Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 8, 1998
Publication Date: N/A
Interpretive Summary: Water resources in the Midwest USA are often contaminated with agricultural chemicals. This study examined the movement of the herbicides atrazine and metribuzin through the soil into subsurface tile drainage lines and into shallow ground water. Subsurface drainage eventually enters the surface water system. The site is within the Walnut Creek watershed and is cropped with corn and soybeans with banded herbicides. In soil, most of the herbicide was retained in the upper 45 cm, with herbicides detected in less than 20% of the samples below this depth. Atrazine was far more persistent than metribuzin in soil. This pattern was also seen in drainage water and ground water, where concentrations of atrazine were well above those of metribuzin. Both herbicides were detected in water from the tile drainage line, but average concentrations in any month did not exceed the standards for drinking water quality. Movement of herbicides into drainage water was dependent upon seasonal and year to year variations in rainfall. Only 1.4% of the ground water samples contained concentrations of atrazine above the 3 parts per billion limit and only 30% contained detectable concentrations. These results indicate that there is a low risk of water contamination beneath these soils because farmers use herbicides in small quantities and the herbicides are retained in the soil.
Technical Abstract: Transport of herbicides in subsurface drainage can result in unacceptable levels of contamination in surface waters. This study was conducted to assess 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 ground water. The site was farmed in a corn and soybean rotation with banded herbicide applications. Monthly-flow weighted average concentrations of atrazine in subsurface drainage water did not exceed 3 ug L**-1 and annual losses ranged from 0.02 to 2.16 g ha**-1 yr**-1 during the four year study. Less than 3% of the ground water samples contained atrazine concentrations exceeding the 3 ug L**-1 MCL. Atrazine was more frequently detected in ground water beneath the lowest parts of the field, despite greater sorption to soils in that area compared to other areas in the field. Metribuzin was also found in ground water, but only half as frequently as atrazine. The patterns observed in subsurface drainage and ground water reflected the persistence of atrazine and metribuzin in soil. Atrazine was detected in >90% of surface soil samples up to 23 months after application, whereas metribuzin was rarely detected during the second year following application. Atrazine was found far more commonly than metribuzin in soil deeper than 30 cm. Transport of herbicides in subsurface drainage does not result in contamination that exceeds regulatory action levels at this site.