Groundwater Training Course SOPAC, April Island Groundwater Resources

Transcription

1 Groundwater Training Course SOPAC, April 2005 Island Groundwater Resources

2 Outline of Presentation Freshwater resources of islands Water use Factors affecting freshwater resources Water resource management measures

3 Freshwater Resources Naturally occurring: Surface Water Groundwater Rainwater Other approaches: Desalination Importation of Islands Use of seawater or brackish water for some uses Use of coconuts for drinking (in emergencies)

4 Surface Water Resources RIVERS & STREAMS surface subterranean (in karstic formations) SPRINGS on island at coastline submarine LAKES & SWAMPS fresh brackish

5 Groundwater Resources PERCHED AQUIFERS (High Islands) HORIZONTAL VERTICAL (Dyke-confined) BASAL AQUIFERS (High & Low Islands) COASTAL AQUIFERS FRESHWATER LENSES

6 Groundwater Resources (HIGH ISLAND example) Example from the Hawaiian Islands showing 3 types of groundwater aquifers * perched (horizontal) * perched (vertical) * basal

7 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 )

8 Groundwater Resources (Coastal Zone Conditions in some HIGH ISLANDS) Basal aquifer - note THIN transition zone

9 Summary data for Pacific Island Countries

10 Water uses Water uses Water supply (most important use) Tourism (selected islands) Mining (selected islands) Irrigated agriculture (limited) Hydropower (limited)

11 Summary water use data for Pacific Island Countries

12 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

13 Island Area Definitions LARGE SMALL VERY SMALL 'TINY AREA > 2,000 km 2, or WIDTH > 10 km # AREA < 2,000 km 2, or WIDTH < 10 km # AREA < 100 km2, or WIDTH < 3 km AREA < 20 km 2, or WIDTH < 1 km # UNESCO (1991), Hydrology and Water Resources of Small Islands: a practical guide

14 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

15 Small islands- Limited Resources Betio, Tarawa atoll, Kiribati 15,000 people on 0.8 km 2 of land Many have. Small land area Large population or other demands Malé island (capital of the Maldives) 70,000 people on 1.3 km 2 of land Very limited or depleted water resources

16 Topography High Island: Groundwater & Surface Water Resources Low Island: Groundwater Resources only

17 AFFECTED BY: Local Climate - 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)

18 Mean Annual Rainfall in the Pacific Ocean High rainfall zone in south-west; Low rainfall zone in equatorial east (average conditions)

19 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

20 Annual Rainfall Variability, Betio,, Tarawa, Kiribati 5,000 4,000 Annual Rainfall ( ) Mean = 1,876 mm Cv = 0.46 Annual Rainfall ( ) Mean = 2,181 mm Cv = 0.51 Annual rainall (mm) 3,000 2,000 1, 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

21 Local Climate: Effects of Topography on Rainfall Different orographic effects for islands of varying heights A. Very high B. High C. Low

22 Local Climate: Effects of Topography on Rainfall Rarotonga Cook Islands Annual Rainfall pattern (High rainfall gradient)

23 Geology MAIN TYPES: Volcanic Limestone Coral atoll and cay Mixed (most common) ALSO: Bedrock Unconsolidated (Sand, Mud)

24 Geology Examples of Volcanic, Limestone and Coral Islands

25 Geology Evolution of volcanic island... to mixed volcanic and coral island.. to coral atoll

26 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

27 Mangaia Raised limestone rim Central volcanic area

28 Island types for Pacific Island Countries

29 Key features:- Hydrogeology - location and types of aquifers (basal, perched) - permeability - porosity / specific yield - major structural features (e.g. fractures, dykes, karst, unconformities)

30 Hydrogeology of a Typical Coral Island Increasing permeability with depth Fine lagoon sand/silt Coarse rubble Unconformity > Increasing permeability >

31 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)

32 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

33 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

34 Water Resources Management (cont d) Effective use of available groundwater resources: adequate water resources assessment, monitoring and analysis to ensure sustainability of extractions use of different classes of water (e.g. rainwater for drinking, cooking; groundwater for other uses) use of appropriate technology - especially in particularly fragile areas (e.g. infiltration galleries for water supply)

35 Groundwater Pumping (on small coral islands and coastal zones of high islands) Dug wells Boreholes Infiltration galleries

36 Solar Pumping (at infiltration gallery, Lifuka,, Tonga)

37 Water Resources Management (cont d) Protection of existing groundwater resources: groundwater protection zones or reserves controls over groundwater pumping and surface water extraction control over human settlement (e.g. edge of islands for houses, centre for water and playing fields) appropriate sanitation & solid waste disposal systems

38 Water Resources Management (cont d) Demand management measures: leakage control consumer education & awareness low water use fittings & appliances metering & charging for water on basis of usage drought management policies use of dry composting toilets

39 Water Resources Management (cont d) Use of other sources in extreme circumstances: desalination (main supply, drinking water only) importation (barges, pipelines, bottles) use of treated wastewater for some uses (e.g. irrigation) use of seawater for some uses (e.g. flushing water, fire fighting) Special measures: coastal protection land reclamation to protect/increase the area available for groundwater (freshwater lens) occurrence

40 Possible land reclamation in some islands Potential to build islands which will form freshwater lenses over time (e.g. Tarawa atoll)

41 Further information on island water resources