of Moore Park and breaking though to the Lachlan Water Reserve (now referred to as ‘Busby’s Pond’) at a location near the Robertson Road Gate. Tunnelling in the Hawkesbury Sandstone bedrock was largely carried out by hand, with limited sections advanced by blasting with black powder. Spoil removal and ventilation was facilitated through 28 shafts, initially dug down along the alignment to assess rock weathering and water table conditions.

The tunnel took 10 years to complete, being finally commissioned in 1837. It remained in use into the 1880’s, when pollution issues at the source and along the tunnel alignment began affecting desirability of the water for drinking water. By 1902 it was only able to be used for street washing. It is of note that, whilst the tunnel took a long time to

HYDROGEOLOGY OF THE BOTANY BASIN RK HATLEY

74 Australian Geomechanics Vol 39 No 3 September 2004

construct, it was nevertheless an adequate supply of water to the city well before it was completed, by virtue of groundwater inflow from the bedrock sandstones.

While the Busby’s Bore was supplanted as the primary water supply to the city in the 1860’s when the Upper Nepean Scheme was embraced, the swamps, stretching from the Lachlan Water Reserve in Centennial Park to Botany Bay, have remained a source of water supplies up to the present. Several dams were constructed on the lower ponds adjacent to Botany Bay, and were used to provide water and mechanical power to a wool mill and, subsequently, a flour mill in the mid 1800’s. Through the second half of that century, the surface water supplies provided by the swamps were further dammed and the water pumped to the Paddington Reservoir using steam powered pumping systems. Busby’s Bore was refurbished in the 1870’s to improve the supplies harvested from newly constructed dams in the Lachlan Water Reserve.

It was at this time that interest in developing the groundwater resources within the Botany Basin was sparked. A number of trial wells were dug in the north Kensington area and supplies up to 2.4 m3/hour demonstrated. Estimates of the potential for groundwater resources in the northern portion of the Botany Basin were made by the authorities’ engineers and it was concluded that they could provide the City’s requirement of the time from as few as three similar wells. Even though further exploration of the northern Botany Basin demonstrated the viability of the resource, the groundwater resources were not developed and exploited until the following century.

During the late 1880’s, a sewerage disposal scheme was constructed which exploited the favourable hydrogeological characteristics of the Botany sands for the disposal of sewage (Henry, 1939). This sewage farm was located in the north-western shoreline of Botany Bay, near the Cooks River mouth. During its period of operation, sewage was discharged to the infiltration ponds on the permeable dune sands. The sewage farm was closed once the second of Sydney’s ocean outfalls was completed at Long Bay in 1916 (the first being at North Bondi). A similar sewage disposal farm operated at Rockdale and, together with the Cooks River farm, was discharging 15ML/day to the Botany Basin sands aquifers (Henry, 1939) by the early 1900’s.

As the urbanisation and accompanying industrialisation of the Botany Basin progressed through the 1900’s, the basin became more and more impacted. Its hydrogeology behaviour and the quality of the resources it provides have been significantly modified by changing land uses, the spread of habitation and the growth of light through to heavy industry across the basin. These changes have put great strain on the resource to the point that the aquifer is considered to be ‘stressed’. In particular, the growth of substantial petro-chemical and related industrial centres in the Banksmeadow and Kurnell areas, petroleum storage, paper manufacturing, galvanising and metals works and other general manufacturing development have all contributed to the stressed condition the basin is now in. Landfill in the Lakes and lower Cooks River areas has also played a part in the degradation of the resource. Sand mining, land reclamation and groundwater have also impacted the hydrogeological condition of the basin.

3 GEOLOGY Preferential erosion occurring at the coincidence of the Tertiary coastline and two major river drainages in the vicinity of the Botany Bay, created a topographic depression within the older Triassic bedrock formations, comprising the Wianamatta and Hawkesbury Group rocks, as illustrated in Figure 1 (Albani and Rickwood, 1998; Griffin, 1963). These basement rocks, into which the Botany Basin was sculptured, comprise interbedded shales and sandstones of the Ashfield Shale Formation overlying medium to coarse grained quartz sandstones of the Hawkesbury Group. The erosional characteristics and structural situation of the latter largely determined the extent and configuration of the basin. The geological and hydrogeological characteristics of these basement formations are considered in more detail elsewhere in this volume.

The basin persisted into the late Quaternary Period when sea level fluctuation conditions favoured the persistence of sedimentary deposition over erosion. Within the sedimentary column enduring until the present, deposition of the basal sediments began within estuarine environments (Mulholland, 1942) which developed initially and gave rise to the laying down of fine grained alluvial clay and silt sediments in the deeply incised paleochannels which characterise the basin floor topography (Figure 1) at the time. As the basin progressively filled, the depositional environment changed, and, after being interrupted by numerous erosional episodes, sediments became coarser grained in nature, pre-dominantly fine to medium grained sands of alluvial and aeolian origin. Interspersed in the upper portion of the sequence are frequent lenses of peat which developed in inter-distributary lagoonal areas.

The Botany Basin sedimentary column is punctuated by evidence of numerous geological features which suggest substantial sea level and climatic changes. McNally and Branagan (1998) note that “like other areas of coastal Quaternary sediments, the Botany Basin has been subjected to as many as 20 major cycles of sea level rise and fall over the past 2 million years, and to many more smaller fluctuations.” These authors, however, note that it is only sediments



deposited in during the past 120,000 years for which we have records, corresponding to the Last interglacial and Last Glacial stages of the Pleistocene and the Holocene periods.

