California Department of Pesticide Regulation Bioassessment Pilot Study: Identifying Correlations between Macroinvertebrate Communities and Pesticides and Other Environmental Variables of Agricultural Runoff

This project was a collaborative effort between the California Department of Pesticide Regulation (DPR) and the Central Valley Regional Water Quality Control Board (CVRWQCB). The agencies combined limited monitoring resources to jointly collect biological, pesticide and water quality data. The primary objective of this study was to identify potential adverse impacts to the aquatic environment from pesticides and/or other elements in agricultural runoff. The approach was to compare benthic macroinvertebrate (BMI) communities at sites directly receiving agricultural runoff to physically similar sites with lesser agricultural runoff inputs. Two waterways in San Joaquin County, California, were sampled. They are used for both irrigation supply and drainage and are not considered fish habitats. Two sites in each waterway were sampled, and all four sites monitored had similar physical habitats, having been historically modified for agricultural activities. The waterways were channelized, had banks and riparian zones with little vegetation, and had substrate consisted solely of mud and/or sand. Samples were collected from each waterway at a site where irrigation supply entered (input) and near the end of a waterway after receiving discharges from agricultural drainage (output). All sampling occurred from February 15, 2005 through September 18, 2005. Sites were monitored semi-weekly for pesticides and nutrients. Water quality parameters consisting of temperature, pH, specific conductance (EC), dissolved oxygen (DO) and turbidity were monitored hourly, and aquatic macroinvertebrates were collected monthly. All sites were lacking in pollutant sensitive taxa (Ephemeroptera, Plecoptera, Trichoptera). Dominant taxa found at all sites were Chironomidae, Gastropoda, Oligochaeta, and Amphipoda species. There were significant differences between Inlet1 and Outlet1 in taxa richness, abundance and community structure. At Inlet2 and Outlet2, there were no significant differences in any of the measured BMI variables. There were significant differences in various water quality parameters between input and output sites in the same waterways. The organophosphate pesticides chlorpyrifos and dimethoate were the two most frequently detected pesticides. While occasional exceedances of the chlorpyrifos LC50 for Ceriodaphnia dubia occurred, dimethoate concentrations did not exceed any known aquatic toxicity benchmarks. The pyrethroids lambda cyhalothrin and bifenthrin were detected in two bed sediment samples but did not exceed any known aquatic toxicity benchmarks. Pesticide sampling was limited in duration, occurring only during the summer sampling period, and no spatial or temporal correlation between pesticide detections and BMI variables was evident. The various pesticide, nutrient, and basic water quality variables were highly collinear, demonstrating the difficulty in inferring causality based on their correlation with BMI variables. However, it was clear that low DO concentration was related to BMI community structure. At Outlet1, extreme continuous exceedances of the DO water quality criterion was related to the presence of low DO tolerant taxa. Other water quality variables such as elevated EC or turbidity may also have had an effect, but that conclusion is far more tentative.