TO ANTICIPATE IMPACTS OF RECLAMATION OF JAKARTA BAY

- A groundwater conservation's perspective - ^*)

by : Soetrisno S., Satriyo H., and Haryadi T. ^**)

2.Groundwater Setting and Present Situation of Water Supply

Greater Jakarta is located in the groundwater basin which is so called the Jakarta Groundwater Basin.

The bottom of the basin is formed by impermeable Miocene sediments which also crop out at the southern boundary of the system. The basin fill which consist of marine Pliocene and Quarternary sand and delta sediments is between 0 and more than 300 m thick. The thickness of single sandy aquifer layers intercalated with a predominantly silty/clayey sequence is only between 1 and 5 m and is only 20% of the total fill. Fine sand and silt is very frequence component of 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 Quarternary sequence of 250 m thickness are some 250 m² day^-1 near the coast and increase 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 Quarternary aquifer may generally be 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).

Confined aquifers may extend to Jakarta bay and Seribu Archipelago as proved by drilling in some islands (Untung Jawa, Pramuka) of archipelago and marine geology research in Java Sea (Sarmili at.al, 1996).

Under natural flow conditions the recharge area of the deep aquifer system was situated 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) below the water table of the unconfined shallow aquifer system.

Piezometric head in North Jakarta changed from 12.5 m above sea level in 1910 to about sea level in 70’s, then lowered drastically to 30 - 50 m below sea level in 90’s.

Groundwater quality in general was very good at the beginning of this century but in the top layers of 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 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 predicted 750 million m³year^-1. Although the volumes of groundwater abstracted are small in comparison to those of surface water, at present it is of considerable economic and social importance, because about 70% of the population in Greater Jakarta and the majority of the industries in Jabotabek rely on this resources.

Water demand of Greater Jakarta for various utilities, based on the recent information, is fulfilled from surface water after being treated by Jakarta Municipal Water Supply (PAM Jaya), or directly treated by people from groundwater which being abstracted through deep wells, driven wells, dug wells, and from spring water coming from West Java. At the end of 1994 PAM Jaya has supplied to 337.000 consumer, about 20% of total Jakarta population (Kirmanto, 1995). Maximum production capacity was 18.065 litre/second.

Report of Jabotabek Water Resources Management Study (JWRMS), 1994, noted that on 1990 water demand for northern part of Jakarta was 11.54 m³ sec^-1 being provided by means of piping system 2.88 m³ sec^-1 meanwhile the rest of demand about 8.65 m³ sec^-1 was still dependent on groundwater.

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 1995, deep groundwater abstraction was estimated 54 million m³ year^-1 which was about 50% higher than that of registered wells (32.2 million m³ year^-1).

The highly demand of groundwater usage has led to lowering piezometric head and increasing salinization as mentioned above, and presumably the main cause of land-subsidence in several locations in North Jakarta. Rate of subsidence was highest recorded of 34 cm year^-1 from 1979 - 1991 years of observation.

Referring to the present usage of groundwater and its impacts, therefore, to meet the demand of water needs in reclaimed land in the future must not be relied on groundwater resources anymore.

As the impacts of groundwater abstraction can be predicted, soon in 1985 Directorate of Environmental Geology (DEG) and Federal Institute of Geosciences and Natural Resources, Germany, proposed scenarios of optimisation of groundwater abstraction in Jakarta basin, based on the result of groundwater modelling.

If the total groundwater abstraction could be limited to 47 m³ year^-1 and in 1995 reduced abstraction in northern part of Jakarta (an area extended to 10 km from shore line) to 10% of total abstraction, then to stop pumping in that area in 2005, hence, starting in the year 1995 piezometric head of groundwater in North Jakarta will recover and become stable at about 5 m below sea level in the year 2010 (Schmidt and Tirtomihardjo, 1986). However, to implement that scenarios was not the simple thing as figured by present usage of groundwater as mentioned above.

Groundwater abstraction in North Jakarta, therefore, has strictly to be limited in order to halt continuous degradation of that resources. For that purpose DEG since 1994, stated that no new groundwater abstraction should be permitted in the area which extend in the north of line stretched from areas of Kebonjeruk in the west, Sudirman in the central, Jatinegara and Pulogadung in the east, as depicted on Groundwater Conservation Map of Jakarta (Tirtomihardjo and Wibowo, 1994).