HERBICIDE PERSISTENCE AND MOBILITY IN RECHARGE LAKE WATERSHED IN YORK, NEBRASKA

Elevated levels of herbicides in surface and groundwater are a concern in the cornbelt in the USA. This study was conducted to interpret the herbicide behavior in a watershed system using data collected from ru noff, Recharge Lake, and groundwater influenced by agriculture and lak e seepage. The York Ground Water Recharge Project was constructed on a tributary of Beaver Creek, which drains a 3327-ha watershed of primar ily row-cropped heavily irrigated farmland. The estimated average runo ff is 1.48 x 10(6) m(3) yr(-1) under a precipitation norm of 635 mm yr (-1). Maximum atrazine 2-chloro-4-ethylamino-6-isopropylamino-s-triazi ne) inputs to Recharge Lake occurred in May and June runoff events and resulted in average lake concentrations of 36 and 17 mu g L(-1) in 19 93 and 1994, respectively. Only about 0.28 and 0.19% of total applied atrazine was lost to runoff in 1993 and 1994, respectively. The deethy latrazine (DEA) to atrazine molar ratio (DAR) decreased rapidly over a period of several hours in runoff samples, which is consistent with i nputs from recently atrazine-treated soil. After the spring runoff, he rbicide levels in Recharge Lake rose rapidly in response to the runoff event, then diminished gradually over a period of months. Atrazine co ncentrations in Recharge Lake decreased exponentially with time. Degra dation half-lives were 237 d (r = 0.93) in 1993 and 209 d (r = 0.91) i n 1994. Adjusted DEA concentrations in Recharge Lake remained relative ly constant, indicating little evidence for biotic degradation and sug gesting that abiotic degradation of atrazine to hydroxyatrazine -hydro xy-4-ethylamino-6-isopropylamino-s-triazine) was the most likely major degradative pathway in Recharge Lake.