Concentrations of Herbicides in Wetlands on Organic and Minimum‐Tillage Farms

Wetlands are abundant throughout the agricultural landscape of central Saskatchewan, Canada, and the biota present in these wetlands may be vulnerable to the toxic effects of pesticides used on nearby crops. We hypothesized that herbicide concentrations would be higher in wetlands on minimum‐tillage farms than on organic (no herbicide use) farms, and that the principal transport mechanisms of runoff versus atmospheric deposition could be identified based on the concentrations in these two wetland types. To test these hypotheses, 29 herbicides were monitored for 5 yr in 16 wetlands on minimum‐tillage farms and in seven wetlands on organic farms. Twenty herbicides were detected in wetlands on minimum‐tillage farms versus 12 in wetlands on organic farms. Clopyralid, MCPA, 2,4‐D, bromoxynil, dichlorprop, and dicamba were detected at a >50% frequency in wetlands on both minimum‐tillage and organic farms. Concentrations of clopyralid were significantly higher in wetlands on minimum‐tillage farms than in those on organic farms, whereas no significant difference was observed for any of the other five herbicides. Glyphosate, including its degradation product AMPA, was detected in >50% frequency only in wetlands on minimum‐tillage farms where the mean concentration (1278 ng L⁻¹) was higher than the concentration of other herbicides. Mass applied, vapor pressure, and water solubility were important determinants of herbicide concentrations and detections in wetlands. Herbicide concentrations in all but two samples were less than their respective Canadian guideline for protection of aquatic life, suggesting that, overall, individual herbicide concentrations in the wetlands were not toxic to biota. CORE IDEAS: Wetland biota may be at risk from toxic effects of herbicides. We assessed 29 herbicides in wetlands on minimum‐tillage and organic farms. Fewer herbicides and lower concentrations were detected in wetlands on organic farms. Detections were related to relative use, herbicide volatility, and water solubility. Herbicides in wetlands on minimum‐tillage farms were usually below toxic thresholds