Evaluation of Chemical and Physical Processes Controlling DBCP Fate and Transport in the Eastern San Joaquin Valley, California

K.R . Burow and N.M. Dubrovsky, and S.Y. Panshin

In:
EOS, Transactions of the American Geophysical Union, v. 76, no. 46, Supplement, p. 186-187.

Abstract:
Future use of the sole-source aquifer in the eastern San Joaquin Valley will depend in part on how long 1,2-dibromo-3-chloropropane (DBCP), an agricultural fumigant not used for at least 15 years, persists at concentrations above the maximum contaminant level of 0.2 mg/L. Field data indicate that overall concentrations in ground water have decreased by one order of magnitude since the late 1970's. Laboratory experiments by earlier investigators show that DBCP transformed under conditions similar to in situ, with an estimated half-life ranging from 6.1 (pH 7.8, 21.1°C) to 141 years (pH 7.0, 15°C). A detailed hydrogeologic investigation is being done to assess the relative importance of: 1) chemical transformation, 2) dispersion, and 3) pumping and irrigation return flow in affecting DBCP concentrations. Water samples were collected from 20 multi-level monitoring wells installed along a 3-1/2-mile transect. DBCP concentrations in these samples ranged from less than the detection limit of 0.03 mg/L to a maximum of 3.7 mg/L. Chlorofluorocarbon age-dating results indicate that peak DBCP concentrations are in ground water recharged in the late 1970's. The primary transformation product identified in laboratory studies, 2-bromoallyl alcohol (BAA), was not detected in any of the 20 ground-water samples; the detection limit for BAA is 0.04 mg/L. The absence of BAA in ground water suggests that transformation to BAA may be insignificant by comparison with physical processes in controlling observed concentration trends; the estimated in-situ hydrolysis half-life is much greater than laboratory-determined values. Two-dimensional numerical flow and transport modeling is being used to test hypotheses addressing input function, degradation rate, dispersion, and in a relative sense, effects of irrigation pumpage and recharge on DBCP concentrations in the aquifer. A detailed characterization of the hydrostratigraphy was completed to model the effects of dispersion caused by aquifer heterogeneity.