Simazine, diuron, and bromacil are the most frequently detected currently registered pesticides in California groundwater. These herbicides have been used for several decades in Fresno and Tulare counties, California; however, previous data are inadequate to determine whether the detections are a result of recent or historical applications (i.e., within the last decade, or 20-30 yr ago). Chlorofluorocarbon (CFC) groundwater age-dating was used in conjunction with one-dimensional transport modeling to address this question. The estimated times 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 applications 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 simazine degradates deethylsimazine (DES; 2-amino-4-chloro-6-ethylamino-s-triazine) and diaminochlorotriazine (DACT; 2,4-diamino-6-chloro-s-triazine) to total triazine concentrations (defined here as simazine + DES + DACT) in 30 domestic wells. The N-dealkylated s- chlorotriazine degradates DES and DACT substantially contribute to total triazine concentrations in Fresno and Tulare 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.