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 km inland from Sigatoka town on the SW of Viti Levu and covering an area of around 30 km ² –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

Dubalevu vertical dipole data

Silt loam 0 Gravelly silt Fresh siltstone Weathered or fractured sandstone Fresh sandstone Recent Alluvium Cici Sandstone Geology Depth (m) Dubalevu 10/07 Fractured Aquifer Alluvial Aquifer

Geology Depth (m) Silt loam Weathered siltstone Fresh siltstone Weathered or fractured sandstone Fresh sandstone Dubalevu 10/10 Fractured Aquifer Recent Alluvium Cici Sandstone

Location Map - Bilalevu

Bilalevu

Bilalevu Vertical dipole Data

Bilalevu 10/ Recent Alluvium Tari Formation Depth (m) Alluvial Aquifer Sandy Gravel Sandy silt Fresh sandstone

Bilalevu 10/ Recent Alluvium Tari Formation Depth (m) Fractured Aquifer Fresh sandstone Sandy silt Weathered Sandstone & Mudstone 51 Fresh sandstone

Preliminary Groundwater Chemical Results

Discussion Presence of two aquifers –Alluvial –Fractured bedrock Heterogeneity of geological systems Water chemistry dominated by calcium- bicarbonate 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!!!!