Abstract: Ground Water Plume Migration in a Complex Hydrogeologic Setting at Lawrence Livermore National Laboratory (LLNL), Site 300

MADRID, VIC, Weiss Associates, Emeryville, CA; ZAFER DEMIR, Weiss Associates;RICHARD LANDGRAF and FRED HOFFMAN, Lawrence Livermore National Laboratory

We are evaluating the influence of two separate ground water flow systems on plume migration in a complex hydrogeologic setting at Lawrence Livermore National Laboratory, Site 300. Site 300 is an 11-square mile Department of Energy experimental test facility and superfund site located in the semi-arid, rugged terrain of the Altamont Hills near Tracy, California.

Ground water flow at Site 300 occurs mainly in two distinct aquifer types, alluvial and bedrock. The Quaternary alluvial aquifers are 10-30 feet-thick and consist of sand and gravel deposited in southeast- to northeast-trending canyons that ultimately flow into the San Joaquin Valley. The bedrock aquifers occur in the Miocene Neroly Formation, a 500+ foot-thick, fluviallacustrine sequence consisting of interbedded claystone, siltstone, sandstone, and conglomerate.

The alluvial aquifers are characterized by high hydraulic conductivity and flow velocity. Ground water flow is controlled by channel geometry and contaminant transport is dominated by advective and dispersive processes due to high flow velocities. In contrast, the bedrock aquifers are characterized by low hydraulic conductivity and low ground water flow velocity. Ground water flow is controlled by bedrock structure and contaminant transport is strongly influenced by diffusive and sorptive processes due to very low flow velocities.

Plume migration at the site is influenced by the seasonal interaction between these two flow systems. Evidence demonstrating the interaction between these two flow systems and their impact on trichloroethylene and tritium plumes will be presented to illustrate these concepts.

Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California