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Hydrogeology of the Middle Sigatoka Valley and Human
Impacts on Groundwater Systems – Preliminary Results
Amini Loco
Mineral Resources Department
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Overview• Introduction & Rationale
• Objectives
• Investigation Methodologies
• Literature Review - Groundwater Utilisation History and Site Geology
• Results & Brief Discussions– Geophysics, Groundwater Drilling and Groundwater Chemistry
• Remaining Investigations
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Introduction
• This project looks at an intensive, semi-commercial to commercial farming community – The Middle Sigatoka Valley, where groundwater is used for both irrigation and portable purposes and attempts to identify impacts of human activities on groundwater systems.
• The project will be focusing on Dubalevu and Bilalevu areas
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Study Area In Viti Levu
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Study area – 1:50000 Topo Map
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Objectives• Characterize underlying geological materials and
structural control in terms groundwater potential and groundwater chemistry
• Investigate the status of groundwater utilization in the area
• Determine the sustainable yield of the underlying aquifer(s)
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Investigation Methodology
• Literature Review
• Engineering geology and geomorphologic mapping
• Near-surface geophysical methods
• Preliminary Groundwater Chemical sampling and analysis
• Groundwater drilling using mud-rotary GEMCO H22A rig and assessment of lithological composition of drilled-cuttings
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Background - Literature Review• Middle Sigatoka Valley is:
– Located 15-17 km inland from Sigatoka town on the SW of Viti Levu and covering an area of around 30 km²
– Characterized by topographic lows and flats along the reaches of the Sigatoka River and bounded by highly dissected and rugged hills
– Intensive commercial landuse and high groundwater utilisation for irrigation and portable purpose
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1:10,000 IKONOS SATELITE IMAGE
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Existing wells - Groundwater Utilization
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Site Geological Framework • Geological System underlying the area include:
– Interbeeded, thinly-moderately thickly bedded brown mudstone, bluish-grey sandstone with lenses of dacite and dacitic tuffs of the Tari Formation on the South - SE
– 250 m Uplifted Qalimare Limestone on the NE
– Steeply Late Miocene Cici Sandstone and Takaro Conglomerate and Rudite covering W-NW of the area
– Recent Alluvium materials occupying the topographic lows and flats (15 – 25m thickness) characterized by a number of terraces and alluvial fans composed of angular and gravels sandy silt
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Geology Map
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Dubalevu Cross-section
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Bilalevu Cross Section
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Alluvial Deposits
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Bilalevu – Alluvial Flats
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Dubalevu – Abandoned meander
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Uplifted Qalimare Limestone
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Fractured Limestone
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Geophysical Survey and Results• Electrical Resistivity and Electromagnetic
methods– Supersting 8 channels IP– EM-34-3 receiver and transmitter coils
• Aims include:– determine depth to bedrock and thickness of alluvial
materials– Identify any structural features (e.g. faults and folds)– to select potential drillholes
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Supersting Resistivity Equipment
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EM34-3 Equipment
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Location Map - Dubalevu
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Dubalevu – Dipole Dipole Strong Gradient (DDSG) Profile
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Dubalevu vertical dipole data
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Silt loam
0
Gravelly silt
Fresh siltstone
Weathered or fractured sandstone
34
33
Fresh sandstone
22
11
6
Rece
nt A
lluvi
umCi
ci S
ands
tone
Geology Depth (m)
Dubalevu 10/07
Frac
ture
d Aq
uife
rAl
luvi
al
Aqui
fer
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35.6
0
33
24
18
8
Geology Depth (m)
Silt loam
Weathered siltstone
Fresh siltstone
Weathered or fractured sandstone
Fresh sandstone
Dubalevu 10/10
Frac
ture
d Aq
uife
r
Rece
nt A
lluvi
umCi
ci S
ands
tone
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Location Map - Bilalevu
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Bilalevu
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Bilalevu Vertical dipole Data
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Bilalevu 10/15
0
35.
17
11
Rece
nt
Allu
vium
Tari
Form
ation
Depth (m)
Allu
vial
Aq
uife
r
Sandy Gravel
Sandy silt
Fresh sandstone
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Bilalevu 10/14
0
55
39
16
Rece
nt
Allu
vium
Tari
Form
ation
Depth (m)
Frac
ture
d Aq
uife
r
Fresh sandstone
Sandy silt
Weathered Sandstone & Mudstone
51
Fresh sandstone
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Preliminary Groundwater Chemical ResultsNovember Sampling Stiff Diagram
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10IONS
00.511.522.533.544.555.566.577.588.599.510CATIONS
Na+K
Ca
Mg
Cl
HCO3+CO3
SO4
Sigatoka River, Dubalevu
Na+K
Ca
Mg
Cl
HCO3+CO3
SO4
Pumping Well 4
Na+K
Ca
Mg
Cl
HCO3+CO3
SO4
Pumping Well 2
Na+K
Ca
Mg
Cl
HCO3+CO3
SO4
Sigatoka River, Bilalevu
Na+K
Ca
Mg
Cl
HCO3+CO3
SO4
Oxbow Lake, Bilalevu
Na+K
Ca
Mg
Cl
HCO3+CO3
SO4
Bulatale Store
Na+K
Ca
Mg
Cl
HCO3+CO3
SO4
Tubakeli Settlement
Na+K
Ca
Mg
Cl
HCO3+CO3
SO4
AGriculture Station, Dubalevu
Na+K
Ca
Mg
Cl
HCO3+CO3
SO4
Ami Chand
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Discussion• Presence of two aquifers
– Alluvial– Fractured bedrock
• Heterogeneity of geological systems
• Water chemistry dominated by calcium-bicarbonate waters
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Future direction????• River gauging of the Sigatoka River
– Aims to establish the losses/gains from and into adjacent Sigatoka River
• Pumping tests
• Groundwater Isotope analysis and dating
• Water balance and Conceptual Groundwater Model
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Acknowledgements• The Fiji Government through the Ministry of Fijian Affairs Board Scholarship Mineral Resources
Department for financially support in my studies and making the project a reality
• SOPAC for permitting the use of their new Resistivity and EM equipment and with the field leadership and supervision Mr Peter Sinclair
• Mineral Resources Department Staffs, particularly the Hydrogeology section staffs, for their field assistance throughout the past months
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Thank you!!!!