3 GROUNDWATER SETTING

3.1 Jakarta

Greater Jakarta is located in the groundwater basin known as the Jakarta Groundwater Basin.

The base of the aquifer system is formed by impermeable Miocene sediments which also crop out at the southern boundary of the basin. The basin fill, which consist of marine Pliocene and Quaternary sand and delta sediments, is up to 300 m thick. Individual sand horizons are typically 1 - 5 m thick and comprise only 20% of the total fill deposits. Silts and clays separate these horizons. Fine sand and silt is very frequent component of these aquifers (Schmidt et al, 1985)

The horizontal permeability (Kh) of tested layers was found to be between 0.1 m day^-1 and 40 m day^-1. Transmissivity (T) for the entire Quaternary sequence of 250 m thickness are some 250 m² day^-1 near the coast but increases to about 500 m²/day near hinge line about 20 km to the south. The vertical permeability (Kv) is estimated as varying between 1/100 and 1/5000 of the horizontal permeability. The Quaternary deposits may be conveniently divided into three aquifer systems on the basis of the hydraulic characteristics and depths; these are: (1) Phreatic Aquifer System (0-40 m), (2) Upper Confined Aquifer System (40-140 m), and (3) Lower Confined Aquifer System (>140 m).

Under natural flow conditions the recharge area of the deep aquifer system is located in the hilly area at elevations of between 25 and 200 m. Discharge from the confined aquifer to the natural base level in the flat coastal area occurred mainly by upward leakage, evapotranpiration and outflow to the surface water system. Today, recharge to the deep aquifer system, other than horizontal inflow, may occur throughout the city area by downward leakage, as head levels of the confined aquifer system have dropped regionally (2 - 4.6 m year^-1) to below the water table of the unconfined shallow aquifer system.

Groundwater quality in general was very good at the beginning of this century although the top aquifer within the coastal plain deposits was slightly brackish to saline. At present, the exploitation of deep groundwater has caused salinization of the deep aquifer, whereas the rapid urbanization and the consequent high population densities within the urban areas have led to the contamination of the shallow aquifer.

The supply of water is a pressing problem in Greater Jakarta. The 1995 total water demand is estimated at 750 million m³year^-1. Although the volumes of groundwater presently abstracted for public supply are small in comparison to those of surface water, groundwater is of considerable economic and social importance, because about 70% of the population in Greater Jakarta and the majority of the industries rely on this resource.

The groundwater contribution to the actual supply is about 250 million m³ year^-1 and is mainly abstracted from innumerable shallow wells (80%) and more than 3,000 deep wells (20%). Between 1900 and 1950, groundwater abstraction was below 10 million m³ year^-1 but since that time, mainly after 1970, it has steadily increase in step with the growth in population and industrial development. In the year 1994, deep groundwater abstraction was estimated to be 53 million m³ year^-1 which was about 50% higher than could be accounted for by registered wells (33.8 million m³ year^-1).

3.2 Bandung

The multi-layer aquifer configuration of the Bandung basin may be simplified into two hydrogeologic systems. The shallow aquifers which are unconfined, and occur within the upper 40 m are commonly exploited by dug wells or driven wells and have a high vulnerability to pollution. These aquifers are composed of both volcanic product, from the volcanic complexes bordering this basin, and from lake sediments which were deposited within the central part of the basin.

The deep aquifers which are semi confined to confined, are present at depths between 40 m and 150 m. The most prominent aquifer is the Cibeureum aquifer which is comprised mainly of arenaceous tuff and volcanic breccias of young volcanic deposits. The permeability of this aquifer is moderate to high within the coarse pyroclastics and within the lava flows, producing an average transmissivity of between 111 and 877 m² day^-1. Due to its high productivity, the Cibeureum aquifer is the most widely utilised aquifer in the basin and is intensively exploited, primarily by textile industries.

The other aquifers are the Cikapundung aquifer, which consists of old volcanic deposits, and the Kosambi aquifer, which is composed of mostly fine grained lake deposits. Both aquifers have low to moderate transmissivities (112 to 150 m² day^-1), and therefore are not so widely exploited. The northern area of the basin which is situated at elevations between 1050 and 1300 m above mean sea level (amsl) is considered to be the main recharge area of the groundwater exploited in the basin; this was based on a study of the natural stable isotope content of groundwater (Geyh 1990). Groundwater through flow from an elevation of 1000 m amsl to the basin is calculated to be about 107 Mm³ year^-1 (Schmidt and Tirtomihardjo 1991), of which 70% comes from the north.

The chemical composition of groundwater is characterised by low to moderate salinity (< 1000 m S cm^-1), high HCO^-₃ concentration (45 - 90 meq%), low concentration of SO^-2₄ (<20 meq%), and high concentrations of Fe (exceeding 1 mg l^-1) in practically all areas of the basin.

Due to excessive abstraction by industries, the current piezometric head of groundwater in the basin has declined markedly. In the early 1900’s the piezometric level in the areas which are now occupied by industries, was usually present between 20 and 25 m above surface, while now it lies generally more than 50 m below the surface. In the area where many textile industries are concentrated, a cone of depression has developed. The piezometric head as recorded by 48 observation wells throughout the basin, has been declining continuously almost over the entire basin at a rate of between 2 - 4 m year^-1.

Lowering of the piezometric heads has changed the groundwater flow system because the piezometric head is now generally below the phreatic head almost everywhere within Greater Bandung. Downward recharge from the upper system to the deep system, therefore, has occured, making the deep aquifer system vulnerable to pollution. However, up to now there is no evidence of pollution in the deep system.

Since the early development of the city of Bandung in the late 19^th century, groundwater resources have played an important role in supplying water demand. Following the industrial era in the 1970’s, Bandung became a great urban area which attracted people from surrounding rural areas. Groundwater abstraction in 1970’s was 10.5 Mm³ year^-1, and the number of wells was less than 300.

By 1995, municipal water supply abstracted 6.7 Mm³ year^-1 of groundwater. Together with spring and surface water, it supplied about 43% of drinking water demand in the Greater Bandung. The remaining water users rely on groundwater resources. In the same period, 66.9 Mm³ year^-1 of groundwater has been abstracted from deeper aquifers, 80% of which is used by the industrial sector. This figure was recorded from 2225 licensed wells, while it was believed that many additional illegal wells tapped the same aquifer. It is estimated that 80 Mm³ year^-1 of groundwater was pumped by the textile industries alone in 1995.

Local domestic supply of groundwater is from dug wells or shallow boreholes. There is no exact figure concerning the total abstraction from the shallow aquifer. However, assuming that 60% of the total population of 3.5 million use 90 l per capita per day of groundwater, it is estimated that 69 Mm³ per annum of groundwater was abstracted from the shallow aquifers in Greater Bandung in 1995.

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