Figure 1: Paleotopography of the Botany Basin floor (Albani and Rickwood, 1998, after Griffin, 1963).

Figure 2: Schematic section illustrating the development of the Botany Basin sediments.



The differentiation of sediment types and associations within the Botany Basin sedimentary sequence lead several authors to distinguish up to four stratigraphic units within that sequence, from the basal Pleistocene Sediment (Qps), through the Botany Sands’ (Qpb) to the upper Holocene (Qhs) unit. Griffin (1963), Smart (1974), Albani (1978), Roy (1980) and Yu (1994) have variously described these stratigraphic units, diagrammatically presented in Figure 3, using a variety of designations, including ‘Clay Beds Unit’, ‘Sand Beds Unit’ and ‘Dune Sands Unit’ (Smart 1974); Units A-D (Thorne, 1985); and Units 1-3 (Roy, 1983). A summary of the lithologies of these subdivisions is provided in the text which follows. The reader is referred to the works of these authors (and the excellent summary provided by McNally and Branagan, 1998) for in depth discussion on the various geological units which comprise the Botany Basin’s sedimentary column.

Figure 3: Schematic of Botany Basin stratigraphic subdivisions based on depositional environments and sea level changes through the Pleistocene and Holocene Periods (Albani and Rickwood, 1998).

A Basal Unit comprising sediments laid down in a deep, open tidal environment and including fluvial and aeolian medium grained dense sands with gravel lenses, grading “… upward into marine sand with estuarine shells and … interbedded with peaty estuarine muds” (Bish et al., 2000) and lignite lenses (Unit 4 in Figure 3). The unit varies from 0 m to 30 m thickness.

The sediments of the Second Unit were laid down in shallow restricted lagoonal environments (presumed to be interglacial estuarine and later fluvial deposits) and includes clay and clay-rich quartz sand lenses (Unit 3 in Figure 3).. Peat lenses developed in more swampy areas within the lagoon environments while the clean sands were deposited as channel fill sediments. Subaerial desiccation of the muds in the upper portion of the unit resulted in the development of a characteristic texture of fissure and crack structures. This unit reaches a maximum thickness of 15 m.

Sediments of the Third Unit (the Botany Sands) dominate the Botany Basin sedimentary column and were unconformably deposited over the clays and sands of the underlying Second Unit deposits, and comprise uniformly graded (well-sorted), clean, poorly cemented fine to medium grained quartz sands laid down in an subaerial depositional environment (aeolian and littoral dune and beach sands) which affect the Botany Basin area. Lenses and bands of inter-dunal peat and organic clay also occur through the unit (Unit 2 in Figure 3).

Sand dune structures dominate the sand beds laid down in this unit. The unit varies from 0 m to 30 m thickness, with an average thickness of 15m. Bish et al. (2000) notes that the top of this unit is “…. marked by an erosional discontinuity of intermittently cemented (and very hard) sandy material forming a horizon called ‘Waterloo Rock’ or Coffee Rock’”. However, McNally and Branagan (1998) refer to these weakly cemented sands as being a facies of the Holocene Sediments (Upper Unit) and consider them to represent “… a ‘hardpan’ or humicrete duricrust formed by evapotranspiration within the capillary zone above a shallow water table.”

An Upper Unit (Fourth Unit or Qps) comprising a heterogenous mix of fine to medium grained loose sands with interbedded silts and clay beds (soft muds), most commonly developed in the proximity to Botany Bay, caps off the Botany Basin sequence (Unit 1 in Figure 3).. They are younger than the Botany Sands and exist close to the present day land surface (McNally and Branagan, 1998). These sediments were laid down in a variety of depositional environments prevalent during the Holocene period, ranging from “… transgressive dune, estuarine bay, beach ridge, tidal flat and terrestrial swamp” (Bish et al., 2000) environments. The unit varies from 0 m to >20 m in thickness.



In the past century or more, filling and drainage of the low-lying and coastal areas in and around the Botany Bay area has increased the land surface area, making more land available for urban and industrial development. This has largely focused on the Botany and Mascot areas, making way for the development of Port Botany and Sydney Airport. Filling involved the ‘dray and scoop’ or ‘dredging and hydraulic placement’ of Botany Sands (dune sands) materials over these areas. Some 50 million tonnes of Botany Sands are estimated to have been dredged from Botany Bay and placed on the low-lying and coastal areas around the bay.

4 HYDROGEOLOGY By its very nature, the unconsolidated, largely uncemented sands beds of the Botany Basin sequence generally make for good aquifer materials. And, as described earlier, these have been exploited at various intensities since the arrival of European settlers in the 18th century. They remain an important groundwater resource to the present.

On a macroscopic scale, two primary groundwater systems operate within the Botany Basin: the groundwater system which operates within the bedrock sequences which constitute the Botany Basin erosional depression and the second, which operates within the unconsolidated sediments of the Botany Sandbed Aquifer (Merrick, 1998) units laid down within the depositional basin, the geology of which has been described previously. While this paper focuses on the hydrogeology of the Botany Sandbed Aquifer (BSA), the interaction between this aquifer system and its encapsulating bedrock formations is considered, although not widely understood.

Recharge to the BSA is primarily through direct rainfall infiltration with minor contributions attributable to irrigation, leaky service mains and flow from the underlying bedrock units. This is at its greatest in those areas of the basin which are least affected by urban development, namely the open space areas including Centennial Park, Moore Park, Queen Park, Heffron Park, Snape Park, Roland Park, Randwick Racecourse, and the 5 major golf courses and parks which surround the wetland and ponds of the East Lakes area (Lachlan Ponds) or Southern Zone nature reserves. Less significant contributions, both direct and indirect, come from the numerous smaller open areas, residential backyards and runoff from the covered areas. Recharge from these sources has been estimated at between 6% over estuarine sediments to 37% over sandy sediments (Merrick, 1994).

