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Little Topashaw Creek - Seasonal Variations of Pesticides - Results
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Laboratory analysis of storm event samples


Water collection site showing pumping sampler hose and in-situ logger



Primary agricultural land use in the near drainage contributing area upstream of the sampled reach includes cotton (Gossypium sp.), sweet potato (Ipomoea batatas), corn (Zea sp.), sorghum (Sorghum pp.), wheat (Triticum sp.), and soybeans (Glycine sp.).


During the sampling period, 1176 storm water samples were collected from 58 high flow storm events. There were 2038 pesticide detections greater than or equal to 0.1 mg L-1, equivalent to 10.2% of the possible 19,992 detections. On a quarter year basis, detections per storm event were highest for the second quarter of 2001 (with 111), followed by the fourth quarter of 2001 (with 81). Of the 58 storm events, 22 (38%) occurred in the second quarter, followed by 14 (24%), 13 (22%), and 9 (16%) in the first, third, and fourth quarters, respectively. Overall, most detections were for p,p'-DDT (542) and metabolites (129), totaling 671, followed by methylparathion (366), atrazine (277), and metolachlor (198).


Of the herbicides detected, atrazine and metolachlor had the highest mean concentrations in high flow storm runoff in the second quarter of 2001 with values of 2.50?7.53 and 1.28?3.13 ?g L-1, respectively. Among the current-use insecticides in high flow storm water in the second quarter of 2001, bifenthrin and 8-cyhalothrin had the highest mean concentrations with values of 0.70?2.88 and 0.40?0.52 ?g L-1, respectively. However, the most frequently detected current-use insecticides were methylparathion and chlorfenapyr, with 53 and 49 detections, respectively, followed closely by bifenthrin with 41 detections. The annual seasonal "spring flush" in many US stream and river systems likely accounts for these phenomena.