From the 195O's to the early 1970's expansion of sanitary sewerage in southwest Nassau County contributed to progressive declines in ground-water levels. Since the early 197O's, however, 10 years after the area was fully sewered, water levels have not declined significantly, which suggests that the water table may have reached a new equilibrium position. Double-mass-curve analyses show that during 1953-76 the average weighted ground-water levels in a 32-square-mile (83-square-kilometer) part of the sewered area declined 12.2 feet (3.73 meters) more than those in the unsewered area to the east. However, by 1973 this decline was 13.5 feet (4.1 meters). Finite-difference digital-model results indicate that 3.6 feet (1.1 meters) of the relative 1953-76 decline was due to pumping in adjacent Queens County and that most of the remaining decline was a result of sewerage. Streamflow within the sewered area decreased in response to the lowered ground-water levels, and ground-water levels in the adjacent unsewered area were also lowered because of the sewerage.
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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