Secondary and, indeed subordinate, components of recharge are considered to be those which are derived from the surrounding bedrock geology and from those sourced from outside the basin. These include irrigation water, leaky stormwater, water- and sewer-mains, and creeks draining off the surrounding basin geology. The hydrogeological interaction of the bedrock with the BSA is not well understood, with general hydraulic principles suggesting it should be one dominated by one-way recharge from the latter to the former (through limited upward leakage). However, numerical groundwater modelling undertaken by Merrick (1994) suggests that downward percolation into the underlying bedrock generally prevails. With the construction of a number of infrastructure road, tunnelling and building projects locally penetrating the contact of the BSA sediments and the bedrock, investigating and understanding the hydrogeological interaction of both groundwater systems is important. Two case studies involving the investigation and management of the groundwater issues impacting on two such projects are presented.

In reviewing the hydrogeology of the BSA, this paper accepts the aerial subdivisions proposed by the Department of Infrastructure, Planning and Natural Resource (DIPNR) when considering the Botany Basin ‘Groundwater Management Area’ (GWMA) 018, namely, the Northern Zone, the Western Zone and the Southern Zone (Bish et al., 2000) as illustrated in Figure 4.

The major components of groundwater flow within the BSA in these three zones of the Botany Basin are largely inward towards Botany Bay, the centre of the basin, and considered on a local scale, flow within each of the zones broadly follows the physiographic features (topography) and surface drainages and ponds which occur across the basin. Water table levels with the BSA vary from 0 m depth, occurring where the ponds and lakes later manifest themselves as windows into the groundwater table to up to 25 m depth in the Kurnell Peninsula area. Jankowski and Beck (1998) note that over the majority of the basin water levels are within 9 m of the surface, which is particularly the case down the central axis of the Northern Zone, where the water table can be within 2 m of the surface. The BSA is generally unconfined, particularly at the outer edge of the basin, becoming both semi-confined and unconfined closer to Botany Bay, where discontinuous bands, beds and lenses of clay, silt, peat or Waterloo Rock (coffee rock) locally create conditions which fully or partially confine the groundwater. Jankowski and Beck (1998) reference a pumping test undertaken at David Phillips Field by Webb and Watson (1979) which demonstrated the existence of 3 separate aquifers at that location, namely a water table (unconfined) aquifer to 7.7 m, underlain by 2 semi-confined aquifers at 7.7 m – 17 m and 17 – 30 m, each being separated by peaty clay layers.



Figure 4: Geological Map of the Botany Basin showing GWMA zones (Bish et al., 2000, McKibbin and Russell, 2001, 2002).

In the case of the Northern Zone, flow is generally in the southerly and south-westerly direction from the elevated dominant recharge areas located against the Paddington – Randwick – La Perouse ridge, discharging to Lachlan Ponds, Alexandria Channel-Cooks River, in the case of the shallow water table systems, and Botany Bay, in the case of both the shallow and deeper regional groundwater systems. Along this north-south flow path from the dominant recharge to the discharge areas, the shallow hydrogeological systems exist in dynamic equilibrium with the wetland lakes and ponds, and drainages, each acting as ‘flow through’ systems, being discharge (dominantly) or recharge areas under differing climatic conditions (Jankowski and Knight, 1991; Merrick 1994; Jankowski and Acworth, 1998). The average hydraulic gradient through the Northern Zone is 1:200 (Jankowski and Beck, 1998), steepening locally in the areas of recharge and flattening through the central-southern lakes areas.

Within the Western Zone, groundwater flow is largely eastward, from the elevated bedrock basin edge recharge areas to Botany Bay to the east, with a minor component to flow to the Cooks and St Georges Rivers.

In the Southern Zone, flow is largely inward to Botany Bay (westward, north-westward and northward as the shoreline shape dictates) with a subordinate component of flow in a south-east direction from a groundwater recharge divide along the centre axis of the Kurnell Peninsula towards a discharge area in Bates Bay.

Data from which the generalised groundwater flow and water level contour patterns within the Botany Basin (largely from information from the dominant aquifer unit, the BSA) have been gathered, compiled and contoured by a number



of authors in recent times, including Shiel (1942), Mulholland (1942), Griffin (1963), Cornell (1964), Wallis (1967), Hawke (1973), Smart (1974), Johnson (1975), Davies and Merrick (1994), Merrick (1994, 1995, 1998a, 1998b), Merrick and Knight (1997) and Bish et al., (2000). The basis for the most recent of these compilations has been data collected since the early 1970s by the Department of Infrastructure, Planning and Natural Resources (DIPNR, formerly the Department of Land and Water Conservation, DLWC) from the regular monitoring of a network of monitoring wells across the basin. Currently DIPNR monitor 41 monitoring wells, largely located within the Northern Zone (Bish et al., 2000) where they are unevenly, irregularly distributed across that area.

