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ARS Home » Midwest Area » Columbia, Missouri » Cropping Systems and Water Quality Research » Research » Publications at this Location » Publication #233785
Title: Herbicide transport trends in Goodwater Creek experimental watershed II: acetochlor, alachlor, metolachlor, and metribuzin

Author
item Lerch, Robert
item Sadler, Edward
item Baffaut, Claire
item Kitchen, Newell
item Sudduth, Kenneth - Ken

Submitted to: Journal of the American Water Resources Association
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/12/2010
Publication Date: 12/6/2010
Citation: Lerch, R.N., Sadler, E.J., Baffaut, C., Kitchen, N.R., Sudduth, K.A. 2010. Herbicide transport trends in Goodwater Creek experimental watershed II: acetochlor, alachlor, metolachlor, and metribuzin. Journal of the American Water Resources Association. 47(2):224-238.

Interpretive Summary: Farmers in the Midwestern United States continue to be reliant on soil-applied herbicides for weed control in row crop production, and herbicide contamination of surface waters. Runoff-prone watersheds remain an environmental problem. The primary objective of this study was to analyze trends in concentration and mass of acetochlor, alachlor, metolachlor, and metribuzin in Goodwater Creek Experimental Watershed (GCEW) from 1992 to 2006. These herbicides are mainly used for weed control in corn and soybean. A secondary objective was to document the effects of best management practices (BMPs) implemented within GCEW over the last 15 years on trends in herbicide transport. Changes in herbicide use were the main reason for observed changes in herbicide concentration. For instance, alachlor use decreased 74 percent from 1992 to 2006, resulting in major decreases in alachlor concentration over the last six years of the study. Smaller decreases in metolachlor and metribuzin use also decreased concentrations, but much less than that observed for alachlor. Acetochlor use in GCEW began in 1995, and by 2000 it was the second or third most heavily used herbicide in the watershed. Accordingly, acetochlor concentrations steadily increased from 1998 to 2006. The mass of these herbicides transported to the stream was generally related to the amount of stream flow in the second quarter of the year when these herbicides were applied to fields. This indicated that the timing of spring runoff events was the key factor controlling annual variation in herbicide mass transported to the stream. Yearly, from 0.2 percent to 3.7 percent of the herbicides applied within GCEW were lost from the outlet, with metolachlor having the highest loss and alachlor the lowest. Losses of metolachlor and atrazine (see the companion report on atrazine) at the watershed scale suggested that farmers in the watershed typically did not incorporate these soil-applied herbicides. Despite extensive education and outreach efforts in the watershed, conservation best management practices (BMPs) within GCEW were mainly implemented to control erosion, rather than reduce herbicide transport. Thus, these BMPs had no discernable impact on reducing herbicide transport. Overall, changes in herbicide use and second quarter stream flow had the greatest effect on trends in concentration and annual mass transport.

Technical Abstract: Farmers in the Midwestern United States continue to be reliant on soil-applied herbicides for weed control in row crop production, and herbicide contamination of surface waters. Runoff-prone watersheds remain an environmental problem. The primary objective of this study was to analyze trends in concentration and load of acetochlor, alachlor, metolachlor, and metribuzin in Goodwater Creek Experimental Watershed (GCEW) from 1992 to 2006. A secondary objective was to document the effects of best management practices (BMPs) implemented within GCEW over the last 15 years on trends in herbicide transport. The major decrease in alachlor use and increase in acetochlor use caused shifts in flow-weighted concentrations that were observed over the entire concentration range. The smaller decrease in metolachlor use led to a consistent decreasing time trend only for the upper end of the concentration distribution. Metribuzin also showed moderate decreases in concentration with time, especially since 1998. Annual loads were generally correlated to second quarter discharge, indicating that the timing of spring runoff events was the key factor controlling annual variation in herbicide load. Median relative herbicide loads, as a percent of applied, were 3.7% for metolachlor, 1.3% for metribuzin, 0.36% for acetochlor, and 0.18% for alachlor. Areal losses rates of atrazine and metolachlor at the watershed scale (on a treated area basis) suggested that farmers in the watershed typically did not incorporate these soil-applied herbicides. Despite extensive education and outreach efforts in the watershed, conservation BMPs within GCEW were mainly implemented to control erosion, rather than reduce herbicide transport. Thus, these BMPs had no discernable impact on reducing herbicide transport in GCEW. Overall, changes in herbicide use and second quarter discharge had the greatest effect on trends in flow-weighted concentration and annual load.