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Variability in the distribution and dissipation of the herbicide thifensulfuron-methyl in a prairie wetland

Surface runoff, atmospheric transport, spray drift, and other transport processes result in frequent herbicide detections in prairie dugouts and wetlands. Thifensulfuron-methyl is a commonly used sulfonylurea herbicide with high phytotoxicity that could pose a potential threat to wetland ecosystems. The dissipation of thifensulfuron-methyl in a prairie wetland was monitored in the water column and wetland sediments during four months following herbicide addition. Variability in herbicide concentrations relative to physico-chemical characteristics of the wetland was evaluated by collecting water samples for herbicide analysis from two locations near the middle of the wetland and at two depths (0.5 m [1.6 ft] below the surface and 0.1 m [0.3 ft] above the wetland floor) at each location. Sediment samples were collected beneath the water sampling locations and at two sites near the wetland margin. Water temperature was constantly monitored throughout the study period, and a range of wetland properties was measured at each sampling. Higher temperatures and greater diurnal fluctuations were measured at 0.5 m depth than at 0.1 m from the bottom. Conversely, electrical conductivity of the wetland water was consistently greater at depth than near the surface of the wetland. Concentrations of thifensulfuron-methyl were consistently higher at 0.5 m depth than at 0.1 m from the bottom. Dissipation of thifensulfuron-methyl was biphasic at both depths, dissipating more rapidly initially (most of the herbicide was lost from the water column after 7 days) such that initial time for 50% dissipation values were around 2 days. Dissipation in the second phase proceeded at a much slower rate, taking three to five weeks for 50% of the residual herbicide to dissipate (quantifiable concentrations were still measured after 128 days). Thifensulfuron-methyl was detected in sediments collected near the wetland edge but not in samples taken from the deeper parts of the wetland. The physico-chemical environment of the wetland (temperature, pH, and dissolved oxygen [DO]) affected the distribution of thifensulfuron-methyl and should be considered in the development of sampling regimes and pesticide fate models.