Hydrogeology of the Middle Sigatoka Valley and Human Impacts on Groundwater Systems – Preliminary Results Amini Loco Mineral Resources Department 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 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 Study Area In Viti Levu Study area – 1:50000 Topo Map 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) 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 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 1:10,000 IKONOS SATELITE IMAGE Existing wells - Groundwater Utilization 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 Geology Map Dubalevu Cross-section Bilalevu Cross Section Alluvial Deposits Bilalevu – Alluvial Flats Dubalevu – Abandoned meander Uplifted Qalimare Limestone Fractured Limestone 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 Supersting Resistivity Equipment EM34-3 Equipment Location Map - Dubalevu Dubalevu – Dipole Dipole Strong Gradient (DDSG) Profile 0 420 410 400 390 380 370 360 350 340 330 320 310 300 290 280 270 260 250 240 230 220 210 200 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 Conductivity (mS/cm) Dubalevu vertical dipole data Dubalevu tapered vertical dipole data 70 60 50 40 vz 10 tapered vz 20 tapered 30 vz 40 tapered 20 10 0 Dubalevu 10/07 Depth (m) 0 Silt loam 6 Gravelly silt 11 Alluvial Aquifer Recent Alluvium Geology 22 Weathered or fractured sandstone 33 34 Fresh sandstone Fractured Aquifer Cici Sandstone Fresh siltstone Dubalevu 10/10 Recent Alluvium Geology Depth (m) 0 Silt loam 8 Weathered siltstone 18 Fresh siltstone Weathered or fractured sandstone 33 35.6 Fresh sandstone Fractured Aquifer Cici Sandstone 24 Location Map - Bilalevu Bilalevu Bilalevu Vertical dipole Data Bilalevu EM3405 tappered vertical dipole analysis 50 30 vz 10 tappered vz 20 tapparerd vz 40 tappered 20 10 reading distance (m) 340 330 320 310 300 290 280 270 260 250 240 230 220 210 200 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0 0 Conductivity (mS/cm) 40 Bilalevu 10/15 Depth (m) 0 11 Sandy Gravel Tari Formation 17 Fresh sandstone 35. Alluvial Aquifer Recent Alluvium Sandy silt Bilalevu 10/14 Recent Alluvium Depth (m) 0 Sandy silt 16 Fresh sandstone Weathered Sandstone & Mudstone 51 55 Fresh sandstone Fractured Aquifer Tari Formation 39 Preliminary Groundwater Chemical Results November Sampling Stif f Diagram CATIONS 10 9.5 9 8.5 IONS 8 7.5 7 6.5 6 5.5 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 Na+K 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10 Cl Ca HCO3+CO3 Mg SO4 Sigatoka Riv er, Dubalev u Na+K Cl Ca HCO3+CO3 Mg SO4 Pumping Well 4 Na+K Cl Ca HCO3+CO3 Mg SO4 Pumping Well 2 Na+K Cl Ca HCO3+CO3 Mg SO4 Sigatoka Riv er, Bilalev u Na+K Cl Ca HCO3+CO3 Mg SO4 Oxbow Lake, Bilalev u Na+K Cl Ca HCO3+CO3 Mg SO4 Bulatale Store Na+K Cl Ca HCO3+CO3 Mg SO4 Tubakeli Settlement Na+K Cl Ca HCO3+CO3 Mg SO4 AGriculture Station, Dubalev u Na+K Cl Ca HCO3+CO3 Mg SO4 Ami Chand Discussion • Presence of two aquifers – Alluvial – Fractured bedrock • Heterogeneity of geological systems • Water chemistry dominated by calciumbicarbonate waters 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 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 Thank you!!!!