Analyses of pesticide distribution among surface-water samples from the Hudson River basin indicate that the concentrations of the herbicides atrazine, metolachlor, and cyanazine in streams are related to land use, time of application, and rainfall-runoff events thereafter. These three herbicides are used primarily for weed control in agricultural settings and are among the most widely used pesticides in the Hudson River basin. In a synoptic study, water samples were collected during base-flow conditions at 44 stream sites during May and June 1994. More than half (25) of these sites are classified as agricultural sites and represent watersheds that are more than 20 percent agricultural land; the remaining 19 sites represent watersheds with a variety of land use. Samples were also collected for pesticide analysis at two agricultural sites - the Mohawk River at Cohoes, N. Y. and Canajoharie Creek near Canajoharie, N. Y., weekly from May through early July and monthly from August through September. Canajoharie Creek, a tributary to the Mohawk River, drains a small watershed of 150 km2, of which more than 60 percent is agricultural land. The Mohawk River at Cohoes drains a large watershed of more than 8,900 km2, about 34 percent of which is agricultural land.
Results of the synoptic study indicate that herbicide detection is significantly correlated with land use and quantity applied. Metolachlor and atrazine were detected in 64 percent and 60 percent, respectively, of the samples of runoff from agricultural sites and in less than 11 percent of the samples of runoff from non agricultural sites. The detection limit for metolachlor is 0.009 uugg/L and for atrazine is 0.017 ug/L. Most of the runoff samples with detectable atrazine concentrations were collected from watersheds in central parts of the Mohawk Valley and in tributaries to Schoharie Creek, in areas with the highest herbicide-application rates in the Hudson River basin. Desethylatrazine (detection limit 0.005 ug/L), a degradation product of atrazine, and cyanazine (detection limit 0.013 ug/L) were detected in 48 percent and 24 percent of the agricultural watersheds, respectively, but were not detected in non agricultural watersheds.
Analyses of samples from Canajohaire Creek and the Mohawk River indicate that herbicide concentrations are strongly related to the timing of application and stormflows thereafter. Farmers in the Mohawk River basin generally apply atrazine between mid-May and mid-June. Before June, atrazine concentrations at the Canajoharie Creek site were less than 0.04 ug/L; from mid-June through July, they exceeded 0.3 ug/L. The concentrations increased after runoff producing storms in June and July; the stream-water sample with the highest concentration (4.3 ug/L) was obtained in early July during the a large stormflow and represents the highest instantaneous discharge of any sample collected during these months. Atrazine concentrations in streamwater at the Mohawk River site at Cohoes were generally lower than those in Canajoharie Creek and exceeded 0.04 ug/L only from late June through August. The peak atrazine concentration (0.37 ug/L) at the Cohoes site was reported in early July after a large stormflow.
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