Groundwater Training Course
SOPAC, April 2005
Island Groundwater Resources
Outline of Presentation
• Freshwater resources of islands
• Water use
• Factors affecting freshwater resources
• Water resource management measures
Freshwater Resources
of Islands
• Naturally occurring:
• Surface Water
•
Groundwater
• Rainwater
• Other approaches:
•
•
•
•
Desalination
Importation
Use of seawater or brackish water for some uses
Use of coconuts for drinking (in emergencies)
Surface Water Resources
• RIVERS & STREAMS
♦ surface
♦ subterranean (in
karstic formations)
• SPRINGS
♦ on island
♦ at coastline
♦ submarine
• LAKES & SWAMPS
♦ fresh
♦ brackish
Groundwater Resources
• PERCHED AQUIFERS
(High Islands)
♦ ‘HORIZONTAL’
♦ ‘VERTICAL’(Dyke-confined)
• BASAL AQUIFERS
(High & Low Islands)
♦ COASTAL AQUIFERS
♦ ‘FRESHWATER LENSES’
Groundwater Resources
(HIGH ISLAND example)
Example from the Hawaiian Islands
showing 3 types of groundwater aquifers
* perched (horizontal)
* perched (vertical)
* basal
Groundwater Resources
(LOW ISLAND example)
Example of small coral island
from a typical atoll with thick
transition zone
* basal aquifer only
(often called a ‘freshwater lens’)
Groundwater Resources
(Coastal Zone Conditions in some HIGH ISLANDS)
Basal aquifer - note THIN transition zone
Summary data for Pacific Island Countries
Water uses
•
•
•
•
•
Water supply (most important use)
Tourism (selected islands)
Mining (selected islands)
Irrigated agriculture (limited)
Hydropower (limited)
Summary water use data for Pacific Island Countries
Factors which influence Fresh Groundwater
Occurrence on Islands
• AREA & SHAPE
• TOPOGRAPHY
• GEOLOGY & HYDROGEOLOGY
• CLIMATE & HYDROLOGY
• SOILS
• VEGETATION
• EXTREME EVENTS
(e.g. Earthquakes, cyclones, waves, droughts)
• HUMAN SETTLEMENT & ACTIVITIES
) Pumping - location, method and volume
) Pollution - biological, chemical
Island Area Definitions
• LARGE
AREA > 2,000 km2, or
WIDTH > 10 km
• SMALL
# AREA
• VERY SMALL
# AREA
• 'TINY’
< 2,000 km2, or
WIDTH < 10 km
< 100 km2, or
WIDTH < 3 km
AREA < 20 km2, or
WIDTH < 1 km
# UNESCO (1991), Hydrology and Water Resources of Small Islands: a practical guide
Size and Shape
• Larger islands generally have greater groundwater
resources potential than smaller islands
•Wider islands generally have greater groundwater
resources potential than narrower islands
Small islands- Limited Resources
Betio, Tarawa atoll, Kiribati
15,000 people on 0.8 km2 of land
Many have….
Malé island
(capital of the Maldives)
70,000 people on 1.3 km2 of land
• Small land area
• Large population or other demands
• Very limited or depleted water resources
Topography
High Island:
Groundwater & Surface
Water Resources
Low Island:
Groundwater
Resources only
Local Climate
• AFFECTED BY:
- Geographical Location
- Proximity to Large Land Mass(es)
- Orographic Effects (High islands)
• SOME KEY FEATURES:
- Cyclones (random)
- Monsoons (annual cycle)
- El Niño - La Niña episodes (2-7 year cycle)
- Climatic Variability
- Climatic Change (over many years)
Mean Annual Rainfall
in the Pacific Ocean
High rainfall zone in south-west; Low rainfall zone in equatorial east (average conditions)
Impacts of El Niño and La Niña Episodes on Rainfall
El Niño conditions
Wetter and Drier
regions associated
with
El Niño and
La Niña episodes
La Niña conditions
Annual Rainfall Variability, Betio, Tarawa, Kiribati
5,000
Annual Rainfall (1954-1978)
Mean = 1,876 mm
Cv
= 0.46
Annual rainall (mm)
4,000
Annual Rainfall (1979-2004)
Mean = 2,181 mm
Cv
= 0.51
3,000
2,000
1,000
Year
Notes:
(a) Low rainfall in La Niña periods, high rainfall in El Niño periods
(b) In last 26 years: (a) 16% higher mean (average) rainfall,
(b) 13% higher variability (Cv), than in previous 25 years
Also: lowest 3 year rainfall and 4 highest annual rainfalls
2004
2002
2000
1998
1996
1994
1992
1990
1988
1986
1984
1982
1980
1978
1976
1974
1972
1970
1968
1966
1964
1962
1960
1958
1956
1954
0
Local Climate: Effects of Topography on Rainfall
