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HOGA ISLAND: GROUNDWATER RESOURCE PROJECT
Background
Hoga Island has been identified as a suitable location for future (eco)tourism developments, particularly with respect to its marine environment. Local people are in favour of increasing tourism on account of the additional income that visitors would bring to the local economy. To avoid uncontrolled and unsustainable expansion of tourist facilities, Hoga is subject to proposals that any tourist activities or new developments be licenced on an individual basis subject to acceptable environmental impact assessments being completed. Environmental constraints will limit the number of people that can be accommodated on the island without causing adverse environmental – and possibly economic – impacts. The availability of fresh water is one of the environmental factors that could define this upper limit on the island’s population. The main source of fresh water on Hoga is groundwater.
Groundwater is a fragile resource in continental
land areas. On small islands, and particularly carbonate islands, the
groundwater resource is at even greater risk of being made unusable. Three
factors affect the availability of island groundwater:
(i) the limited quantity of stored fresh water;
(ii) the extent of saline intrusion;
(iii) the risk of contamination by pollution.
This project aimed to establish the nature of the groundwater aquifer on Hoga island in terms of the hydrogeological controls and constraints. This would enable an assessment of the implications of present and future abstractions of water from the aquifer in terms of (a) increased demand for water by increased tourist populations and (b) how the abstraction is managed. It was anticipated that a number of additional related issues would come to light which would require further detailed investigations.
Methodology
The nature of the research question and the absence of any prior information relating to Hoga Island required the adoption of a research strategy which comprised a number of different component investigations, each requiring a particular technique. The most fundamental requirement for an investigation of this type is a map of the study area. No detailed map of Hoga was known to exist, so I had to make one. This was done using a Garmin hand-held GPS unit, providing a spatial accuracy of around 5 m, to register the coordinates of points around the coastline of the island. The coastline defined by the ‘High Water Mark’ was mapped either from a boat 5 m offshore from rocky cliffs, or by walking along the upper edges of the sandy beaches. Additional observations and notes made during the survey provided further relevant information.
Reconnaissance surveys of the interior of the island were then required, in order to provide an indication of the nature of the terrain and the geology and to facilitate the planning of subsequent detailed investigations. These surveys simply involved walking through the interior of the island with a local guide, recording any relevant details of the ground conditions that were encountered and logging their positions using the GPS. Similar but more detailed ‘unstructured’ surveys were undertaken in the vicinity of the Operation Wallacea base where an extensive network of karst features characterise the ground surface.
An additional component of these preliminary surveys involved locating and identifying freshwater within the ground throughout the island. Electrical conductivity of the water, which increases as the total dissolved salts in the water increase, was used to distinguish between seawater (off the scale at >1000 mS) and fresh or nearly fresh water (typically <20 mS). A bucket was used to extract water from all of the island’s wells and any natural holes where water was found, and a digital conductivity meter with an electrolytic probe was used to test the water in each case. All locations where fresh water was found were logged using the GPS and the conductivity readings were recorded.
Further information concerning the likely nature and extent of the aquifer required more specific survey data. A natural hole in the rock near the centre of the island was found to contain fresh water at 3 m depth. This location should in theory be indicative of the maximum elevation of the groundwater table within the island, so the actual elevation of the surface of the water had to be determined. This was done by conducting a ‘level survey’ from ‘High Water Mark’ on the beach on the west coast, along the shortest possible line to the central hole. This also provided a value for a possible maximum elevation for the island, although both this and the water table elevation need to be related to mean sea level rather than ‘High Water Mark’: this correction will be made later when the relevant tidal measurements have been obtained from another scientific team. An additional topographic survey was also attempted, crossing the island at the latitude of the central hole. The purpose of this was to characterise the cross-sectional geometry of the rock mass containing the aquifer and thus identify any additional constraints on the possible extent of the aquifer.
Current and future demands on the aquifer were assessed by conducting a survey of abstraction from the well near the Operation Wallacea base, from the first collections of water at 03:30 until 17:00, with an estimate being made of abstractions during the final hour of daylight (the well is not used in the evenings). The volume of water abstracted was combined with the known imports of fresh water from Kaledupa and this total was used to further estimate the abstraction of water from the two wells belonging to the village at the north end of the island. From this a theoretical per capita demand curve for fresh water can be derived.
Results
Hoga Island comprises a roughly triangular landmass up to 1.8 km long and wide, with a narrow peninsula extending out from this some 1.6 km towards the southeast. The total area of the island is 3.42 sq. km. It is formed from coral limestone and has a maximum elevation above mean sea level of around 5 m. 43% of the 12.6 km coastline comprises sandy beaches, the remainder being characterised by overhanging cliffs around 1 m high above ‘High Water Mark’. The eastern half of the island seems to be more highly karstified than the western half, with more caves and holes dissecting the ground surface. Most of the holes on the eastern side were dry, although their elevations are not known. There are generally fewer holes or other dissolution features in the western half of the island, although some do occur. The entire island displays a structural feature characterised by linear fractures trending 160-340 degrees. Where cliffs define the coastline there are extensive shallow caves extending inland. Similar caves occur in the interior of the island, probably originating during a period of higher relative sea level.
The aquifer water table in the natural hole in the centre of the island was found to be 7 cm above ‘High Water Mark’, which is estimated to be equivalent to around 80 cm above mean sea level. According to the Ghyben-Herzberg principle, which describes the effects of the density difference between seawater and fresh water, this suggests that the fresh water aquifer may be around 30 m thick in the centre of the island. This is very large compared with other, larger carbonate islands in other parts of the tropics, and suggests that there may be little karstic dissolution of the limestone below the present sea level. The lower permeability associated with diffuse flow through the lower rocks would limit natural drainage from the aquifer to the sea.
In the absence of rainfall data for this part of southeast Sulawesi, a conservative minimum of 1500 mm p.a. can be used to form the basis of any water resource calculations. Up to 30% of this might be expected to reach the aquifer, i.e. around 500 mm p.a.. If the aquifer is estimated to have an area of only 2.25 sq. km (allowing for possible loss of capacity around the eastern and southern margins due to marine penetration), this would equate to an annual input of just over 1.1 million cubic metres of rainwater into the aquifer. In an undisturbed system, this input will be balanced by natural drainage of water from the aquifer into the sea by diffuse flow through the rock, uptake of water directly from the aquifer by penetrating tree roots and any other natural losses (there is a possibility that fresh water discharges periodically from the reef wall offshore).
Present water demands on Hoga are estimated to be around 50 l per person per day, on the basis of actual abstractions from the existing Operation Wallacea well and the daily import of drinking water from Kaledupa for an estimated population in the southwest of the island of 150 people. If this consumption rate is applied to the estimated population of 80 in the village at the north end of the island (which is probably unrealistically high), then the total freshwater demand for the island at present is around 11,500 l/day, or just over 4000 cubic metres p.a.. This represents 0.4% of the annual water budget of the aquifer, and is sufficiently low that the total present demand and a significantly increased demand could be met from the aquifer, provided the resource is managed appropriately.
These preliminary results make no allowance for the seasonality of rainfall and of (eco)tourist visitors at present, nor do they incorporate other potentially limiting factors such as the incidence of drought and the unknown actual extent of the aquifer. These issues will be considered at greater length in a final report which will take the form of a scientific publication.
Final report
It is anticipated that a paper entitled Groundwater in a small tropical carbonate island: estimating a sustainable limit to tourism development in terms of freshwater abstraction will be completed and submitted to an appropriate journal for publication by the end of March 2002 by Dr Alan Dykes, University of Huddersfield.