Water Quality in Walnut Creek Watershed: Herbicides in Soils, Subsurface Drainage, and Groundwater

Herbicide transport in subsurface drainage can result in unacceptable levels of contamination in surface waters. This study assessed the extent of atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3-,5-triazine-2,4-diamine] and metribuzin [4-amino-6-(1,1-dimethyethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one transport to subsurface drainage and shallow groundwater. A corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] rotation was used with corn receiving banded atrazine applications of 459 g ha -1 in 1992 and 561 g ha -1 in 1994. Soybean were treated with metribuzin at 420 g ha -1 in 1993 and 1995. Monthly flow-weighted average concentrations of atrazine in drainage water did not exceed 3 μg L -1 and annual losses ranged from 0.02 to 2.16 g ha -1 during the 4-yr study. Less than 3% of the groundwater samples contained atrazine concentrations exceeding the 3 μg L -1 maximum contaminant level for drinking water (MCL). Atrazine was detected more frequently in groundwater beneath the lowest parts of the field, despite greater than average sorption to soils in that area. Metribuzin was also found in groundwater, but only half as frequently as atrazine. The patterns observed in subsurface drainage and groundwater reflected the persistence of atrazine and metribuzin in soil. Atrazine was detected in >90% of surface soil samples up to 23 mo after application, whereas metribuzin was rarely detected during the second year following application. Atrazine was found far more commonly than metribuzin in soil below 30 cm depth.