BOREHOLE GEOPHYSICS AND DEVELOPMENT OF A CONCEPTUAL HYDROGEOLOGICAL MODEL: CASE STUDY OF PLUME MIGRATION IN MARTINSBURG SHALE AT THE AVTEX FIBERS SUPERFUND SITE

The Avtex Fibers Superfund Site is a former viscose rayon-manufacturing facility that operated from 1940 until 1989, adjacent to the Shenandoah River in Front Royal, Virginia. An extensive remediation program is underway to address abandoned plant buildings and over 120 acres of waste impoundments. Of primary concern are three viscose waste basins that are the source of a dense aqueous-phase liquid (DAPL) plume that has migrated from the basins into the Martinsburg Formation and beneath the Shenandoah River. The DAPL leachate has a high pH (12–13 s.u.), a density of 1.06 g/cm³, and contains high concentrations of carbon disulfide, ammonia, arsenic, and antimony. Plume migration is strongly controlled by high-permeability structural features that align with fold axes and bedding strike, but which diverge from the piezometric head gradient by nearly 45 degrees.

To develop an understanding of plume migration, hydrogeological investigations at the site have included structural geologic mapping, logging of over 1400 feet of core, downhole geophysics, geochemical sampling, and hydraulic testing and pumping test evaluations. Five critical deep bedrock wells were cored, with core recoveries exceeding 95%. Geophysical logging included caliper, natural gamma, SP, resistivity, heat-pulse flowmeter, and acoustic televiewer tools. The data produced from these efforts were integrated to develop a highly refined conceptual model that defines meso-scale folding and associated faulting, zones of increased fluid transmissivity related to fold hinges, reverse fault zones, and bedding planes. The evidence indicates that the interaction of high-permeability structural features—including bedding plane partings, fault zones, and extensional joints within west-verging, overturned folds—control plume migration. Because of the great depth of the plume, the geophysical logs provide a more cost-effective method of mapping aquifer features, such as vertical flow gradients and flow zones, than could be attained by routine subsurface investigations.