Loading of the Herbicide Diuron into the California Water System

Abstract

Diuron, a preemergent or early postemergent herbicide, predominantly applied in late fall and early winter in California, is the third most applied herbicide in the state. Previous studies showed that a small percentage (0.6 to 4.4%) of the applied diuron runs off during subsequent storms resulting in surface water concentrations high enough to cause algal toxicity. The goal of this study was to augment the existing sporadic monitoring results for diuron in surface waters to produce a high-resolution temporal record of diuron concentrations at a single point on the California Aqueduct (State Water Project). This location integrates runoff from many northern California locations and serves as a drinking water supply for millions of southern California residents. We analyzed a half-year time series of weekly samples and found a 2-month-long peak of diuron promptly following the strongest rainstorms of the winter. Such a pulse might be completely missed by infrequent (such as annual) sampling. Insignificant amounts of the principal biological breakdown products of diuron were observed during the diuron peak supporting the presumption that limited transformation of the compound occurs under environmental conditions. The total load of diuron transported through this system from January through June of 2004 was approximately 680 kg. Diuron's direct effects on aquatic organisms and indirect effects on human health via degradation products (including the potent carcinogen N-nitrosodimethylamine, which may be formed during drinking water disinfection from the dimethylamine precursor in the structure of diuron) are also of concern.