Evaporation after application is an important dissipation pathway for many pesticide active ingredients. Once in the atmosphere, they are subject to advective and diffusive transport, photochemical oxidation and degradation, sorption on aerosols and particulate matter, and wet and dry deposition. Most studies that have evaluated deposition have focused on collection and analysis of rain and in some studies adverse ecological consequences have been documented. Thus to comprehensively assess risks of pesticide use measurements that determine the rate of pesticide wet deposition are needed within agricultural watersheds. We measured a suite of commonly used pesticides in event-based rain samples for 3 years within a 123-ha agricultural watershed in south central Georgia. We also monitored land-use within the watershed so that we could estimate the amounts of pesticides that were used during crop production. The median number of pesticides detected in samples was six with the fungicide, chlorothalonil, and the insecticide endosulfan detected most frequently and in the highest amounts. The highest concentrations and rates of deposition of these and other pesticides monitored was in samples collected during growing seasons when the pesticides were being applied. Comparison of measured concentrations to toxicity thresholds for aquatic life showed that a relatively high percentage of samples could have adverse impact. The greatest potential impact was linked to endosulfan since it is highly toxic to fish and aquatic invertebrates although risks are expected to diminish since the compound is no longer licensed for use. We also found the wet deposition of an herbicide used on many crops was more than 5 times greater than in surface runoff that was measured in a companion study. This provides further support for our conclusion that evaluation of the amounts of pesticides that may be deposited by rain is needed to effectively assess risks of pesticide use.<br/>

Assessing pesticide wet depostion risks in agricultural watersheds

254th National Meeting (2017)

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