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Description
Recent studies have shown that aquifer heterogeneities on all scales can effect contaminant migration and must be considered in order to effectively characterize and remediate groundwater contamination. The purpose of this study is to evaluate the effect of heterogeneity caused by buried-stream-channel deposits on contaminant migration in a multiple-aquifer system in the North Santa Clara Valley subbasin. Three aquifers were defined during this study that are made up of laterally discontinuous channel deposits composed primarily of sand and gravel within a matrix of finer-grained soils. Trichloroethylene, and other chlorinated solvents, have been introduced into the each of the aquifers as a result of historical industrial activities in the area. The heterogeneity within the aquifers created by lateral variations in grain-size appears to strongly influence contaminant migration downgradient of known sources. This study has shown that high hydraulic conductivity materials associated with the occurrence of buried stream-channel deposits tend to act as preferred pathways for groundwater flow and contaminant transport. The degree to which buried stream-channel deposits influence contaminant transport depends on the degree of continuity and interconnectedness of high hydraulic conductivity materials. The complexity of contaminant distribution in heterogeneous multiple-aquifer systems can greatly impact the time and expense associated with field characterization if not dealt with properly. In order to effectively characterize sites with spatially variable hydraulic conductivity, the continuity and interconnectedness of permeable deposits must be defined. Factors that can complicate the effective characterization of complex aquifer systems include lack of pertinent geologic data, placement and density of monitor wells, and the presence of multiple sources. Reconnaissance investigative techniques and/or geostatistical methods should be employed, as appropriate, to assist in reducing the guesswork involved in field characterization.
