DEFINITION OF HYDROSTRATIGRAPHIC UNITS WITHIN THE ALLUVIAL SEDIMENTS AT LAWRENCE LIVERMORE NATIONAL LABORATORY

At Lawrence Livermore National Laboratory (LLNL) Superfund site, the properties of ground water flow were used to define a series of hydrostratigraphic units (HSUs) within a thick sequence of previously-undivided, heterogeneous alluvial sediments. A methodology using multiple independent data sets was relied upon to define the hydrostratigraphic unit boundaries. The methodology employs an iterative process to minimize uncertainty in the correlations. Monitoring of the ground water system under stressed conditions during extraction well pumping and long-term pumping tests provided the most effective data set for identifying and verifying HSU boundaries. Hydrostratigraphic analysis identified low-permeability horizons within the alluvial sequence that exert significant control over ground-water flow and contaminant transport. These geologic features, which inhibit vertical hydraulic communication and contaminant migration, form the boundaries of the HSUs. At LLNL, the HSUs generally consist of a hydraulically interconnected network of higher-permeability deposits set within finer-grained, lower-permeability sediments. By identifying the primary hydraulic controls within the LLNL alluvial sequence, a hydrostratigraphic framework consistent with ground water flow and contaminant transport processes was established. The HSU framework has allowed for the mapping of a complex network of co-mingled plumes, each of which can be traced back to their respective source areas. Ground water cleanup systems at the site have been designed to treat and capture individual contaminant plumes, and are optimized with respect to their location, geometry, and mobility. This has proven a successful strategy for implementing the ground water cleanup based on the site remediation history. Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.