A groundbreaking project, initiated in the early 1990s and published by Merrick (1994), set about developing a better understanding of the hydrogeological behaviour and groundwater flow conditions within the Botany Basin. This project set about constructing and calibrating a regional numerical groundwater flow model for the Botany Basin. The model focused on the Northern Zone and used the Aquifem-1 finite element code (Townley and Wilson, 1980). This regional numerical model has served as a basis for a number of localised groundwater models to allow predictive assessments of groundwater behaviour to be made (Merrick, 1994). These have included the studies to assess the possible impact to the local groundwater environments of the Sydney Airports third runway, the expansion of Port Botany (Merrick, 1998a) and the Eastern Distributor (Jewell, 1997) and Orica Botany Groundwater Cleanup Project (unpublished, 2004). In the case of the Eastern Distributor, a local model was used to assess the impact of dewatering, required to provide a dry working environment where the Eastern Distributor southern tunnel exit was constructed below the water table, on the local groundwater table. The local effects of lowering of the water table through dewatering pumping formed part of the assessment of likely settlement impacts of the residential areas in the vicinity of the tunnel exit. This will be discussed in greater detail in the case studies presented later in the text.

Davies and Merrick (1994) and Merrick (1994) published the details and various results of the Botany Basin model, which include its usefulness in “… quantifying recharge and discharge components of the water budget” (Merrick, 1998a) for the Northern Zone of the basin, and “… demand forecasting and optimisation models for conjunctive management of the water resources of the Botany Basin”, respectively.

Reclamation works, primarily located along the southern portion of the Northern Zone, along the Cooks River – Sydney Airport – Port Botany shoreline, have impacted the shallow groundwater flow conditions in this area. The heterogeneity of the soils underlying these reclaimed lands influences both the lateral and vertical groundwater flow patterns. Merrick (1998) has used numerical groundwater modelling methods to assess the impact of proposed (at that time) land reclamation works in the Penrhyn Estuary area (immediately west of Port Botany). The outcome of the modelling demonstrated that minor groundwater mounding behind the works could be anticipated and that maintaining the existing drainages was the key to avoiding the rise in groundwater levels and changes in groundwater velocity. Merrick (1998) also noted that the latter was an important consideration in avoiding waterlogging ground up-gradient of the work and mobilising contaminants in the shallow groundwater system.

Through the BSA sequence, groundwater flow direction and rate is influenced by the basal paleo-topography, hydrogeological characteristics of the various layered sediment types comprising the sedimentary column, and their horizontal and vertical distribution. The paleochannels (Figure 1) which form prominent subcrop topographic features influence the local deep groundwater flow patterns (Albani and Rickwood, 1998), particularly in the lowermost aquifer beds. Bish et al. (2000) note that the Lakes Valley paleochannel located in the central portion of the Northern Zone provides a preferred pathway for groundwater and contaminant flow in that area of the basin. Within the BSA, the unconformities regionally and locally influence the groundwater flow direction and rate, particularly where the unconformable contacts have a significant height and lithological variability. However, it is within the coarse grained permeable sands comprising the Botany Sands (the Third Unit) that the majority of accessible groundwater storage is held and where groundwater flux is at its greatest.

On a micro-scale, Jankowski and Beck (1998) have undertaken detailed investigations using 49 bundled multi-level piezometers installed over a rectangular portion of the Eastlakes Experimental Site, 7 m by 11 m in size, to assess the effect of minor changes of lithology and mineralogy of the BSA within the topmost 4 m of saturated aquifer on the hydraulic properties and water quality of the groundwater therein. The outcome of the assessment was to demonstrate that at this scale minor changes in these aquifer characteristics, which themselves reflect differing depositional environments, resulted in substantial variation in hydraulic conductivity and groundwater chemistry. These authors note that hydraulic conductivity ranges of over 50 m/day over 0.5 m may result from “small variations in grain size range and changes of a few percent between grain size populations …” and that “… chemical reactions resulting from water-sediment interaction between groundwater and different lithological units produces significant changes in groundwater chemistry”, including marked changes in pH, electrical conductivity, Na+, Ca2+, SO4

-, and HCO3- over distances of 0.5

m to 1.0 m. This, the authors also note, is likely to have considerable influence of contaminant and contaminant migration, frequently encountered across the basin.



Figure 5: Map of the Botany Basin showing inferred groundwater flow directions in the Botany Sandbed Aquifer unit (Bish et al., 2000, McKibbin and Russell, 2002).

5 HYDROGEOLOGICAL PROPERTIES The hydrogeological properties of the Quaternary sediments within the Botany Basin are best known from those units which are most exploitable for their groundwater resources or have been impacted by the urban settlement. This largely applies to the shallowest unit within the Botany Basin sequence, namely the BSA unit. Considerable hydrogeological data has been collected from this aquifer unit by researchers, government agencies (including DIPNR), scientific and engineering consultancies and private companies and individuals. Much of this information is confidential and has rarely been published. However, there is sufficient information which has been published, much of which has been compiled and summarised by Bish et al. (2000) and which is reproduced in Table 1.

From the compilation of published data prepared by Bish et al. (2000) (Tables 1 and 2) the hydrogeological properties of the BSA can be broadly summarised as follows:

• Average thickness of 15m to 20m, up to 53m in the deeper paleochannels; • Recharge by rainfall infiltration estimates of 6% over estuarine sediments to 37% over sandy sediments

(DIPNR report that they consistently derived a value of 30% in other coastal geological systems in NSW); • Hydraulic gradient of 0.003 to 0.01; • Estimated range of porosities of 0.33 to 0.40; • Variable storage coefficients of 0.0004 to 0.26;



• Hydraulic conductivities of 1.4 m/day to 85 m/day; • Transmissivities of 230 m3/day/m to 630 m3/day/m and • Specific yields / storage coefficients of 0.17 to 0.26.

