The California Department of Pesticide Regulation initiated regulations on pesticide use in 1989 to mitigate groundwater contamination by atrazine and subsequently for simazine, diuron, bromacil, and norflurazon. Annual water samples from 2000 to 2012 were obtained from 67 domestic drinking water wells known to contain pesticide residues and located in vulnerable areas of Fresno and Tulare counties (59 distinct land sections). Decreasing trends in concentration over time were measured for total simazine residue, diuron, and bromacil. In contrast, increased trends were observed for norflurazon and its degradate. A large addition of regulated sections of land occurred in 1999 when use of simazine, diuron, and bromacil became regulated in 49, 32, and 22 of the land sections. Pesticide use data for 1999 indicated a dramatic shift in use from the regulated pesticides to norflurazon which was not yet listed for regulation. Previous age-dating of 30 domestic wells in this area using chlorofluorocarbons indicated an average recharge age of 7 years. Comparison of temporal changes in annual pesticide use to the onset of trends confirmed effects occurring within a ten-year time span in a number of the sections. Data available for total well depth for a subset of the wells indicated that changes should have occurred over a longer timeframe had wells been screened at the lowest depth. Although there was variation in well construction information, available data indicated a number of wells with large lengths of perforated screen and gravel packing around the casing. These features could have provided routes for mixing of aquifer water throughout the borehole. Thus, knowledge of well construction and the effect on mixing and extraction of water from a borehole are factors to consider when modeling fate of pesticides in the subsurface, especially with respect to predicting concentrations of solutes in well water.

Relating pesticide use and well construction to trends in pesticide concentration in domestic wells monitored in vulnerable areas of Fresno and Tulare Counties, California

248th National Meeting (2014)

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