Spatial and temporal distribution of herbicides and herbicide degradates in a shallow glacial drift aquifer/surface water system, south‐central Michigan

Abstract

AbstractHerbicide and herbicide degradate compounds were measured in tile drains, monitoring wells, domestic wells, and in the receiving stream at a farm in south‐central Michigan, in which corn, soybeans, and livestock are the major products. Immunoassay screens of two herbicide groups (triazines and alachlor) in 2000 showed a decrease in the number of detections from June to September, after the well‐known spring flush, of 48% to 26% for alachlor and 62% to 22% for the triazines. Gas chromatography/mass spectrometry and liquid chromatography/mass spectrometry analyses of 2001 samples documented a fall flush as well, mobilizing relatively high levels of both parent and degradate compounds to the tile drain system. The compounds included both metolachlor and atrazine, which were applied on the farm earlier that year, as well as the degradates of alachlor, which had not been applied on the farm for several years. In four tile drain samples, metolachlor was detected at concentrations ranging from 18 to 25 μg/L, whereas atrazine ranged from nondetect to 2.25 μg/L. Contrary to previous studies, metolachlor concentrations were higher than metolachlor ethane sulfonic acid, by a factor of 3 to 7. The high soil permeability, high water table, and shallow screened intervals of monitoring wells directly below the point of application may explain the lesser degree of breakdown at the point of sampling. Although most detections were below maximum contaminant level or health advisory levels, the unknown effects of degradates and the aggregate levels of compounds measured in many samples may pose higher health risks than for individual compounds.