TABLE 1: Aquifer properties of the Botany Sandbed Aquifer (from Bish et al., 2000 for DIPNR)

Source Location Test Type Porosity

(%)

Hydraulic Gradient

Trans-missivity

(m3/day/m)

Hydraulic

Conductivity

(m/day)

Storage

Coefficient

Specific

Yield

Sy

Cotton (1937) Pump Test 85 0.24 0.24

Sheil (1942) 70

Mulholland (1942)

33

Griffin (1963) 0.008 80 0.262

Swan (1971) Daceyville 230 0.4 - 0.0022

Smart (1974) Banksmeadow industrial area

40 25 0.26

Kalf & McKibbin (1974)

UNSW David Phillips Oval

300 0.003

Webb & Watson (1979)

Deeper sand aquifer, UNSW

280 Kh = 20 – 30

Kv = 0.012

0.001 - 0.003

Brodie (1988) Laboratory test

12 - 29

Nieweglowski (1988)

629 21

Jankowski & Knight (1990)

Slug test 1.9 - 8.6

Hitchcock (1991) Pump test 591 0.024 – 0.058

Hitchcock (1991) Rising falling head test

5.2 - 10.1

Dudgeon (1993) Centennial Park 0.01 - 0.008

20 - 35

Evans (1993) East Lakes 12.9 - 15

Constant head test

19 - 38

Merrick (1994) Moore Park Modelling 28 0.2

East Botany industrial area

20 0.03

Alexandra Canal

14 0.075

Lavitt (1994) Banksmeadow area

Laboratory test

25-31 20

Starr (1996) Kurnell Landfill 30

Jankowski et.al. (1997)

Eastlakes Falling head test

0.003 - 0.009

1.8 - 50

Maunsell (1997) 15 - 85

Jewell (1997) Centennial Park Pump test 0.008 400 40

E S Marks Field 140 17

Moore Park 171 - 703 18 – 80

mean 34

0.11- 0.25

Zetland 110 - 670 11-47

mean 18

0.20

It is worthwhile noting that in developing the numerical groundwater model for the Northern Zone, Merrick (1994) utilised storage coefficients values of 0.2, 0.075 and 0.03 for the northern, southern and East Botany area, respectively, to achieve an appropriately calibrated model.



Bish et al. (2000), in reviewing the status of the BSA for DIPNR (falling within Groundwater Management Area 018) has compiled a summary of the characteristics of the 3 zones, which define the Botany Basin. These statistics are presented in Table 2

Table 2: Summary of BSA in the Northern, Western and Southern Zones (after Bish et al., 2000 for DIPNR)

Parameter Northern Zone Western Zone Southern Zone

Area 61.5 km2 10.0 km2 23.0 km2

Sediment Thickness <80 m <30 m <30 m

Aquifer Thickness <35 m <20 m <20 m

Volume of Rainfall Recharge (estimated dry period to wet period, or calculated average)*

22 ML/d to 44 ML/d

[Merrick 1994]

23 ML/d (average)

[based on Merrick 1994 estimate for the Northern Zone]

8.4 ML/d (average)

[based on Merrick 1994 estimate for the Northern Zone]

Yield 1.0 - 41 L/s (Av 5 L/s) 0.05 - 3.4 L/s (Av 0.5 L/s) 0.1 - 15 L/s (Av. 2 L/s)

Depth to Water Table 0 - 35 m, (Av <5 m) 0.7 - 7.6 m, (Av 3.4 m) 0.3 - 11 m, (Av 3 m)

Salinity 130 - 600 µS/cm 130 - 600 µS/cm 325 - 1490 µS/cm

Number of Licensed Extraction Points (1998/99)

491 179 63

Number of Licences (1998/99) 430 168 25

Usage (1998/99) n/a n/a n/a

Allocation (1998/99) 4,182 ML 10 ML 667 ML

Sustainable Yield 14,297 ML/year 2,325 ML/year 5,893 ML/year

Land Uses Proportions

Residential 40% 70% 10%

Industrial / Commercial 40% 10% 60%

Public Open Space, Park and Reserves

20% 20% 30%

* Johnson (1981) estimated the total amount of recharge to the Northern Zone as 63 ML/day, assuming a significant source of recharge other than rainfall.

6 HYDROGEOCHEMISTRY From the very early days of European settlement in Sydney, the value of the BSA as a high quality water resource was recognised. Its low salinity and shallow depth made it a useful source of drinking, stock water and, later, industrial water and energy.

Jankowski and Yu (1998) and Bish et al. (2000) reviewed the hydrogeochemistry of the Botany Sands Aquifer, highlighting the fact that the water quality is highly variable, being characterised by:

• Electrical conductivities (EC) of between 100 µS/cm and 8,000 µS/cm (typical <1,500 µS/cm); • pH of 4.3 to 8.9; • Acidic groundwaters are fresh and of low salinity, while alkaline groundwaters are variably

contaminated; • Sulphate (SO4

2-) concentration to 5 - >100 mg/L due to groundwater flow through pyritic peaty beds and lenses;

• Nitrate and ammonia concentrations which are largely negligible, but locally can have ammonia and nitrate concentrations up to 38 mg/L and 87 mg/L (respectively) where they are impacted by the use of fertilisers and other nutrient sources such as leaky sewer mains, industrial facilities, pre-existing (now closed) landfill and night soil depots;

• The fresh water, arising largely from rainfall flux to the water table, are of a Cl-SO4-Na type, while groundwaters affected by shell debris are of the HCO3- and Ca2+ types;



• Contaminated groundwater rich in HCO3- and cations arise through impacts of other contaminants.

