Bambang Nurbianto, Jakarta – Due to a greatly increased amount of construction and over-exploitation of groundwater, the city's land has subsided up to 91 centimeters in recent years, a city official said on Tuesday.
Speaking at a hearing with the City Council's Commission C for economic affairs, head of the city mining agency, Muzahiem Mochtar, said that based on a study, the land subsidence in various parts of Jakarta during the 1993 to 1999 period ranged from four centimeters to 91 centimeters, depending on each location.
The 4-centimeter drop occurred on Jl. Lubang Buaya in East Jakarta, while the 91-centimeter sinkage occurred on Jl. Sunter Kemayoran in North Jakarta.
Other significant drops occurred on Jl. Baru Arah Sunter (76.1 cm), Jl. Mundu Koja Utara (52 cm), Jl. Gadang Tanjung Priok (42 cm), and Jl. Kampung Malaka III (27 cm), which are all in North Jakarta.
He said the continuing process of subsidence was also a serious factor that contributed to the worsening of annual floods in the city.
"The continuing subsidence of land here could worsen the environmental problems, including the possibility that the annual floods in the city will worsen in the future," he said. Muzahiem said the impact of groundwater exploitation on land subsidence accounted for about 19 percent, while the weight of buildings on the land was responsible for about 80 percent, and one percent was due to natural processes.
"Although the impact of the groundwater exploitation on land subsidence is only 19 percent, it should be stopped. Otherwise, it will significantly lower the ground surface (in relation to sea-level)," he added.
"Exploitation of groundwater should be minimized to a sustainable level. If groundwater is over-exploited, not only the groundwater will disappear, but it will also hasten land subsidence," he reiterated.
He said that only 7.5 percent of groundwater use was regulated by his office, while the rest was still unregulated.
He pointed out that 44 percent of the potable water needed in Jakarta was supplied by the city-owned company PAM Jaya, which uses river water.
He said that there were several ways to curb the use of groundwater. He suggested that taxes for groundwater use should be increased. "That way, we could discourage companies from wasting groundwater," he said.
He also suggested that PAM Jaya should be allowed to expand its coverage because it could guarantee the water supply for the whole city.
Haris Pindratno of the Indonesian Geological Expert Association said the 2010 city masterplan needed to be revised as the masterplan did not consider geological factors.
"Because there is no geological consideration, the capability of land to support the buildings was never taken into account. And based on the study it is proven that the weight of buildings significantly contributes to subsidence," he added.