Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/5/1998
Publication Date: N/A
Interpretive Summary: Contamination of shallow groundwater resources with agricultural chemicals is a potential threat to human health. Alachlor is a herbicide that is widely applied to corn in the Midwest to control annual grasses and leafy weeds. Alachlor is degraded in soil by microbes, and the rate of degradation is primarily controlled by moisture and temperature and whether or not the alachlor is adsorbed onto soil particles. This study was designed to answer the following questions: (i) how fast is alachlor degraded in soil? (ii) at what soil depth can most of the alachlor be found? (iii) how much of the alachlor is leached through the soil with water movement? (iv) are these answers different for no-till and chisel plow tillage of soil? Results of these studies show that 50% of the alachlor was degraded in the top foot of soil in about 2 days and that the degradation rate was slightly faster in no-till than chisel plow. Regardless of tillage, most of the alachlor was always found in the top 4 inches of soil. Alachlor leached through no-till soil more quickly and in larger amounts than through chisel plow soil, although the amount leached in no-till was only 1.6% of the amount applied. These results indicate that alachlor degrades fairly rapidly in soil and that most of it is bound to soil in the top 4 inches. Only very small amounts of alachlor are lost through leaching, suggesting that alachlor has a relatively low potential for contaminating groundwater under these Midwest soils.
Technical Abstract: One soil column laboratory experiment and a two-year field-sampling study evaluated the overall dissipation of alachlor and the effect of no-till and chisel-plow tillages on alachlor leaching and dissipation. In the top 30-cm layer of soil, the overall half-life of alachlor was roughly 2 d, and the time to 90% dissipation ranged from 17 to 30 d. In no-till soil, alachlor dissipated slightly faster, and more was transported into the 10-to-30-cm soil layer. The magnitude of the tillage effect was dependent on weather conditions, especially rainfall, that favored movement into and dissipation within the soil structure, rather than dissipation on the soil surface. Most of the alachlor present in the soil, regardless of tillage, was found in the top 10 cm at all times. Of the alachlor applied to 30-cm-tall soil columns, only 0.4% was removed by water flowing from chisel-plow columns and 1.6% from no-till columns. The results show that tillage was not a reliable indicator of alachlor dissipation and leaching and that alachlor leaching was a minor dissipation process.