On the basis of the latter, Jankowski and Yu (1998) recognised three major hydrogeochemical zones within the Northern Zone, largely arising from the impact of urban/industrial pollution. These authors identify a ‘Zone A’ (Centennial Park – Randwick – Maroubra – East Lakes area) in the north and central areas which is characterised by the freshest waters (least contaminated), low in major ion concentration and slightly acidic. Average ECs of 168 in the northern portion of Zone A, becoming more saline southward, to 158 µS/cm and 213 µS/cm, with 1,130 µS/cm being associated with Astrolabe Park (landfill). Water of ‘Zone B’ (Botany – Banksmeadow – Hillsdale industrial area) was identified as having the most saline water (average ECs of 1,699–1,718 µS/cm), highest inorganic and organic contaminant concentrations and variable acid to alkaline quality. Water of ‘Zone C’ (Alexandria) is characterised by high salinity (average EC of 605 µS/cm), rich in HCO3- and Ca, and high concentrations of all ions.

Figure 6: Hydrochemical zonation of the groundwaters of the Botany Sandbed aquifer (after Jankowski and Yu, 1998).

The severity of point and non-point (diffuse) source contamination in specific areas of the Northern Zone, alluded to in the previous discussion, have lead the regulatory authorities to designate high risk area based on an aquifer risk assessment undertaken by the DIPNR (previously DLWC) in 1999. A Ground Water Protection Zone was declared by the DLWC under the Water Act of 1912 across part of the Northern Zone. Zone 1 of this GWPZ is centred on the Botany-Banksmeadow industrial area (Figure 7). Within this zone groundwater extraction is excluded/embargoed. A buffer zone, Zone 2 surrounds Zone 1, where groundwater extraction is restricted to prevent induced contaminant migration through pumpage.



An exception to the Zone 1 embargoed area will be the proposed large scale groundwater pump-and-treat system proposed for remediation of a halogenated hydrocarbon plume originating from the Orica site at Botany.

A further protection zone has been designated in the vicinity of the Elgas underground cavern gas storage facility at Port Botany; no drilling or groundwater extraction is permitted in this area. This facility comprises 4 galleries making up a propane gas storage cavern of 130,000 m3 capacity, excavated to a depth of 130 m below ground level into the Hawkesbury Sandstone bedrock beneath Molineaux Point. The gas is stored at 841 kPa in the low permeability sandstone bedrock and is secured from leakage by a water curtain above the crown of the cavern and which envelops the storage.

Figure 7: DIPNR Groundwater Management Map showing Ground Water Protection Zones (after DIPNR, 2003).

7 CASE STUDY 1 – THE M5 VENTIALTION TUNNEL AND SHAFT, TURRELLA In preparation for designing the ventilation shaft and tunnel for the M5 freeway system tunnels, the construction consortium sought to assess the impact the shaft sinking and tunnelling might have on the water table in the local



aquifer beds and, if drawn down, the settlements which arise. Then the structure was to be designed so that these impacts were minimised.

The ventilation systems for the eastern end of the main M5 tunnel comprise 680 m of sub-horizontal tunnel, rising to a subterranean fan room via a 40 m deep shaft and on to an above-ground exhaust stack. The shaft crosses the contact between the underlying Hawkesbury Sandstone bedrock and the Quaternary Alluvial sediments of the Botany Basin adjacent to Wolli Creek.

At this location, the sediments comprise an alluvial sequence of basal clays and sandy clays overlain by silty or clayey sands with lenses of clay, peat and clean sands. The underlying Hawkesbury Sandstone bedrock is typical of that which is encountered around Sydney. It does, however, show preferential fracturing in the vicinity of the Wolli Creek alignment. Away from the creek bed these characteristics are much less pronounced.

A program of investigation was embarked upon to assess the likely hydraulic behaviour of each of the substrata types which the tunnel and shaft would penetrate and their hydraulic connectivity. This involved the installation of a network of 7 multi-level piezometers (screened in the alluvium and the sandstone) together with a deep pumping well, installed, sealed and screened in the sandstone. The well and piezometers were slug tested prior to conducting a suite of pumping tests (at rates of 1.4 L/s to 3.3 L/s), including a 14 day long constant rate test (at 2.6 L/s and 2.9 L/s).

The analysis of the resulting complex data set revealed that:

• The transmissivity, storativity and hydraulic conductivity of the unweathered highly fractured sandstone in the proximity of Wolli Creek paleochannel were calculated to be 14 m3/day/m, 0.001 and 7.5 x 10-6 m/s, respectively, while further away from the creek they were less fractured and less permeable;

• The mean hydraulic conductivities of the alluvial sediments were calculated to range from 2.1 to 9.5 x 10-6 m/s, being similar to that of the underlying sandstone;

• The sandstone is in moderate to good, but variable, hydraulic communication with the overlying alluvium, despite the presence of a basal layer of fine grained sediments and

• Depending on the nature of the basal sediments, piezometer water levels showed limited to moderate responses to groundwater extraction from the underlying sandstone.

Using the data gathered by the hydraulic program, a local numerical groundwater model (MODFLOW finite difference model) was constructed to simulate the groundwater conditions in the Turrella area, and the effects of shaft and tunnel construction. The model was calibrated against the tidal information, short term water table monitoring and the pumping test data.

On the basis of the modelling, it was possible to conclude that:

• Because the shaft and tunnel (close to the creek) appear to be in a zone of permeable fractured sandstone, drawdown in the bedrock would cause drawdown in the overlying alluvium over a large area along the Wolli Creek Valley;

• The drawdown through leakage into the vulnerable portion of an unsealed shaft and tunnel would, in the long term, cause equilibrium drawdowns of up to 2 m which, in turn, could result in settlement of up to 50 mm (particularly where the peat layers are present);

• As a consequence of these outcomes, a recommendation that the shaft and 50 m of the tunnel where it enters the permeable fractured sandstone zone be sealed (as well as any high flow zones within the tunnel up to 200 m from the creek);

• In addition, temporary recharging of the alluvial aquifer during construction was advised and

• Long-term inflows to the last 50 m of the sealed tunnel and the shaft were to be less than 1 L/s in order that settlement effects are minimised to acceptable levels.

