Surface-water and ground-water quality in the East Meadow Brook headwaters area was monitored during 1988-93 to determine the effects of urban stormwater on water quality before, and for 1 year after, the construction of a stormwater-detention basin in 1992. Stormwater samples were collected from the stream during storms. Between storms, water samples were collected from the stream and from a network of monitoring wells in the headwaters area. The detention basin was constructed as part of a pilot project to increase aquifer recharge while decreasing the discharge of contaminated stormwater to coastal waters.

Bacteria and road salt were the major contaminants detected in stormwater samples, and the concentrations of organic compounds and nutrients in the samples rarely exceeded New York State drinking-water standards. Lead and chromium were detected in only a few of the stormwater samples, and cadmium was not detected in any of the samples.

Loads of most inorganic constituents in stormwater reflected the season and the magnitude of the storm and were proportional to the total stormwater volume measured at the headwaters area. Stormwater during the nonwinter (non-road-salting) season had a diluting effect on shallow ground water adjacent to the stream.

Large amounts of sodium and chloride that entered the stream and ground water after road-salt applications to the Westbury drainage area affected the ground-water quality beneath and adjacent to the stream for several months. Concentrations of sodium and chloride in streamwater on March 6, 1989, reached 1,700 mg/L (milligrams per liter) and 2,700 mg/L, respectively, as a result of road salt washed in by stormwater. Median concentrations of sodium and chloride in wells in an area affected by road salt were generally several times higher than concentrations in shallow wells in unaffected suburban areas. Bromide-to-chloride ratios were used to distinguish road salt from atmospherically derived sea salt within the shallow aquifer and indicated that ground water was affected by road salt to a depth of 14 meters.