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

Dubalevu vertical dipole data

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

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

Location Map - Bilalevu

Bilalevu

Bilalevu Vertical dipole Data

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

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

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

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!!!!