The value of undertaking investigations to this level of thoroughness has mitigated against potential settlement problems arising in the future. A comprehensive knowledge of the geological situation, a detailed testing program, a thorough understanding of the hydrogeological characteristics and behaviour of the aquifers concerned and the use of modelling techniques provided the confidence in the selected solution.



8 CASE STUDY 2 - EASTERN DISTRIBUTOR The second selected case study deals with a similar problem but on a larger scale. The Eastern Distributor forms part of the planned “Sydney Orbital” road network and is a stretch of 6 km of motorway connecting the southern end of the tunnel / bridge harbour crossings at Woolloomooloo, to the Southern Suburbs of Sydney. The road comprises a northern tunnelled section of motorway (1.7 km in length) beneath the inner Eastern Suburbs of Sydney, then emerges to the south into a 4.3 km length of lowered “parkway”, generally 4 or 5 metres below ground level, and extends to the suburb of Zetland, where it meets the Southern Cross Drive motorway. A second shorter tunnel section is located close to the southern end of the parkway.

The southern exit of the tunnelled section of motorway to the Eastern Distributor penetrates the basin bedrock sandstone contact with the overlying BSA and continues as a subsurface roadway within the unconsolidated sediments of the BSA until it meets Southern Cross Drive. Within both the sandstone bedrock and alluvium sections of the motorway, construction was below the local water tables within each aquifer. For the sandstone, being a low permeability rock unit at this location, groundwater inflows were considered manageable and of low risk to existing structures. However, with the very permeable aeolian sands which comprise the BSA at this location, construction below the water table was recognised as problematic. Dewatering, in the short term for construction and in the long term for the life time of the structure, was considered to pose a significant risk due to likely settlement effects on the densely built areas located primarily along the west of the subsurface parkway section of the alignment.

As part of the design of the motorway, 3 options were considered and tested, including:

• A tanked, water tight structure with vertical sides;

• A permanently drained excavation with vertical sides and

• A tanked structure to partial height, with provision for drainage of an overlying battered excavation during periods of high water table.

An intensive program of hydrogeological investigations to assess the appropriateness of each of these options was commissioned and carried out by Jewell (1997).

The investigation involved background research of hydrogeological information on the area and relevant reports on dewatering, design, implementation and interpretation of two aquifer pumping tests in the northern (Moore Park) and southern (Zetland) areas of the alignment, prediction of likely dewatering rates, use of analytical and numerical modelling methods to assess and predict local and regional impacts of construction and permanent dewatering on the surrounding areas and make recommendations for the design of the dewatering and reinjection systems.

Jewell (1997) found that the BSA along the alignment of the motorway comprises 3 main aquifer units, namely, an upper aquifer unit of aeolian sands of 15 m thickness, an intermediate unit comprising 5 m of peaty clay and silty clay beds and a lower aquifer unit of interbedded estuarine-alluvial sand, clay and peat beds. Shallow groundwater flow was demonstrated to be in a south-westerly to southerly direction from the areas of high recharge in the Moore Park area to inferred regional flow discharge points at the Alexandria Canal and Botany Bay. The water table along most of the subsurface alignment of the parkway is approximately 5 m below ground surface, with historic information indicating water table fluctuations of up to 2.2 m. Drilling at the southern end of the alignment (Zetland) indicated that a bedrock ridge was present, causing a sharp steepening hydraulic gradient at this location.

A 350 mm diameter pumping well and 8 multi-level piezometers, located at 10, 20, 50 and 80 or 150 metres distance from the pumping well, and orientated approximately north-south and east-west, were installed at each of the Moore Park and Zetland test locations. Water pumped from the Zetland test was reinjected into the BSA through injection wells, to test the viability of this method of mitigating local drawdown effects. The test pumping wells were pumped at a constant rate for 34 hours at 8.5 L/s at the Moore Park location and 7 days at 5 L/s at the Moore Park location. Water level data was collected using ‘dipper’ probes and data loggers during the pumping and recovery tests. The constant rate and recovery monitoring data collected was analysed using the Cooper and Jacob (1946) and Theis (1935) methods, respectively. Calculated hydraulic conductivity values ranging from 18 m/d to 80 m/d (mean 34 m/d) for the Moore Park Area and 11 m/d to 47 m/d (mean 18 m/d) and specific yields of 0.10 to 0.25 (Moore Park). Pronounced lateral and vertical anisotropy and a marked lateral heterogeneity were noted in the results.

The regional and local groundwater systems were modelled by Jewell (1997) to assess the effect of groundwater extraction. The regional effects of dewatering were modelled utilising the finite element numerical model (Aquifem-1), prepared by Merrick (1994) for the northern Botany Basin, a model successfully used for the third runway at Sydney Airport and the southern railway (the airport link line) pre-construction assessments. Two local models were constructed to evaluate the dewatering impacts at the local scale, and included analytical model (based on Edelman, 1972) to calculate drawdown along an infinite line sink, and a 2-dimensional (section) finite difference (MODFLOW)



model to incorporate the aquifer layering, variabilities in hydraulic conductivities, rainfall recharge and the partially penetrating diaphragm walls.

