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 to that original project was aimed at demonstrating the utility of electrical resistivity imaging for monitoring the long-term impacts of bioremediation. Resistivity imaging was performed at the original demonstration site approximately 7 years after bioamendment injections were conducted. There were three primary objectives:

1. Identify the long-term geophysical footprint of active bioremediation at a volatile organic compound-contaminated site. By doing so, assess the long-term spatial extent of the altered zone using surface-based geophysical imaging techniques.
2. Determine the significance of the geophysical footprint with respect to solid phase mineral transformations and/or biofilms induced by the treatment process. This would enable geophysical alterations within the treatment zone to be interpreted in terms of biogeochemical impacts, which can then be mapped in space and time using geophysical imaging with limited sampling.
3. 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.

Electrical geophysical imaging is a method of remotely estimating the distribution of subsurface electrical properties by 1) measuring the response of the subsurface to a low-frequency current transmission, and 2) recovering the distribution of electrical properties that gave rise to the response. In a typical electrical geophysical survey, four-electrode measurements are used. Two of these serve as current electrodes, and two of these as potential (or measurement) electrodes. These electrodes are typically made of metal (predominantly stainless steel, even though graphite electrodes are also used). A voltage (typically 10-200 V) is applied to the current electrodes, resulting in an induced potential distribution within the subsurface, which is measured across the potential electrodes. Many such measurements are strategically collected over an array of 10’s to 100’s of electrodes to generate a full survey. Each data set comprising a survey is then analyzed via tomographic inversion to produce an estimate (or image) of subsurface conductive and capacitive properties. Repeating this process in a continuous sequence constitutes time-lapse imaging, whereby changes in electrical properties are interpreted in terms of some subsurface process of interest. The goal of this project was to assess the utility of static electrical geophysical imaging for studying the long-term impacts of bioremediation, as stated by the project objectives.

Objective #1: Identify the long-term 3D spatial extent of the biogeochemically altered zone

Borehole logs, particularly magnetic susceptibility, successfully identified anomalous regions along wellbores in treated zones that were in clear contrast with corresponding measurements in untreated zones. The team considered this objective to have been successfully demonstrated.

Objective #2: Determine the significance of the geophysical footprint with respect to treatment induced biogeochemical transformations

This effort identified geochemical contrasts between the untreated and treated zones that clearly originated with the bioremediation injections that occurred in 2008. It also established evidence of the relationships between biogeochemical and geophysical anomalies, including the relationship between magnetic susceptibility and Fe²⁺ and Fe³⁺ ratios, and between precipitated minerals (or biofilms) and surface conductivity. However, those relationships were not conclusive enough to interpret the geophysical data in terms of specific biogeochemical impacts. Therefore, the team considered this objective only partially achieved.

Objective#3: Map biogeochemical impacts based on geophysical footprints

Objective 3 was the culminating objective of the demonstration and required successful demonstration of both objectives 1 and objectives 2. Because objective 2 was not fully demonstrated, objective 3 was not achieved.

For this project, the team was unable to collect geophysical data that permitted the change in geophysical properties to be directly imaged in the same way they were for ER-200717, a former ESTCP project. This is because the demobilization and well abandonment procedure used at the conclusion of ER-200717 did not permit re-occupation of the original monitoring/ERT boreholes, and therefore did not permit deployment of electrodes in the same positions as used in ER-200717. Equivalent electrode positions are required for time-lapse imaging.  Consequently, the team was not able to subtract pre-treatment geophysical properties from the geophysical images and were not able to produce the changes in geophysical properties caused by the bioremediation process alone.  The team was required to interpret both the geophysical and geochemical with confounding influence of background, pre-treatment heterogeneity, rather than interpreting changes in geophysical properties with changes in geochemical properties caused exclusively by bioremediation processes. The team considers the confounding influence of background heterogeneity (both geochemical and geophysical) to be primary cause of its failure to achieve objectives 2 and 3.

Kessouri, P., T. Johnson, F.D. Day-Lewis C. Wang, D. Ntarliagiannis, and L.D. Slater. 2022. Post-Remediation Geophysical Assessment: Investigating Long-Term Electrical Geophysical Signatures Resulting from Bioremediation at a Chlorinated Solvent Contaminated Site. Journal of Environmental Management, 302(A):113944. doi.org/10.1016/j.jenvman.2021.113944.