Discharge and suspended sediment concentrations had an important but secondary role in associated insecticide concentrations. As expected, current-use herbicide and insecticide types and concentrations were lowest in storm events occurring during fall and winter months irrespective of the magnitude of the storm event, ensuing discharge, and associated suspended sediment concentrations. Historic-use pesticide and metabolite storm event concentrations fluctuated with suspended sediment concentrations but were at measurable levels in each storm event throughout the year. In the current study, pesticide concentrations were transient within and among storm events, and this pattern has been observed in other studies as well (Knight and Cooper 1996; Coupe 2000). For example, storm samples from March 2001 had detectable amounts of only one type of herbicide (metolachlor) and no detectable amounts of methyl parathion in contrast to April and May 2001.

Aquatic organisms within Little Topashaw Creek are exposed to transient pulses of pesticide mixtures that vary in frequency and magnitude depending upon agricultural practices, sedimentation and discharge.