Groundwater of the Jakarta Basin aquifer is heavily exploited for drinking water. As a result, the piezometric head has dropped dramatically. Extensive hydrogeologic and numeric model studies have been made to find a reliable basis for managing available groundwater resources. Environmental carbon (14C, 13C) and other isotope analyses (180, 2H, 3H) were made. Two sampling strategies were employed, which show that using well-defined and representative sampling sites, no matter how few, is the only way to obtain reliable geoscientific information. Large quantities of data from randomly distributed samples of uncertain origin is not recommended.