Seven herbicides (2,4-D, 2,4-D methyl ester, bromacil, dicamba, diuron, imazaquin, and sulfometuron), four insecticides (carbaryl, diazinon, imidacloprid, and malathion), two fungicides (metalaxyl and myclobutanil), and caffeine (an indicator of wastewater) were detected in at least one sample from the Kisco River at concentrations above 0.1 ug/L (micrograms per liter). Four of these compounds - 2,4-D, 2,4-D methyl ester, dicamba, and metalaxyl - were detected in at least one sample from the Kisco River at a concentration above 1 ug/L. Only three herbicides (2,4-D, imazethapyr, and prometon) and caffeine were detected at concentrations above 0.1 ug/L in one or more of the Middle Branch Croton River samples, and no compounds were detected above 0.4 ug/L in Middle Branch Croton River samples. No samples contained concentrations of pesticides that exceeded human health-based water-quality standards. However, samples from the Kisco River contained four insecticides (carbaryl, chlorpyrifos, diazinon, and malathion) and one herbicide (2,4-D) in concentrations that exceeded water-quality criteria for the protection of aquatic life. Aquatic-life protection criteria were generally exceeded only in stormflow samples collected in June, September, and December 2000. No samples from the Middle Branch Croton River contained target compounds that exceeded water-quality criteria for the protection of aquatic life.
Pesticide concentrations were generally higher, and the numbers of compounds generally larger in samples from the Kisco River than in samples from the Middle Branch Croton River, probably because the Kisco River watershed has a greater population density and is more extensively developed. The highest concentrations of most compounds in both streams were detected in stormflow samples collected in June, September, and December 2000. This indicates that stormflow sampling is essential in assessments of pesticide occurrence in streams that drain developed lands. The lowest concentrations of most compounds at both sites were detected in baseflow samples collected from October 2000 through February 2001, although the concentrations of several compounds increased substantially during stormflows at the Kisco River site in November and December, 2000.
