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Demonstrating a Biogeophysics Strategy for Minimally Invasive Post Remediation Performance Assessment
Dr. Tim Johnson | Pacific Northwest National Laboratory
ESTCP project ER-200717, which was led by the Naval Facilities Engineering Service Center, demonstrated the use of geophysical techniques to provide near real-time, noninvasive, and cost-effective information on the spatial and temporal distribution of amendments. The technology used electrical resistivity measurements from a series of wells to detect changes in electrical conductivity. Electrical resistivity monitoring is particularly useful for enhanced bioremediation because the amendment solutions used for bioremediation increase the electrical conductivity of the subsurface significantly above the background conductivity. Time-lapse electrical resistivity monitoring can delineate where amendments were initially delivered, as well as track their migration and depletion over time. Near real-time information is particularly valuable because it can allow modifications and/or additional injections while equipment is still present on site.
This follow-on effort is aimed at demonstrating the utility of electrical resistivity imaging for monitoring the long-term impacts of bioremediation. In 2016, resistivity imaging will be performed at the original demonstration site approximately 7 years after bioamendment injections were conducted. The specific objectives of this follow-on demonstration are:
- Identify the long-term geophysical footprint of active bioremediation at a volatile organic compound (VOC) contaminated site. This will demonstrate assessment of the long-term spatial extent of the altered zone using surface-based geophysical imaging techniques.
- Determine the significance of the geophysical footprint with respect to solid phase mineral transformations and/or biofilms induced by the treatment process. This will enable interpretation of geophysical alterations within the treatment zone in terms of biogeochemical impacts, which can then be mapped in space and time using geophysical imaging with limited sampling.
- Demonstrate the use of 1 and 2 above to map gradients in the geophysical footprints of biostimulation along a transect crossing the boundary of the treatment area at an active remediation site, and interpret those gradients in terms of long-term biogeochemical impacts.
This effort will involve returning to the Brandywine DRMO to assess the long-term impacts of bioremediation using the same methods as those used in the HPMS system, but with a focus on understanding amendment effects rather than real-time monitoring of amendment emplacement. Specifically, electrical resistivity imaging surveys will be conducted both inside and outside of the original treatment zone, including the area originally monitored under ER-200717. Limited geochemical sampling and laboratory core analysis on both treated and untreated sediment will be conducted to identify the geophysical signatures of biomineralization. These signatures will then be used to assess the field-scale geophysical images in terms of long-term bioremediation impacts.
Current technologies for assessing the success of emplacement primarily rely on direct measurements in wells. Such measurements are expensive and time-consuming, and they provide only limited spatial and temporal information. As demonstrated under the original project, the HPMS provides two main benefits: first, far fewer wells will be required for understanding amendment distributions, particularly when surface electrodes are deployed, leading to significant cost savings (20 to 50% or greater per site), and second, rapid identification of missed target zones will enable optimal amendment application and thus increase efficiency. Overall, the system will provide low-cost, near-real-time, volumetric information that will reduce overall monitoring cost and increase monitoring performance. (Anticipated Project Completion - 2018)