A LARGE-SCALE RETROSPECTIVE GROUND-WATER MONITORING STUDY FOR METOLACHLOR

A large-scale retrospective ground water monitoring study for metolachlor, 2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide, was conducted. The study consisted of the collection and analysis of ground water samples from 240 existing wells (203 domestic farm wells and 37 monitoring wells) in selected areas of four states: the Dougherty Plains, Georgia; McLean County, Illinois; Floyd and Mitchell counties, Iowa; and the Central Sands area of Wisconsin. These locations were selected on the basis of high metolachlor use and hydrogeological vulnerability. The wells selected were all screened in the uppermost aquifer, were relatively shallow, and were located close to fields where metolachlor has historically been used. Pesticide handling and storage practices on the farms where the wells were located were documented, and wells were selected to avoid obvious point sources. The wells selected for monitoring included drinking water, irrigation and monitoring wells. Monitoring was at quarterly intervals for a one-year period. Metolachlor was detected in 89 of 920 samples (9.7 percent) from 39 of 240 wells (16.3 percent) with a screening level (quantitation level) of 0.1 ppb. Ten out of 240 wells sampled (4.2 percent) showed detections in all sampling rounds. Of the samples analyzed, 5.1 percent had detections in the range between 0.1 and 0.5 ppb. Thus, non-detections plus detections below 0.5 ppb accounted for 95.4 percent of all samples, and detections below 0.5 ppb represented more than half of all the detections found in the study (47 out of 89 detections). In the 31 samples where metolachlor was detected above 1.0 ppb, every detection could be related to a known or suspected point source. Based on the frequency and magnitude of metolachlor detections in this study, there appears to be no significant measurable field leaching of metolachlor to ground water under conditions of high product use and high ground water vulnerability. Instead, measurable detections of metolachlor, particularly those exceeding 1.0 ppb, appear to be associated with point sources.