Twenty five pesticides or pesticide metabolites were detected in samples collected from May, 1998 through January, 1999 in three small public-supply reservoirs in western New York. Samples were collected at tributaries and reservoir outlets for comparison with samples from the water-supply intakes. No samples from public-water-supply intakes exceeded any Federal or State water-quality standards, although some samples from tributaries did exceed a few standards. The maximum concentrations of the most frequently detected pesticides in water-supply intake samples were between 10 and 50 percent of the lowest applicable water quality standard. Pesticides that exceeded water-quality standards at the tributary sites were the herbicides atrazine, alachlor, and cyanazine, and the insecticide p,p’-DDE. Land use in the watersheds that surround these reservoirs is largely agricultural; thus, the results do not necessarily represent conditions in other water-supply reservoirs in New York State.

The most frequently detected pesticides or pesticide metabolites were the corn herbicides atrazine and metolachlor, and two metabolites of metolachlor - metolachlor ethanesulfonic acid (ESA) and metolachlor oxanilic acid (OA). More than half of the samples from the three water-supply intake sites contained at least one of these compounds at concentrations greater than 0.2 µg/L (micrograms per liter); the concentrations ranged from 0.01 to nearly 10 µg/L. Many samples contained metabolites of other commonly used herbicides at concentrations greater than those of their parent compounds. Only two insecticides or insecticide metabolites were detected (carbofuran and p,p’-DDE and concentrations of these compounds were less than 0.1 µg/L.

The total concentration of pesticides and metabolites at the three water-supply intake sites are correlated with land use. The highest concentrations were in the watershed with the greatest percentage of row-crop land use,and the lowest concentrations were in the watershed with the smallest amount of row-crop land use. Total median concentration of all pesticides and metabolites at the water-supply- intake sites (median sum of detected pesticide and metabolite concentrations in all samples) was 10.0 µg/L for the LeRoy intake (25 percent row-crop land use), 2.99 µg/L for the Perry intake (21 percent row-crop land use), and 1.28 µg/L for the Hornell intake (12 percent row-crop land use). The decline in concentrations and numbers of pesticides and metabolites detected after the first storm runoff following pesticide application (the spring flush) was generally much greater in the tributaries than at the water-supply intakes, particularly for the LeRoy site. The lower variability at the intake sites is attributed to storage within the reservoir and may also be the result of sustained release of pesticides and metabolites from ground water that discharges to the reservoirs.