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Amini Loco Mineral Resources Department

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

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Amini Loco Mineral Resources Department

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  1. Hydrogeology of the Middle Sigatoka Valley and Human Impacts on Groundwater Systems – Preliminary Results Amini Loco Mineral Resources Department

  2. 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

  3. 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

  4. Study Area In Viti Levu

  5. Study area – 1:50000 Topo Map

  6. 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)

  7. 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

  8. 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

  9. 1:10,000 IKONOS SATELITE IMAGE

  10. Existing wells - Groundwater Utilization

  11. 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

  12. Geology Map

  13. Dubalevu Cross-section

  14. Bilalevu Cross Section

  15. Alluvial Deposits

  16. Bilalevu – Alluvial Flats

  17. Dubalevu – Abandoned meander

  18. Uplifted Qalimare Limestone

  19. Fractured Limestone

  20. 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

  21. Supersting Resistivity Equipment

  22. EM34-3 Equipment

  23. Location Map - Dubalevu

  24. Dubalevu – Dipole Dipole Strong Gradient (DDSG) Profile

  25. Dubalevu vertical dipole data

  26. Dubalevu 10/07 Geology Depth (m) 0 Silt loam Recent Alluvium 6 Alluvial Aquifer Gravelly silt 11 Fresh siltstone 22 Cici Sandstone Weathered or fractured sandstone Fractured Aquifer 33 Fresh sandstone 34

  27. Dubalevu 10/10 Depth (m) Geology 0 Silt loam 8 Recent Alluvium Weathered siltstone 18 Fresh siltstone 24 Weathered or fractured sandstone Cici Sandstone Fractured Aquifer 33 35.6 Fresh sandstone

  28. Location Map - Bilalevu

  29. Bilalevu

  30. Bilalevu Vertical dipole Data

  31. Bilalevu 10/15 Depth (m) 0 Sandy silt Recent Alluvium 11 Alluvial Aquifer Sandy Gravel 17 Tari Formation Fresh sandstone 35.

  32. Bilalevu 10/14 Depth (m) 0 Recent Alluvium Sandy silt 16 Fresh sandstone 39 Tari Formation Weathered Sandstone & Mudstone Fractured Aquifer 51 Fresh sandstone 55

  33. Preliminary Groundwater Chemical Results

  34. Discussion • Presence of two aquifers • Alluvial • Fractured bedrock • Heterogeneity of geological systems • Water chemistry dominated by calcium-bicarbonate waters

  35. 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

  36. 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

  37. Thank you!!!!

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