Hydrogeology of the Middle Sigatoka Valley and Human Impacts on Groundwater Systems – Preliminary...
-
Upload
emil-howard -
Category
Documents
-
view
218 -
download
2
Transcript of Hydrogeology of the Middle Sigatoka Valley and Human Impacts on Groundwater Systems – Preliminary...
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!!!!