Transcript 06B_02_Loco_STAR Presentation

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
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Conductivity (mS/cm)
Dubalevu vertical dipole data
Dubalevu tapered vertical dipole data
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vz 10 tapered
vz 20 tapered
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vz 40 tapered
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0
Dubalevu 10/07
Depth (m)
0
Silt loam
6
Gravelly silt
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Alluvial
Aquifer
Recent Alluvium
Geology
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Weathered or
fractured sandstone
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Fresh sandstone
Fractured
Aquifer
Cici Sandstone
Fresh siltstone
Dubalevu 10/10
Recent Alluvium
Geology
Depth (m)
0
Silt loam
8
Weathered siltstone
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Fresh siltstone
Weathered or
fractured sandstone
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35.6
Fresh sandstone
Fractured
Aquifer
Cici Sandstone
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Location Map - Bilalevu
Bilalevu
Bilalevu Vertical dipole Data
Bilalevu EM3405 tappered vertical dipole analysis
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vz 10 tappered
vz 20 tapparerd
vz 40 tappered
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10
reading distance (m)
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Conductivity (mS/cm)
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Bilalevu 10/15
Depth (m)
0
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Sandy Gravel
Tari
Formation
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Fresh sandstone
35.
Alluvial
Aquifer
Recent
Alluvium
Sandy silt
Bilalevu 10/14
Recent
Alluvium
Depth (m)
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Sandy silt
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Fresh sandstone
Weathered Sandstone &
Mudstone
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Fresh sandstone
Fractured
Aquifer
Tari
Formation
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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!!!!