Different orographic
effects for islands
of varying heights
A. Very high
B. High
C. Low
Local Climate: Effects of
Topography on Rainfall
Rarotonga
Cook Islands
Annual Rainfall pattern
(High rainfall gradient)
Geology
MAIN TYPES:
•
•
•
•
Volcanic
Limestone
Coral atoll and cay
Mixed (most common)
ALSO:
• Bedrock
• Unconsolidated (Sand, Mud)
Geology
Examples of Volcanic, Limestone and Coral Islands
Geology
Evolution of
volcanic island
..... to mixed
volcanic and
coral island
….. to coral atoll
Mangaia, Cook Islands
(example of mixed geology)
Some statistics
• Area = 52 km2
• Max height = 170m
• Central
eroded
volcanic area with
outer
elevated
limestone plateau
• Water resources
• Surface water
• Groundwater
Mangaia
Raised
limestone rim
Central volcanic
area
Island types for Pacific Island Countries
Hydrogeology
Key features:- location and types of aquifers (basal, perched)
- permeability
- porosity / specific yield
- major structural features (e.g. fractures, dykes, karst,
unconformities)
Hydrogeology of a Typical Coral Island
Fine lagoon
sand/silt
Increasing
permeability
with depth
Unconformity
------> Increasing permeability ------>
Coarse
rubble
Impacts of Human Activities
on Water Resources
These impacts include:
• Saline intrusion of groundwater (due to over-pumping)
• Chemical pollution from:
• hydrocarbon (petrol, diesel) leaks & spills
• solid waste disposal sites
• agricultural chemicals particularly insecticides, herbicides
• industrial discharges
• Biological pollution from:
• inappropriate or poorly maintained sanitation systems
• solid waste disposal sites
• Erosion and Sedimentation (due to land clearing)
High Vulnerability to Pollution
(especially small coral islands)
Unsaturated Zone
Freshwater Zone
Highly vulnerable to:
– chemical & biological pollution due to thin, highly permeable soils
– saline intrusion due to thin freshwater zones
Water Resources Management
Some water resources management approaches:
„ Effective use of available water resources
„ Protection of existing groundwater resources
„ Demand management measures
„ Use of other sources in extreme circumstances
„ Special measures
Water Resources Management (cont’d)
„ Effective use of available groundwater resources:
z adequate water resources assessment, monitoring and analysis
to ensure sustainability of extractions
z use of different classes of water (e.g. rainwater for drinking,
cooking; groundwater for other uses)
z use of appropriate technology - especially in particularly fragile
areas (e.g. infiltration galleries for water supply)
Groundwater Pumping
(on small coral islands and coastal zones of high
islands)
•Dug wells
•Boreholes
•Infiltration
galleries
Solar Pumping
(at infiltration gallery, Lifuka, Tonga)
Water Resources Management (cont’d)
„ Protection of existing groundwater resources:
z groundwater protection zones or ‘reserves’
z controls over groundwater pumping and surface water
extraction
z control over human settlement (e.g. edge of islands for houses,
centre for water and playing fields)
z appropriate sanitation & solid waste disposal systems
Water Resources Management (cont’d)
„ Demand management measures:
z leakage control
z consumer education & awareness
z low water use fittings & appliances
z metering & charging for water on basis of usage
z drought management policies
z use of dry composting toilets
Water Resources Management (cont’d)
„ Use of other sources in extreme circumstances:
z desalination (main supply, drinking water only)
z importation (barges, pipelines, bottles)
z use of treated wastewater for some uses (e.g. irrigation)
z use of seawater for some uses (e.g. flushing water, fire fighting)
„ Special measures:
z coastal protection
z land reclamation to protect/increase the area available for
groundwater (freshwater lens) occurrence
Possible land reclamation in some islands
Potential to
‘build’ islands
which will form
freshwater
lenses over
time (e.g.
Tarawa atoll)
Further information on island water resources