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 runoff, Recharge Lake, and groundwater influenced by agriculture and lake seepage. The York Ground Water Recharge Project was constructed on a tributary of Beaver Creek, which drains a 3327-ha watershed of primarily row-cropped heavily irrigated farm-land. The estimated average runoff is 1.48 x 10 6 m3 yr -1 under a precipitation norm of 635 mm yr -1 . Maximum atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) inputs to Recharge Lake occurred in May and June runoff events and resulted In average lake concentrations of 36 and 17 μg L -1 in 1993 and 1994, respectively. Only about 0.28 and 0.19% of total applied atrazine was lost to runoff in 1993 and 1994, respectively. The deethylatrazine (DEA) to atrazine molar ratio (DAR) decreased rapidly over a period of several hours in runoff samples, which is consistent with inputs from recently atrazine-treated soil. After the spring runoff, herbicide levels in Recharge Lake rose rapidly in response to the runoff event, then diminished gradually over a period of months. Atrazine concentrations in Recharge Lake decreased exponentially with time. Degradation half-lives were 237 d (r = 0.93) in 1993 and 209 d (r = 0.91) in 1994. Adjusted DEA concentrations in Recharge Lake remained relatively constant, indicating little evidence for biotic degradation and suggesting that abiotic degradation of atrazine to hydroxyatrazine (2-hydroxy-4-ethylamino-6-isopropylamino-s-triazine) was the most likely major degradative pathway in Recharge Lake.