Ground-water withdrawals from the aquifers underlying Kings and Queens Counties varied temporally and spatially during the 20th century and caused extreme changes in water levels. The resultant lowering of water levels during periods of heavy pumping caused saltwater intrusion in nearshore areas and the migration of contaminants from land surface into deep aquifers. The recovery of water levels in response to countywide curtailment of pumping has resulted in the flooding of underground structures. Combined withdrawals for public and industrial supply in Kings and Queens Counties were greatest during the 1930's--about 130 million gallons per day. During this period, a large cone of depression developed in the water table in Kings County; within this depression, water levels were about 45 feet lower than in 1903. All pumping for public supply was halted in Kings County in 1947, and in Jamaica (in Queens County) in 1974. Water levels in Kings County had recovered by 1974 and have remained similar to those of 1903 since then, except for minor localized drawdowns due to industrial-supply or dewatering withdrawals. A large cone of depression that had formed in southeastern Queens County before 1974 has now (1997) disappeared. The estimated combined withdrawal for public supply and industrial supply in Kings and Queens Counties in 1996 was only about 50 million gallons per day.

The water-level recoveries in the water-table and confined aquifers generally have resulted in the dilution and dispersion of residual salty and nitrate-contaminated ground water. The majority of recently sampled wells indicate stable or decreasing chloride and nitrate concentrations in all aquifers since 1983. Organic contaminants remain in ground water in Kings, Queens, and Nassau Counties, however; the most commonly detected compounds in 1992-96 were tetrachloroethene, trichloroethene, chloroform, and total trihalomethanes. Water samples from monitoring wells in Kings County indicate a greater number of occurrences of these compounds in the upper glacial aquifer than in the Jameco-Magothy aquifer, whereas samples from public-supply wells in Queens County indicated a greater number of occurrences in the Jameco- Magothy aquifer than in the upper glacial aquifer. This distribution suggests that organic contaminants were not drawn into the deeper aquifers in Kings County before 1947, when their use was limited and deep withdrawals were greatest, and (or) that the longer period of waterlevel recovery in Kings County than in Queens has allowed greater degradation, dilution, and dispersion of any organic contaminants that might have entered the deep aquifers before the cessation of pumping in 1947.