Chlorofluorocarbon Dating of Herbicide-Containing Well Waters in Fresno and Tulare Counties, California

Simaaine, diuron, and bromacil are the most frequently detected currently registered pesticides in California groundwater. These herbicides bave been used for several decades in Fresno and ‘Mare counties, California; however, previous data are inadequate to determine whether the detections are a result of recent or historical applications (i.e., within the hut decade, or 2&3tl yr ago). Chlorofluorocarbon (CFC) groundwater age-dating was used in conjunction with onedimensional transport modeling to address this question. The estlmated tbues between herbicide application and subsequent detection in groundwater samples from 18 domestic wells ranged from 3 to 33 yr~ the aggregate data indicate that more than half of the detections are associated with applimttons in the last decade. The data also suggest that changes in groundwater quality arising from modified management practices will probably not be discernible for at least a decade. A secondary objective of this study was to evaluate the contribution of simaahre degradates deethylsimazine (DES, a-amino4-chloro-6-ethyhrmino-s-triazine) and diaminochlorotriazine (DACT; 2,4.diambro-6-chloro-s-triazine) to total trlazine concentrations (defined here as simazine + DES + DACT) in 30 domestic wells. The N-dealkylated s-chlorotrhuine degradates DES and DACT substantially contribute to total triazine concentrations in Fresno and ‘Mare County groundwater, composing 24 to 100% of the total triazines, with a median of 82%. If s-chlorotriazines display a common mode of toxicological action, the prevalence of triazine degradates in water samples found in this and other studies indicate that drinking water standards based on total s-chlorotriazine concentrations may be most appropriate. C ALIFORNIA is the most productive agricultural state in the nation, with 1994 farm receipts exceeding $20 billion. Although a portion of these revenues were from meat, egg, and milk production, more than 60% came from production of 53 million Mg of field crops, fruits, nuts, and vegetables on 3.2 million ha (California Agricultural Statistics Service, 1995a). In achieving this F. Spurlock, Environmental Hazards Assessment Program, California Dep. of Pest. Regulation, 830 K St., Sacramento, CA 95814-3510. K. Burow and N. Dubrovsky, Water Res. Div., U.S. Geological Survey, Placer Hall, 6000 J St., Sacramento, CA 95819-6129. *Corresponding author ([email protected]). Published in J. Environ. Qual. 29:474-483 (2000). level of productivity, California growers applied 82 million kg of pesticide active ingredient (California Department of Pesticide Regulation, 1996). During the 197Os, the potential for pesticides to move to groundwater in California was underestimated. In 1979, the soil fumigant dibromochloropropane (DBCP) was first detected in groundwater in Lathrop, CA (Peoples et al., 1980). Subsequent studies showed the relatively widespread presence of DBCP in San Joaquin Valley groundwater, and groundwater sampling studies are now conducted by several agencies and organizations (Bartkowiak et al., 1997). In addition to conducting groundwater studies, the Environmental Hazards Assessment Program (EHAP) of the California Department of Pesticide Regulation (CDPR) maintains a statewide Well Inventory Database (WIDB; Bartkowiak et al., 1997). The WIDB is a centralized database of California groundwater monitoring results from private firms and local, state, and federal agencies. Confirmed detections of pesticide active ingredients or degradates have been reported in 1,586 California wells since 1986 (Fig. 1). For the purposes of the WIDB, a confirmed detection is defined as the detection of a compound in two discrete samples from a well. Preemergent herbicides are usually soil-applied, tend to be relatively water-soluble, display low to moderate soil sorption, and possess long soil half-lives. Owing to these characteristics, and their widespread use in a variety of California crop and noncrop sites, preemergent herbicides are the most commonly detected currently registered pesticides in California groundwater. SimaAbbreviations: CDPR, California Department of Pesticide Regulation; CFC, chlorofluorocarbon: DACT, diaminochlorotriazine: DBCP. dibromochloropropane; DES, deethylsimazine; EHAP, Environmental Hazards Assessment Program; ET, evapotranspiration; LC, liquid chromatograph; LEACHM, Leaching Estimation and Chemistrv Model; LGd, limit of quantitation; hCL, maximum contaminant level; MDL, method detection limit; NAWQA, National Water-Quality Assessment Program; SCX, strong cation exchange; USEPA, United States Environmental Protection Agency; USGS, United States Geological Survey; WIDB, Well Inventory Database. SPURLOCK ET AL.: CHLOROFLUOROCARBON DATING OF WELL WATERS 475 zine, diuron, bromacil, and triazine degradates are the most frequently detected herbicide residues in California groundwater (561, 360, 195, and 147 confirmed detections, respectively; Maes ct al., 1993). The largest number of these herbicide detections in groundwater are in Fresno and Tulare counties in the southern San Joaquin Valley; 1 or more of the 3 herbicides (or degradates) have been detected in 509 Fresno and Tulare County wells. The 1994 crop production in these counties included 45 000 ha of citrus and 100 000 ha of fresh market, raisin, and wine grapes (California Agricultural Statistics Service, 1995b). Simazine, diuron, and bromacil are the most common preemergent herbicides used in California citrus production, whereas simazine and diuron are the most common preemergent herbicides applied to grapes. Bromacil is not registered in California for use on grapes. In 1994, grape and citrus applications accounted for 65% of the 300 000 kg active ingredient of simazine, diuron, and bromacil applied in Fresno and Tulare counties (California Department of Pesticide Regulationl996). Several recent studies in the midwestern United States have reported that triazine degradates are frequently present in the surface and groundwaters in areas where atrazine is used (Thurman et al., 1992; Moody and Goolsby, 1993; Kolpin et al., 1995; Kolpin et al., 1996). While little atrazine has been used in Fresno or Tulare counties for at least the last 10 yr, simazine shares two common N-dealkylated degradates with atrazine. The two degradates are 2-amino-4-chloro-6-ethylamino-s-triazine (DES, also known as deisopropyl atrazine), and the di-dealkylated degradate 2,4-diamino-6chloro-s-triazine (DACT) (Fig. 2). The N-dealkylated s-chlorotriazine degradates are similar in structure to the parent herbicides, and limited data suggests they may share a common mode of toxicological action (e.g., Babic-Gojmerac et al., 1989). Consequently, the U.S. Environmental Protection Agency (USEPA) is assuming that the total triazine toxic residue of concern in their assessment of dietary risk for the triazine Special Review (U.S. Environmental Protection Agency, 1994) is the parent triazine plus all metabolites with a triazine ring. In addition, although there are no current National Primary or Secondary Drinking Water Standards for total triazines, total triazines are listed on USEPA’s Drinking Water Contaminant Candidate list (U.S. Environmental Protection Agency, 1998a). The state of Wisconsin has applied atrazine health advisory levels to the total concentration of atrazine plus select degradates [Wisconsin Ground-Water Act 410 (1983), Rule under the Law, Enforcement Standard, Chapter NR, Wisconsin ADM CODE (1991)]. There has been only limited sampling for the triazine degradates DES and DACT in California wells, and further occurrence data are needed. In response to the large number of confirmed herbicide detections, EHAP is developing, testing, and promoting management practices to reduce the movement of herbicides to groundwater. In discussions with the public, growers, commodity groups, and pesticide registrants, questions frequently arise about the extent to 41.00