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MULTIPLE LINES OF FIELD EVIDENCE TO INFORM FRACTURE NETWORK CONNECTIVITY AT A SHALE SITE CONTAMINATED WITH DENSE NON-AQUEOUS PHASE LIQUIDS
Parker, B.L., S.W. Chapman, K.J. Goldstein, and J.A. Cherry.
Groundwater in Fractured Bedrock Environments: Managing Catchment and Subsurface Resources. Geological Society of London, Special Publications 479:1-27(2018)

At a site contaminated with chlorinated solvents in shale, prior borehole testing in eight holes under open-hole ambient and pumping conditions identified 14 flow zones (140 m bedrock interval) with zero to five zones per hole. Cross-hole testing showed few cross-connections between transmissive fractures. The initial conceptual model thus featured a sparse fracture network with few dominant fractures. Detailed profiles (hydraulic head, rock core VOCs, groundwater VOCs from packer and multi-level sampling, cross-hole multi-level monitoring of permanganate injections) collected from several holes revealed a well-connected fracture network with many hydraulically active fractures not influenced by open-hole cross-connection. This contrasting conceptual model contained numerous well-connected horizontal and vertical fractures that allowed chlorinated solvents to penetrate the upper 50-60 m of bedrock as DNAPLs, followed by diffusion-driven mass transfer from fractures into the porous rock matrix, such that nearly all the contaminant mass resided as dissolved and sorbed phases measurable in rock core without cross-contamination during drilling. The difference in the two conceptual models has important implications for source zone and plume attenuation.



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