Contaminated sites in fractured rock are particularly difficult and expensive to remediate, because characterization and monitoring is problematic in the presence of extreme heterogeneity. The performance objectives of this demonstration focused on evaluating: [1] fracture network characterization using a fractured rock geophysics toolbox (FRGT); [2] autonomous monitoring of amendment delivery and subsequent contaminant biodegradation using geophysical technologies that sense beyond the borehole; [3] application of an ‘informed inversion strategy’ to improve the geophysical imaging of fractured rock settings relative to what can currently be achieved with off-the-shelf functionality. Specific performance objectives were largely met, although the physical characteristics of the primary demonstration site, being the Naval Air Warfare Center (NAWC) in W. Trenton, NJ, limited the performance of some methods in the FRGT. The primary benefit of the FRGT is the ability to provide information on variations in physical properties and the fate of amendment injections into fractured rock beyond the vicinity of local borehole observations. The potential impact on DoD operations relates to improved management decisions that can result from an improved understanding of flow and transport processes at fractured rock site, particularly the effectiveness of amendment treatments in targeting contaminants of concern.

In the FRGT, geophysical characterization data are fed into the processing of geophysical monitoring datasets in order to provide appropriate constraints on the inversion and regularization of the data/images resulting in predictions of the transport of amendments and/or progress of biodegradation beyond the vicinity of boreholes. The characterization data include information from established borehole logging instruments that provide high resolution information on physical properties close to the borehole and less established between borehole imaging methods that capture the continuity of structures beyond individual boreholes. The FRGT incorporates multiple geophysical techniques as it is based on the fundamental premise that there is no silver bullet with respect to geophysical technologies and that multiple methods must be tested at a particular site to determine the ones that will provide the most information beyond the boreholes. At the NAWC demonstration site, the most effective technology for characterization and monitoring beyond the borehole was cross-borehole electrical resistivity tomography (ERT). The performance objectives associated with informed inversion therefore focused on advancing the utility of ERT for characterization and monitoring in fractured rock subject to constraints provided by other technology components of the FRGT.

The demonstration focused on characterization and monitoring using a dense array of seven boreholes each drilled through ~70 ft of unweathered rock. A critical part of the demonstration was the design, development and testing of a first-of-its-kind integrated array containing electrodes, packers and injection/sampling ports. This array was constructed to demonstrate in situ ERT monitoring of amendment injections/longer term biodegradation occurring in fractured rock whilst maintaining hydraulically and electrically isolated intervals in all seven boreholes. The demonstration also advanced the functionality of E4D, a high-performance computing code for the inversion of large ERT datasets. Advancements focused on implementation of new regularization constraints that favor site conditions in fractured rock and provide flexibility for incorporating information on the boreholes. Limited laboratory studies were performed to determine the most favorable (from an ERT imaging perspective) amendment substrate from the candidates highlighted as effective at the site. The field demonstration primarily focused on an intensive multi-method downhole and crosshole geophysical characterization campaign followed by a suite of tracer tests and a longer term amendment injection that were imaged with ERT.

The FRGT produced an unprecedented image revealing the continuity of relatively permeable zones within approximately 600 m³ of rock. The images resolve the alternating sequence of laminated and massive mudstones at the site and the results are validated by borehole logging and crosshole hydraulic testing datasets. Time lapse ERT monitoring was able to monitor the evolution of injected tracers and amendments within targeted fracture zones that control the flow and transport characteristics of this site. The time-lapse ERT images capture strong evidence for channelized flow occurring within the fracture zone and provide unique information on the efficacy of targeted amendment emplacement. The images also offer the possibility of estimating fracture surface area impacted by an amendment treatment, with direct implications for the performance of such remediation treatments with respect to remediation goals. Limited testing of the FRGT at a second site, the Eastland Woolen Mill, Corinna, ME, highlighted the potential benefits of directional cross borehole ground penetrating radar for imaging the continuity of major fracture zones beyond borehole walls.

Most of the technologies explored in the FRGT are not subject to any specific regulations beyond what is typical for working in boreholes at contaminated sites and acquiring samples. However, continuous open holes are needed for ERT to be effective in fractured rock. Some states regulate the length of open holes to prevent cross contamination between multiple fractures or aquifers connected to the borehole. In this study, a deviation was readily obtained from the state. The specific borehole technology developed in the course of this research—centered on integrated electrode/sampling/packer arrays—helped to address the open-hole regulatory implementation issue, as fractures were hydraulically isolated during tracer experiments and electrical monitoring. Metal borehole casings would prevent the effective use of most geophysical techniques included in the FRGT, although some of the tools in the FRGT can operate effectively though PVC casing.

This demonstration involved an extensive technology transfer effort where end-user concerns were specifically addressed through lectures, field demonstrations and hands-on Q&A sessions with individuals. In total, the tech-transfer courses directly engaged 230 remediation professionals and regulators via short courses approved for continuing education credits. These efforts revealed that end users were typically poorly equipped to make informed decisions about the likely appropriateness of specific components of the FRGT based on the conditions of a particular site. In order to address this implementation issue, an Excel-based decision support tool was developed to provide recommendations for the selection of specific geophysical techniques for a given project objective and subject to the constraints imposed by the site conditions. Information gained from project ER-2421 was used to inform this project.

Day-Lewis, F. D., Johnson, C. D., Slater, L. D., Robinson, J. L., Williams, J. H., Boyden, C. L., & Werkema, D. (2016). A Fractured Rock Geophysical Toolbox Method. Groundwater, 54(3), 315–316. http://doi.org/10.1111/gwat.12397

Robinson, J., Johnson, T. and Slater, L.D., 2015, Challenges and Opportunities for Fractured Rock Imaging Using 3D Cross Borehole Electrical Resistivity, Geophysics, 80(2), E49-E61

Robinson*, J., Johnson, T. and Slater, L.D., 2013, Evaluation of known-boundary and resistivity constraints for improving cross-borehole DC electrical resistivity imaging of discrete fractures, Geophysics, 78(3), D115-D127, doi: 10.1190/geo2012-0333.1

Robinson*, J., Slater, L., Johnson, T and Binley, A., 2013, Strategies for characterization of fractured rock using cross-borehole electrical imaging tomography, The Leading Edge, 32(7), 784–790

Robinson, J., Slater, L., Johnson, T., Shapiro, A., Tiedeman, C., Ntlargiannis, D., Johnson, C., Day-Lewis, F., Lacombe, P., Imbrigiotta, T. and J. Lane Jr., 2015, Imaging pathways in fractured rock using three-dimensional electrical resistivity tomography, Groundwater, 54(2), DOI: 10.1111/gwat.12356