by: Soetrisno S.

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GEOGRAPHICAL BACKGROUND

Indonesia is the fifth populated country in the world (147 million --census 1980). It extends from latitude 6^o N to 11^o S over a distance of about 2,000 km and from longitude 95^o E to 141^o E over a distance of approximately 5,000 km, covering a total land area of more than 2 million km².

Indonesia consist of more than 13,000 islands, which can mostly be classified as small islands. The islands are divided into three major groups. The most important is the Greater Sunda Complex, which includes the islands of Sumatra, Java, Kalimantan and Sulawesi. The other two groups are the lesser Sundas (Nusa Tenggara), which stretch from Bali to Timor, and the Mollucas, while Irian Jaya form the easternmost part of the Indonesian archipelago.

In general, the islands are built up by highland cores, which stretch towards the sea into more or less wide coastal plains with thick alluvial deposits.

Indonesia has tropical climate which is greatly influenced by mountains and the sea. The annual rainfall is generally more than 2,000 mm. The lowlands are characterized by a high humidity (daily variation from 65 % to 95 %) and moderately temperatures (ranging from 20^o to 30^o C). While most of Indonesia receives adequate rain all the year, southern Sulawesi, Eastern Java and Nusa Tenggara islands have a pronounced dry season, because of the monsoon blowing from Australia. However, it must be noted that rainfall varies according to local conditions.

HYDROGEOLOGIC INVESTIGATIONS

Hydrogeologic investigations of small islands in Indonesia are not yet done extensively. Since eastern part of Indonesia is relatively dry area and often suffer from drought, tje water supply is agreat problem in this area. Thus to solve the problem. groundwater investigation in the eastern part is the first priority and urgently needed.

In small islands, the investigations are mostly hydrogeologic mapping, as a first step towards evaluating and developing the groundwater resources.

Since 1961, the hydrogeologic mapping in small islands, mostly eastern Indonesia, has been done by GSI (geological Survey of Indonesia) in cooperation with West German Bundesanstalt fuer Bodenforschung (BfB). Later on following Indonesia's First Five Year Development Plan in 1969, GSI itself carried out hydrogeologic mapping. Howeve, up to 1984 only a small number of small islands has been covered by mapping. Those are the islands of We and Madura in western Indonesia, the islands of Lombok, Selayar and Solor, and south Maluku archipelago in eastern Indonesia.

GROUNDWATER RESOURCES

The groundwater resources of small islands in Indonesia are greatly affected by the topography, rainfall and geologic conditions.

In the western part, since the rainfall is high and litohologic conditions give relatively good permeability, the groundwater resources of small islands are moderate to high. Only in the Riau archipelago and Bilitung tin island, which are covered by igneous rocks, are the groundwater resources rather poor. However, in most parts of the small islands in western Indonesia, the water supply for domestic use comes from surface water.

The island of We is located on the western border of Indonesia, with a land area of approximately 150 km². This island receives annual rainfall of about 2,100 mm.

Geologically, the island of We is covered mostly by volcanic rocks, and there is ony a small extent of alluvial deposits in the coastal plains of Sabang bay. The aquifers' productivity is classified low to moderate, Springs emerge with various, but generally small, discharges.

From the hydrogeological mapping carried out by GSI and Bfb, some major hydrogeological features of Madura island are known. Its land area is about 4,500km² (population 2,656,803) and its annual rainfall is relatively small at 1,000 - 2,000 mm.

Madura island is mostly covered by karstic limestone of considerable groundwater potential. Some springs with relatively big discharges are often found. Alluvial plains have a limited extent along the southern coast. Here the shallow groundwater is affected by sea water intrusion. In the eastern part, the groundwater resources in several areas are considerably low. However,since the surface water is not adequate for domestic water supply owing to low rainfall, the groundwater resources are most promising. Here, according to Notomihardjo and others (1979), the rate of groundwater recharge was estimated at from the low 0.4 l/se/km² in East Nusa Tenggara to 2.5 l/sec/km² in Maluku.

Lombok island, 4,600km², has annual rainfall ranging from 700 mm in the eastern part to 2,000 mm in the western part.

The groundwater resources of Lombok island have received more attention than those of most other islands in Nusa Tenggara. A reconnaissance hydrogeological map of the island prepared by GSI and BfB in 1972 ia available. The major groundwater areas are shown in this 1 : 400,000 scale map. According to this report, the best groundawter resources are the west coast of Mataram and coastal plains on the north, west and east sides of the island. Groundwater resources are limited on the entire one-third of the south end of the island. This also is where the surface water is in short supply.

