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Co-occurrence of Pesticides and Aquatic Species
The potential co-occurrence of 40 pesticides and 12 threatened/endangered species was investigated for the Sacramento River, San Joaquin River, and Bay-Delta estuary watersheds in northern California (View Map). The 
investigation involved the use of simulation modeling, historical water quality monitoring data, and Geographic Information System (GIS) analysis in a weight-of-evidence context. The pesticides selected for analysis (Table E1) include herbicides, fungicides, and insecticides, and were based on a list of pesticides published by the Central Valley Regional Water Quality Control Board in 2009 estimated to pose the highest overall risk to aquatic life.
Species of concern for this study include nine threatened and endangered fish, amphibians, and invertebrates. Seasonal runs of Chinook salmon were treated as distinct species for this analysis, thereby accounting for essentially 12 unique species.
Methodology
Daily pesticide concentrations were predicted at the Public Land Survey Section (PLSS) section level from runoff, erosion, and drift sources. Pesticides loads were estimated using 10 years of historical pesticide use data obtained from the California Department of Pesticide Regulation’s Pesticide Use and Registration (PUR) database. Application sites represented in the simulations included fruit, vegetable, grain, nuts, rice, and urban/residential landscape maintenance, and structural pest control. Loadings from agricultural uses were predicted using the Pesticide Root Zone Model (PRZM) and the Rice Water Quality Model (RICEWQ). PRZM was also used to estimate runoff from urban/residential applications. Drift was only assumed to occur in agricultural settings and was estimated using a linear equation accounting for application method, application area, and surface water area in the PLSS section.
Approximately 9,115,000 pesticide applications were represented in the simulations, accounting for a total applied mass of 98,279,000 lbs of active ingredient (a.i.) for the 40 chemicals (View Map).
Approximately 14.2% of the applied amount was predicted to reach surface waters via runoff, erosion, drift, and discharge. Runoff from agriculture accounted for over 86% of the mass losses loadings. Erosion and drift from agricultural applications accounted for approximately 5.0% and 4.4% respectively of mass loadings. Another 4.3% was predicted to discharge and run off from rice paddies. Urban runoff accounted for less than 1 percent.
A toxicological threshold was produced for each pesticide. Where available, the threshold was based on the acute benchmark developed by the U.S. Environmental Protection Agency’s Office of Pesticide Programs (OPP) for the most sensitive aquatic non-plant species. An additional safety factor was applied to achieve OPP’s level of concern for threatened and endangered species.
Indicator days (days during which 
at least one pesticide was estimated to exceeded the toxicity threshold) showed distinct spatial-temporal patterns (View Map Series). Indicator days for urban regions were predicted to occur primarily during the late fall through early spring period. Indicator days for agricultural areas occur predominately during the spring/summer crop growing season. Indicator days for rice growing areas were prevalent in the latter part of the crop season.
Species richness was determined to be nearly constant from January to July. From August to December, the species richness varied by month and location (View Map). November was lowest in terms of species richness. The maximum (100th percentile) of species richness was 0.917 (11 out of the 12 species were present in at least one PLSS section) and the 90th percentile was 0.5.
Monthly distributions of species richness were developed for each species for rivers and streams in the study area. Co-occurrence was estimated at the PLSS level by combining monthly distributions of indicator days with monthly distributions of species richness into a multidimensional matrix (View Map).
Results
Areas with very high co-occurrence (exceeding the 90th percentile values for species richness and indicator days) are concentrated in the southern Delta Estuary in San Joaquin County and smaller regions in the Northern Delta Estuary in Sacramento County and western Yolo County. Areas with high co-occurrence (exceeding the 80th percentile values for species richness and indicator days) are present in clusters scattered on the outskirts of the main agricultural areas in the Sacramento River and San Joaquin River Watershed and in the northern section of the Bay-Delta Watershed. Many of these areas have limited or no monitoring data for the forty pesticides evaluated during this study.
Project Publications
Breuer, R., D. Denton, W.M. Williams, C.G. Hoogeweg and M. Zhang. Prioritizing Research Needs for Threatened and Endangered Species in California Watersheds. Read Abstract • Download Poster (7.6mb)
Hoogeweg, C.G., W.M. Williams, R. Breuer, D. Denton, B. Rook and C. Watry. 2011. Spatial and Temporal Quantification of Pesticide Loadings to the Sacramento River, San Joaquin River, and Bay-Delta to Guide Risk Assessment for Sensitive Species. CALFED Science Grant #1055. Nov, 2 2011. 293 pp. Download Report (59mb)
