USE OF GEOPHYSICAL INVESTIGATIONS TO IMPROVE SITE UNDERSTANDING AND REMEDIATION AT A FRACTURED BEDROCK SITE IN CENTRAL MAINE

Characterization of fracture networks in crystalline bedrock terrains where dissolved phase contamination is present is crucial to implementation of effective containment systems. At this site in Central Maine contaminated with dissolved phase chlorinated volatile organic compounds, surface geophysics, angled borings and well installations, extensive borehole geophysical logging, packer interval testing, and groundwater extraction tests allowed development of a comprehensive site conceptual model (SCM). Surface geophysics (a very low frequency [VLF] survey) coupled with fracture trace studies were completed to select initial well locations. Borehole geophysical surveys were conducted using caliper, fluid temperature, fluid resistivity, acoustic televiewer, and heat-pulse flowmeter logs to refine the understanding of the depth, aperture, and orientation of bedrock fractures. The borehole geophysical logging and packer test results were used to characterize likely water-producing rock fractures for final screen interval selection. A pumping test conducted following well installation demonstrated that the characterization of the bedrock was sufficient to construct a containment system to prevent migration of source area groundwater. Once the feasibility of on-site containment was demonstrated, source area containment with off-site institutional controls was approved and selected as the final remedy for the groundwater at the Site. Results of a second, seven-day pumping test conducted following completion of the groundwater hydraulic containment system construction in 2011 were used to confirm locations for additional hydraulic performance monitoring wells. Screen intervals for those wells were selected based on results of borehole geophysical surveys.