Microfracture Surface Characterizations: Implications for In Situ Remedial Methods in Fractured Rock

Purpose: The Bedrock Bioremediation Center (BBC) at the University of New Hampshire is a center specializing in multi-disciplinary research on bioremediation of organically-contaminated bedrock aquifers. The focus of its present work is a field research-based program conducted at Site 32 at the Pease International Tradeport (formerly Pease Air Force Base) in Portsmouth, NH. The U.S. EPA supports the overall mission of the BBC to (i) examine whether microbial communities in organically-contaminated bedrock aquifers are capable of biodegrading the contaminants, (ii) more efficiently and economically characterize the direction of groundwater flow and fracture patterns (size, direction, secondary mineralization) in contaminated bedrock aquifers, (iii) improve and develop new field technologies to control hydraulic and flow conditions in the contaminant zone, (iv) develop laboratory and field methods to estimate and accelerate in situ rates of bioremediation of organic contaminants in bedrock aquifers, and (v) to develop and apply innovative microbial, molecular biology and other advanced techniques to enhance in situ bioremediation and assess the efficacy of remediation strategies. One of the major outreach efforts of the BBC is to transfer information gained during its research to federal, state, and local regulatory agencies and environmental consultants. Background: Site 32 contains a contaminant plume of trichloroethylene (TCE) and its transformation products dichloroethylene (DCE) and vinyl chloride (VC). These are the principal contaminants. The site is situated on a variable thickness upper sand layer overlying a marine clay layer overlying a variable thickness lower sand layer. These unconsolidated layers are situated over the Kittery Formation, a tightly folded, biotite-grade partially metamorphosed sandstone and shale crosscut by numerous porphryitic diabase dikes. The contaminant plume extends downward and laterally northeast ~ 0.5 km via migration through weathered and competent bedrock. The groundwater in the bedrock is predominately contaminated with cis-DCE (280-440 µg/L) with some trans-DCE (26-48 µg/L), TCE (24-59 µg/L), and VC (8-22 µg/L). Since 1997 the overburden has been managed using a sheet pile containment system coupled with pump and treat. The bedrock groundwater zone was given a technical impracticability (TI) waiver. Research Questions: The overarching questions addressed by this portion of the project relate to possible relations between microfracture networks in the bedrock, the surface geochemistry of these microfractures, and the ecology and metabolic activity of attached microbes relative to terminal electron accepting processes and TCE biodegradation. Questions include the following: (1) How does the microfracture surface influence attachment and growth? (2) How does the geochemistry of the microfracture surface influence population ecology and metabolism? (3) What is the relationship between the relatively high specific surface area of the microfracture network and the adjacent relatively open and more voluminous open fracture system? More specifically, how does the microfracture surface influence the dominant terminal electron acceptor processes in the microfracture network? (4) Lastly, what is the precise nature of TCE biodegradative processes within the microfracture network? As part of the overall research plan to better understand these questions, we studied 11 microfractures extracted from competent bedrock cores from two wells at Site 32 (BBC5 and BBC6) so as to characterize, with a variety of surface spectroscopic and microbial techniques, the relation, if any, between microfracture surface geochemistry and the ecology and metabolic activity of attached microbial populations relative to terminal electron accepting processes or to chlorinated solvent biodegradation. Results are highlighted relative to host rock and microfracture mineralogy and geochemistry, groundwater geochemistry, microfracture microbiology, and terminal electron accepting processes