From the modelling, Jewell (1997) showed the regionally dewatering would generate a broad flat depression extending 500 m on either side of the excavation, with predicted drawdowns within this zone being approximately 1 m below average groundwater levels. Jewell (1997) noted that this outcome represented a water level just above the historical low point in the water table elevation (1.0 m to 1.5 m below average water levels). The local analytical modelling predicted drawdowns of 3 m close to the excavation and 1.2 m to 2 m at 300 m from the excavation. The more detailed local numerical model indicated drawdowns of 1.7 m at 200 m distance from the excavation. The modelling also indicated that dewatering pumping rates of 0.25 L/s to 0.5 L/s per metre were initially required to achieve the level of dewatering required for construction. These rates were predicted to decline to equilibrium dewatering rates of 0.1 L/s per metre after 3 months of pumping.

Jewell (1997) concluded that structurally significant drawdown was only likely to occur if the water level in the shallow aquifer was drawn down below the historical low water levels, since pre-consolidation to this level was already considered to have occurred. The modelling studies showed that drawdown to below the historical low water levels would occur between 200 m to 300 m from the excavation, and that the drawdown would be uniform and the gradient shallow, suggesting that differential settlement would be likely to be low.

Conventional vacuum-pumped spearpoints located at 2 m to 4 m spacings were considered appropriate for achieving the required construction dewatering levels.

In concluding the investigation, Jewell (1997) stressed the importance of groundwater monitoring as a means of assessing the impact of the dewatering during the construction and post-construction phases, where this would trigger corrective measures if the impacts deviated from the modelled results. Such corrective measures were then to include reinjection of groundwater, which was demonstrated during the testing works.

Construction of the motorway commenced in August 1997 with the final portions of the project opened in December 2000. At various stages of the construction and operation of the motorway, claims of alleged property damage were made to the constructor and/or road traffic authorities. An ‘independent panel investigation’ was commissioned by the State Government of NSW to assess the claims of alleged damage to properties (inferred to be arising from settlement effects) in the vicinity of the motorway.

After an intensive examination of the information made available to the panel, further geotechnical and hydrogeological review and 2 days of public hearings, their report to the government (Dundon and Train, 2001) concluded:

• that it was likely that the aeolian sand units which largely comprise the BSA which underlie the impacted area reacted rapidly to the water table drawdown by the dewatering program, resulting in prompt settlement effects in the subsoil;

• however, that it is likely that the presence of low permeability clay and peat interbeds within the high permeability sands reacted more slowly to the desiccation effects (drying and shrinkage) induced by the lowering of the water table, causing local differential settlement to occur;

• that the desiccation process in the low permeability materials may take years to occur and may even continue after the surround materials have been re-saturated;

• vibration effects may have accelerated the compaction of the subsurface materials;

• that evidence suggests that while clays and peat layers in the historic drainage channels (of Sheas Creek) in the impacted areas may have been subject to climatic-induced reductions of water table levels, this might not have resulted in significant desiccation shrinkage (and settlement), due to their low permeability, with respect to that in the surrounding subsoil materials and

• that the cumulative effect of groundwater removal and vibration, has caused cracking in specific buildings and this has arisen from accelerated shrinkage of the underlying foundation materials.

This case study highlights the need for intensive investigation of the geological, geotechnical and hydrogeological conditions prior to embarking on construction projects within the Quaternary sediments of the Botany Basin, where construction below the water table is contemplated.

9 CONCLUSIONS This paper has sought to provide a review of the geology and hydrogeology of the Botany Basin. It has done so by considering the macroscopic, mesoscopic and microscopic composition and behaviour of the unconsolidated materials within the basin and finishes with 2 case studies which highlight how the latter impacts on their geomechanical



behaviour. It has considered the basin’s economic and beneficial value, the development within the basin and how this has historically impacted on the basin’s hydrogeology, hydrogeochemistry, water quality and economic value.

The paper has demonstrated that development projects, no matter how large or small, need to consider their impact on the basin’s local and regional condition and the existing development it supports, and employ an appropriate level of investigation to minimise any potential adverse effects.

It is worth noting that DIPNR has a statutory involvement, under the provisions of the Water Act of 1912, where a proposed development intersects a shallow permanent water table and continuous pumping is necessary to lower the water table to permit construction to proceed. DIPNR also stresses that they do not endorse the extraction of groundwater in perpetuity around a development, regarding such developments as unsustainable. DIPNR consider applications for temporary dewatering, where the final development is ‘water tight’ or ‘fully tanked’ to prevent ongoing seepage of water into the structure. They consider technical studies (hydrogeological and geotechnical) and field investigation to be necessary to justify any assertion that a development impinging on the water table will produce a negligible impact on the groundwater flow, water table levels, and any groundwater dependant ecosystem, and will not induce unacceptable settlement in the surrounding area. This process is dealt with through an application for a bore licence under the Water Act and the Development Application process administered by the local council involved.

10 ACKNOWLEGEMENTS The author would like to express his deep gratitude to Dan McKibbin and Greg Russell of DIPNR for their assistance in generously providing advice and unpublished material extensively used in the preparation of this paper. In particular, the Status Report No. 2, Botany Basin Beds (GWMA 018), Botany Basin, NSW (2000), prepared by DIPNR staff, S Bish, S Realica and J Wischusen, whose approval to use some of the summary information provided in this excellent document has greatly assisted the author. Thanks also go to the assistance provided by Mr Noel Merrick of the University of Technology (Sydney), Mr Peter Dundon of Peter Dundon and Associates, Chris Jewell of Chris Jewell and Partners and Diana Yiend of Golder Associates.

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