Before urban development, most ground‐water recharge on Long Island, New York, occurred during the dormant season, when evapotranspiration is low. The use of recharge basins for collection and disposal of urban storm runoff in Nassau County has enabled ground‐water recharge to occur also during the growing season. In contrast, the use of storm sewers to route storm runoff to streams and coastal waters has resulted in a decrease in ground‐water recharge during the dormant season. The net result of these two forms of urban storm‐runoff control has been an increase in annual recharge of about 12 percent in areas served by recharge basins and a decrease of about 10 percent in areas where storm runoff is routed to streams and tidewater. On a countywide basis, annual ground‐water recharge has remained nearly the same as under predevelopment conditions, but its distribution pattern has changed. Redistribution resulted in increased recharge in the eastern and central parts of the county, and decreased recharge in the western and nearshore areas. Model simulation of recharge indicates that the water‐table altitude has increased by as much as 5 ft above predevelopment levels in areas served by recharge basins and declined by as much as 3 feet in areas where stormwater is discharged to streams and tidewater.

A plume of ground water enriched by liquid metal‐plating effluent has formed downgradient from an industrial park in southeast Nassau County, New York. Discharges from the plant to the shallow aquifer began in the 1940's and continue to the present. Core samples of aquifer material from the plume were analyzed by oxalate extraction and dithionite‐citrate‐bicarbonate (DCB) extraction methods for the presence of chromium and cadmium. Results of the extraction indicate that for 1 kilogram of soil, the median concentrations of extracted chromium and cadmium in aquifer material are 7.5 and 1.1 milligrams, respectively, and the maximum concentrations are 19 and 2.3 milligrams, respectively.

Long‐term records of ground‐water quality indicate that in an unsewered area of southeast Nassau County, New York, certain constituent concentrations increased substantially from 1910 to 1975. Most of the increases virtually parallel the rate of population growth and number of houses that discharge waste water through cesspools and septic tanks. Data sites used for this study were two abandoned infiltration galleries in Wantagh and Massapequa that withdraw water from the shallow aquifer. Ranges in constituent concentrations, in milligrams per liter, during 1910–75 were:

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