Prepared in cooperation with the University of California, Davis
U.S. Geological Survey Water-Resource Investigations Report 99-4059
Ground-water samples were collected from 20 monitoring wells installed along a 4.6-kilometer transect. DBCP concentrations in these samples ranged from less than the detection limit of 0.03 mg/L to a maximum of 6.4 mg/L. Results of chlorofluorocarbon (CFC) age dating indicate that DBCP occurs in water that ranges in age from about 2 to 41 years. The primary transformation product BAA, which was identified during previous laboratory studies, was not detected at or greater than 0.03 mg/L in any of the 20 ground-water samples. The lack of detection of BAA indicates that transformation to BAA is insignificant relative to other processes controlling DBCP concentrations. Results from this current study indicate that the in situ hydrolysis half-life for DBCP to BAA is much greater than the laboratory-determined values.
Estimated initial concentrations of DBCP, calculated using CFC-estimated travel times and a half-life of 6.1 years, indicate that maximum initial concentrations are consistent with maximum measured concentrations in ground water. In contrast to initial DBCP concentrations, the estimated initial nitrate concentrations indicate that nitrate concentrations in recharge water have increased with time.
A conceptual two-dimensional numerical flow and transport modeling approach was used to test hypotheses addressing dispersion, transformation rate, and in a relative sense, the effects of ground-water pumping and reapplication of irrigation water on DBCP concentrations in the aquifer. The flow and transport simulations, which represent hypothetical steady-state flow conditions in the aquifer, were used to refine the conceptual understanding of the aquifer system rather than to predict future concentrations of DBCP. Results indicate that dispersion reduces peak concentrations, but this process alone does not account for the apparent decrease in DBCP concentrations in ground water in the eastern San Joaquin Valley. Ground-water pumping and reapplication of irrigation water may affect DBCP concentrations to the extent that this process can be simulated indirectly using first-order decay. Transport simulation results indicate that the in situ “effective” half-life of DBCP caused by processes other than dispersion and transformation to BAA could be on the order of 6 years.
Contents
Abstract
Introduction
Background
Purpose and Scope
Description of Study Area
Chemical and Physical Processes Affecting DBCP Concentrations
Acknowledgments
Well Network and Ground-Water Sampling Methods
Monitoring Well Network
Water-Quality Data Collection And Analysis
Quality-Control Data
Hydrologic and Chemical Characterization
Hydrologic Data
Chemical Data
DBCP, Nitrate, and Specific
Conductance
Chlorofluorocarbon and Tritium
Concentrations
Evaluation of DBCP Input and Processes Affecting DBCP Concentrations
Estimation of Initial Concentrations of DBCP
Chemical Transformation
Dispersion and Ground-Water Pumping and Reapplication
of Irrigation Water
Ground-Water Flow and DBCP Transport Modeling
Simulation of Ground-Water Flow
Simulation of Advective Transport by Particle Tracking
Simulation of DBCP Transport and Decay
Transport Modeling Approach
and the MT3D Model
Simulated DBCP Concentrations
and Comparison with Existing Water-Quality Data
Effects of Averaging and
a Constant Source of DBCP on Simulation Results
Simulation of the Effects
of Dispersion Using Heterogeneous and Homogeneous Hydraulic
Conductivity Distributions
Simulations of Dispersion
Significance of Simulation Results
Summary and Conclusions
References Cited