Monitoring of diazinon concentrations and loadings, and identification of geographic origins consequent to stormwater runoff from orchards in the Sacramento river watershed, U.S.A

Diazinon contamination of California's rivers has resulted in placing several rivers on the federal Clean Water Act 303d list of impaired waterways. Impaired water body listing requires the development of Total Maximum Daily Loads (TMDL). Previous studies identified stormwater related diazinon pulses in California rivers. This study was conducted to monitor diazinon concentrations in the Sacramento River watershed after rainfall events, to ascertain whether pulses could be identified and, if concentrations of concern were observed, to estimate loadings and determine geographic origins of the insecticide. TMDL development requires knowledge of contaminant sources, loadings, and geographic origins. Flow and diazinon concentrations peaked in the Sacramento River at Sacramento after the three largest storms during January and February 1994. Diazinon concentrations peaked consequent to each of three storms. Diazinon concentrations measured in the Sacramento River at Sacramento exceeded the California Department of Fish and Game acute and chronic criteria for protection of aquatic life during January and February for nine and nineteen days, respectively. Multiple exceedances were observed throughout the watershed. Diazinon loading and geographic origin differed with each of the three storms. The design of this study provides a useful template for others attempting to identify loadings and sources of contaminants in surface waters and to rectify aquatic ecosystem contamination from various land use practices.