42-5 Evaluating the Presence of Atrazine, Glyphosate, and Metolachlor within a Midwestern USA Agricultural Watershed

Abstract:

In the midwestern United States, where corn and soybeans are commonly grown, atrazine, glyphosate, and metolachlor are frequently applied as herbicides on agricultural fields. Each of these compounds has the potential to persist in soils, runoff into waterways, and negatively impact aquatic ecosystems. However, testing surface water for atrazine, glyphosate, and metolachlor is expensive (typically $300 or more per sample), and, despite their widespread use, they are not routinely included in laboratory pesticide screening analyses. Consequently, we set out to measure atrazine, glyphosate, and metolachlor concentrations in surface waters of the Little Swan Creek watershed (HUC 040500010402) in south central Michigan to evaluate how lake retention and surface water processes influence contaminant attenuation prior to groundwater recharge. 80% of the land use in this watershed is agricultural, with corn and soybeans as the primary crops. Hence, we expected to find atrazine, glyphosate, and metolachlor in the surface waters of the watershed.

On July 21, 2026, we collected samples under dry weather conditions at six locations chosen to represent watershed segments upstream and downstream of a drainage lake located in the approximate center of the 20,000-acre (81 km²) watershed. Results detected atrazine at relatively low concentrations (0.1 ug/L) in one location downstream of the lake. Other locations were non-detect, indicating concentrations below the reporting limits for all three analytes.

Our results suggest that surface waters in the watershed are not subjected to prolonged high concentrations of atrazine, glyphosate, or metolachlor in mid-summer months. However, wet weather sampling could reveal short-term periods when concentrations may be higher. Alternatively, unseasonally high rainfall in April 2026 may have flushed these herbicides, which are typically applied early in the growing season, through the watershed prior to our sampling. In addition, natural degradation processes could have reduced herbicide concentrations. Future studies should also sample for the highly persistent degradates of these compounds, such as desethylatrazine (atrazine), aminomethylphosphonic acid (glyphosate), and metolachlor ethanesulfonic acid or metolachlor oxanilic acid, to investigate their environmental fate in surface water bodies.

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