Diuron is the third most heavily used herbicide in California. Usage ranged from 1.1 to 1.5 million pounds per year from 1993 to 2002 (California Department of Pesticide Regulation (DPR), Summary of Pesticide Use Report Data, Index by Chemical, http://www.cdpr.ca.gov/docs/pur/pur02rep/02chem.htm). Since 1993, diuron has been widely detected in California’s water supply and nearly always detected at the spring check date that falls around March. The concentrations observed were usually more than 1 μg/L, which is the concentration of acute toxicity for algae and fishes and some of the observations were above 10 μg/L (DPR, Surface Water Data Base, www.cdpr.ca.gov/docs/sw/surfdata.htm).
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