Hydrogeological mapping of Solor island has been done by Directorate of Environmental Geology (DEG), the successor of GSI, in the context of its groundwater development. Solor island is a dry island, has a total land area of approximately 400 km², and receives annual rainfall ranging from 270 mm up to 1,100 mm. Mostly covered by young and old volcanic products, Solor island has considerably low groundwater potential. However, some part of this island is expected to be a promising groundwater development, particularly at the foot of the volcano. Spring discharge is generally small, less than 5 l/sec.

The hydrogeological mapping of Selayar island was carried out by the DEG, in 1983. It is the first step towards evaluation of the groundwater resources of this island.

Selayar island with a total land area of 903 km², is relatively dry with ana annual rainfall of less than 1,500 mm. Owing to geologic conditions (Selayar island is mostly covered by consolidated rocks) and low rainfall, the aquifers' productivity is low to moderate. However, since the northern part is covered by karstic limestone, this area is expected to be most promising for developing groundwater resources for rural water supply.

The small islands in South Maluku, distributed within the Banda Sea and the Arafura Sea, are buit up mostly of volcanic, metamorphic and calcareous rocks. In some volcanic islands, such as islands of Banda, Taren, Romang and Damar, shallow fresh groundwater occurs along their coastal plains. Accumulation of sufficient groundwater may be expected where relatively karstic limestone crops out. Big spring discharge from limestone (400 l/sec) is one of the examples.

GROUNDWATER UTILIZATION

When relatively large quantities of water are required --e.g. for irrigation schemes-- a proper investigation of geology, hydrogeology, recharge, etc. is justified. However, in the small islands of Indonesia, almost all the problems involve water supply for rural or watering stock. The amount of water required is small. The major problem is how to obtain water where it is required, at low cost and with the least possible delay.

Tapping of shallow groundwater through dug wells, and piping springs for rural water supply without treatmnnet, is a common wai in small islands of Indonesia. However, it seems that groundwater use is insufficiently developed, particularly in areas where surface water is relatively plentiful like many small islands in the western part of Indonesia. Very few village people use groundwater for domestic requirements; rural people develop wells where surface water supplies dry up during the dry season.

Groundwater utilization is limited, for the time being, only to domestic uses. However, during the Third Development Plan, in Madura island effort has been made to develop groundwater resources for irrigation by drilling. More than 30 wells have been drilled during this period to irrigate an area of 1,500 ha.

GROUNDWATER DEVELOPMENT PROBLEMS

Some problems have to be faced during the development of groundwater resources of small islands in Indonesia. From the hydrogeological point of view, these problems as follows:

1. The groundwater resources themselves may not be adequate for domestic purposes. This is particularly so in the small islands of the eastern part of indonesia, which are covered by consolidated rocks like small islands spreading along the outer arc system;

2. Salt-water intrusion, especially in the coastal palin. Although wells closer to sea level are expected to produce greater flows, there the possibility of salt-water intrusion should be considered carefully;

3. Anthropogenous pollution; since poor drainage is often found in small islands, particularly in dry season, the pollution of groundwater caused by anthropogenous matter should be considered;

4. There is a lack of geologic maps of small islands, and other thematic maps such as land-use maps, rainfall maps, even topographic maps on favorable scales.

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Reference: 

1. Anonymous, 1980, Groundwater in Indonesia, ESCAP, Committee on Natural Resources, Seventh Session, Bangkok.

2. Meinardi, C.R., 1981, Water Demand and Water Resources of Small Islands; A Literature Review, National Institute for Water Supply, Leidschendam.

3. Mukna, H.S., 1984, Peta Hidrogeologi Indonesia 1 : 250.000 Lembar P. Selayar, Direktorat Geologi Tata Lingkungan, Bandung.

4. Notodihardjo, M., 1979, Water Resources Planning in Indonesia, Ministry of Public Works, Jakarta.

5. Polyakas, K., Livingston, E., and Lubis, Y.M., 1976, Water resources in East Indonesia, Ministry of Public Works, Jakarta.

6.Soekardi, P., and Soetrisno S., 1983, Hydrogeological Map of Indonesia, scale 1 : 2,500,000, Directorate of Environmental Geology, Bandung.

7. Sukrisno dan Soejoto, L., 1983, Prospek Pengembangan Airtanah P. Solor, Direktorat Geologi Tata Lingkungan, Bandung.

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