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Environmental and Engineering Geoscience

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A Conceptual Model and Remediation Strategy for Volatile Organic Compounds in Groundwater in Unconsolidated Sediments: A Lawrence Livermore National Laboratory Case Study

¹ Environmental Restoration Division, Lawrence Livermore National Laboratory, L-530, P.O. Box 808, Livermore, CA 94551
² Weiss Associates, 5801 Christie Avenue, Suite 600, Emeryville, CA 94608
³ Harding ESE, 600 Grand Avenue, Suite 300, Oakland, CA 94610

Field data collected during the first 7 years of pump-and-treat remediation of groundwater containing volatile organic compounds (VOCs) at the Lawrence Livermore National Laboratory (LLNL) Superfund Site in Livermore, California, indicate that groundwater contaminant plumes at this site can be divided into two distinct parts: source areas and distal areas. In source areas, located in the immediate vicinity of the contaminant releases, the contaminants are distributed in relatively high concentrations throughout the vadose zone and below the water table in both fine-grained and coarse-grained sediments. In distal portions of the plume, downgradient of the source areas, the contaminants: 1) are primarily limited to coarse-grained zones, 2) are usually orders of magnitude lower in concentration than in the source areas, and 3) have slightly diffused into the bordering aquitards but should not significantly affect the approach to or achievement of cleanup goals. The cleanup strategy for this distribution of contaminants calls for the hydraulic isolation of the source area followed by aggressive remediation of both the source and distal areas of the plume as needed to achieve remediation objectives most efficiently. In contrast to some of the currently perceived limitations of pump-and-treat remediation, our data and analyses indicate that distal portions of contaminant plumes can be expeditiously remediated, perhaps in less time than it took the contaminants to be transported to their preremediation locations.

Key Words: Hydrogeology • Groundwater Contamination • Remediation • Pump and Treat • Chlorinated Solvent Plumes • Hydrostratigraphy