Borehole Geophysics and Hydrophysical Logging of Fractured Bedrock at The Alark Hard Chrome Facility, Riverside, California

Borehole geophysical investigations were conducted at the Alark Hard Chrome Superfund Site during the spring of 2003 and winter 2004. The Alark site housed a former electroplating facility in Riverside, California. Operations, including the dumping of electroplating solutions into a 40-foot deep dry well and storm drains. These activities released hexavalent chromium solution to the soil and underlying fractured bedrock. Previous investigations have found up to 17 mg/L in a well located about 80 ft southwest of the facility. The most contaminated monitoring wells are screened in granitic bedrock; however, the migration pathway is yet to be determined. Current investigations are being conducted to validate previous investigation results, examine extent of lateral and vertical contamination, and to evaluate the hydraulic characteristics of the fractured bedrock for the purpose of remediation. Data to be presented will highlight results of the borehole geophysical and hydraulic investigations of five existing and three new boreholes. Five existing PVC-cased monitoring wells (MW-2, MW-3, MW-7, MW-8, and MW-9) were<br>investigated in the spring of 2003. Geologic parameters were determined using a natural gamma and electromagnetic induction tool. This tool documented overburden thickness of 11 to 22 feet. Weathered bedrock extends from 17 to 30 feet below ground surface and ranges in thickness from 2 to 20 feet. The screened intervals were subsequently investigated to define hydraulic properties using an advanced hydrophysical tool (NxHpLTM) and a Scanning Colloidal Borescope (SCBS). Zones of preferential flow were identified in MW-2, MW-3 and MW-7.<br>Three new bedrock wells were drilled in December 2003. Each of these wells extends to about 60 feet below the bedrock-alluvium interface. These wells were geophysically investigated in January 2004 with PVC casing extending to just below the top of bedrock. Geophysical investigations included caliper, acoustic and optical televiewer, natural gamma, electromagnetic induction, SP, single point resistivity and long- and short-normal resistivity. Hydrologic evaluation of these wells was conducted using the same hydrophysical tool. Hydraulically conductive intervals identified with hydrophysical logging with were then isolated using an advanced wireline straddle packer (WSP). The WSP provided both interval specific formation fluid sampling and hydrologic information. To evaluate the large-scale hydraulic connections between the tested wells, a pump test was conducted at the conclusion of the WSP testing and sampling.<br>The preliminary results of this study suggest that previously undetected preferential contaminant pathways exist in both the saturated and unsaturated portions of the fractured bedrock. This spatial evaluation of contaminant distribution, permeability and contaminant mass flux, will provide the basis for remedial activities.