Location: Southeast Watershed ResearchTitle: Assessing pesticide wet deposition risk within a small agricultural watershed in the Southeastern Coastal Plain (USA) Author
Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/10/2016
Publication Date: 12/24/2016
Citation: Potter, T.L., Coffin, A.W. 2016. Assessing pesticide wet deposition risk within a small agricultural watershed in the Southeastern Coastal Plain (USA). Science of the Total Environment. 580:158-167. doi:10.1016/j.scitotenv.2016.11.020.
Interpretive Summary: Although not widely appreciated an important dissipation pathway of many pesticides is their evaporation after application and transfer to the atmosphere. Once in the atmosphere, pesticides move with prevailing winds. Rain tends to strip then from air resulting in their deposition on land surfaces. 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. To that end 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 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 in the highest amounts. Residues of these compounds were detected in nearly every sample. 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. Fortunately 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 runoff that was measured in a companion study. This provides further support for the conclusion that evaluation of the amounts of pesticides that may be deposited by rain is needed to effectively assess risks of pesticide use.
Technical Abstract: Pesticide volatilization and deposition with precipitation is widely documented and has been connected to adverse ecological impact. Here we describe a 3-yr study of current use and legacy pesticides in event-based rain samples within a 123-ha agricultural watershed. Crops in farm fields were documented quarterly with data used to estimate target compound use. The median number of pesticide detections in samples was 6. The fungicide, chlorothalonil which was used most intensively was detected in nearly all samples. It had the highest mean and peak concentrations and deposition. Total deposition was ˜0.1 % of the estimated amount applied. The insecticide endosulfan also had relatively high use with behavior mirroring chlorothalonil. There was strong seasonal variation in concentration and depositional dynamics with the highest values measured during growing seasons. Similar behavior was observed with other compounds detected with a general decrease in deposition and mean concentrations as use decreased. Comparison of measured concentrations to values associated with toxic impact on aquatic organisms indicated that chlorothalonil, endosulfan, chlorpyrifos, malathion and atrazine may contribute adverse impact. The number of samples exceeding risk endpoints ranged from 1 to 77 %. The highest number was for endosulfan; however its on-going phase-out is expected to reduce risks. Another finding was that the wet deposition of the herbicide, metolachlor exceeded measured runoff rates in the watershed by 5-fold. The study has demonstrated that localized pesticide wet deposition may present ecological risks and that volatilization and wet deposition is an important pesticide transport pathway at the local scale. Findings point to the need to include wet deposition in assessments of pesticide ecological risk and environmental fate.