Abstract: Structural and Stratigraphic Controls on Contaminant Migration in Miocene Shallow Marine Sandy Siltstones at the Stanford Linear Accelerator Center, Menlo Park, California

WITEBSKY, SUSAN N., Stanford Linear Accelerator Center, Menlo Park, CA; andKENNETH D. EHMAN, Groundworks Environmental, Inc., Santa Clara, CA

Contaminant migration in groundwater through fractured porous media is complex, and its characterization requires a detailed understanding of the site hydrogeologic system. As part of an ongoing remedial investigation at the Stanford Linear Accelerator Center (SLAC), a wide variety of geologic and geophysical data sets have been collected. These data sets have been integrated to develop a hydrogeologic model of the site. Hydrochemical and chemical transport mechanisms were then incorporated into the model to identify the contaminant migration pathways.

The SLAC site is underlain by Miocene shallow marine sediments consisting of fractured, semi-consolidated, sandy siltstones and discontinuous well-cemented sandstone beds and lenses. Various historic activities at SLAC resulted in surface and shallow subsurface releases of chlorinated solvents that have migrated through fractured bedrock into the shallow groundwater. Data collected at the site include detailed geologic outcrop maps, continuous core, surface and downhole geophysics, geotechnical and hydrogeologic measurements, and groundwater chemistry. The integration of all these data sets is critical in developing a model of the hydrogeology and contaminant transport mechanisms. The model demonstrates that the migration of contaminants is more controlled by fractures than stratigraphy. The sandy siltstone bedrock is homogenized by bioturbation and thus is a poor conduit for contaminant migration. Fracture width and density are related to variations in Ethology and larger structural features (anticlines, synclines, and faults) that are mapped on and off site.

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