THE USE OF BOEHOLE GEOPHYSICS FOR HYDRAULIC CHARACTERIZATION TO SUPPORT REMEDIAL DESIGN FOR DNAPL CONTAMINATION IN FRACTURED BEDROCK

Borehole geophysical logging techniques were used to collect hydrogeologic characterization data for the remedial design of a DNAPL contaminated fractured bedrock site. The site is located on the bank of a river adjacent to a waterfall. It is underlain by a thin veneer of unconsolidated glacial deposits and fill, and a sedimentary sequence consisting of well indurated shale and limestone. Investigations have been undertaken to characterize the site hydrogeology and provide the basis for the remedial design. The remedial goal for the site is to stop the migration of contamination, in both dissolved and NAPL phases, to the river.

Geologic data have been collected at the site from rock core and mapping bedrock exposures at the surface and in an existing tunnel. Monitoring wells and multi-level monitoring devices were installed in the unconsolidated deposits and bedrock to measure hydraulic head. Hydraulic testing has been done to estimate the hydraulic properties of the deposits underlying the site. A suite of borehole geophysical logs, consisting of natural gamma, single point resistance, spontaneous potential, caliper, fluid temperature, fluid resistivity, borehole flow meter, and acoustic and optical televiewer logs were collected in several boreholes to identify hydraulically-active fractures and determine the spacing, and orientation of the fractures. Geophysical data were used to help select monitoring intervals for the multi-level monitoring devices.

Two groundwater flow models were developed to integrate the hydrogeologic data collected at the site. A regional-scale equivalent porous media groundwater flow model was constructed, using MODFLOW, to evaluate the groundwater flux across the site and to estimate the flow rates and hydraulic capture zones for various remedial alternatives. A small-scale discrete fracture flow model was developed using the codes FRACMAN and MAFIC to evaluate the potential for hydraulic gradients to stop DNAPL discharge to the river. The fracture orientation data obtained from the geophysical logs and from outcrop and subsurface mapping, were used as the input for generating the fracture network for the discrete fracture flow model.