A Multi‐Year Study of Four Herbicides in Surface Water of a Small Prairie Watershed

Abstract

AbstractTemporal trends of three phenoxy‐herbicides (2,4‐D, dichlorprop and MCPA) and bromoxynil were determined in the South Tobacco Creek, which drains a 70 km2 agricultural watershed near Miami, Manitoba, over a 4‐yr sampling program. The quantities of all pesticides used in the watershed and their application times were accurately recorded with the cooperation of landowners. Herbicide concentrations in South Tobacco Creek ranged from <0.01 ng/L to a maximum concentration of 680 ng/L (for 2,4‐D) and reflected local application times. Spring melt contributions were important when no significant runoff events occurred late in the previous growing season. Discharge of these herbicides in the South Tobacco Creek was calculated to be <0.01% of their quantities used within the watershed. No correlation was found between the water quality parameters: total organic C, total N, and total P and herbicide concentrations in South Tobacco Creek. Total N and P, however, were correlated to water flow, indicating runoff and ground water sources rather than atmospheric. We concluded that detailed temporal patterns of herbicides and nutrients could only be established if high sampling frequency was maintained during such critical periods such as application periods and during major runoff events. An unexpected finding was that herbicide concentrations were not related to runoff losses, but instead they did correspond to elevated levels in precipitation and air measured within the watershed.