Environmental remediation approaches can vary widely for different projects. Based on several factors (potential contaminants, exposure pathways, lithology at the Site, neighboring properties, project stakeholder risk tolerances, timing goals, etc.), there are numerous remediation alternatives to explore. Does the Site require active remediation such as excavation and off-Site disposal or in-situ chemical injections? Or perhaps a passive remediation tactic using natural attenuation and Institutional Controls such as Environmental Restrictive Covenants (ERC) is adequate.
One of the quickest and most proven methods is thermal remediation. Although thermal remediation is perceived to be expensive, a cost analysis comparing this aggressive method with short-term confirmatory sampling versus limited remediation with long-term stewardship needs may prove surprising. Further, as thermal remediation continues to be conducted as part of contaminant cleanup projects, environmental regulators are becoming more familiar and comfortable with this approach.
What is Electric Resistance Heating
Electric Resistance Heating (ERH) is an in-situ thermal remediation technique that heats the soil and groundwater to destroy and/or remove contaminants. Electric current is passed between subsurface electrodes and the resistance to the flow causes heat. The subsurface temperature surpasses the boiling point of water, steam/vapor is formed, and the volatile contaminants are removed. The steam and stripped contaminants are captured within a series of vapor recovery points.
The Use of ERH at a Specific Site in Central Indiana
A former dry-cleaner for several decades, this particular Site was grossly impacted with tetrachloroethylene (PCE) in both subsurface soil and groundwater. In fact, one soil sample contained PCE six orders of magnitude higher than the applicable screening level (SL) and PCE in groundwater exceeded an applicable SL by four orders of magnitude) – all of this after five years of enhanced vacuum extraction (a different type of active remediation)!
Following a very robust subsurface investigation (which included several mobilizations to the Site), obtaining a good understanding of the Site-specific Conceptual Site Model (CSM), evaluating numerous remediation alternatives, and conducting an in-depth cost analysis, ERH was chosen for the remediation at the Site. A collaborative effort was needed from all stakeholders, including the potential responsible party (PRP), insurance carriers, oversight consultants, ERH vendor, and regulatory agency.
Following final design and approval, the subsurface ERH installation (consisting of installing 47 electrodes co-located with shallow vapor recovery ports, a drip-line system, and several deep vapor recovery wells) took approximately two months to complete. The surface installation (including a 20-milimeter insulation barrier to retain heat, a pad-mounted transformer, the Power Control Unit, the vent system, and security fence) took another approximately two months to complete. Security and health and safety checks, which confirmed that no voltage was short-circuiting and energizing near-by metal objects such as the neighboring fence was conducted over a week-long ramp-up procedure.
Following only six months of operation, over 10,000 pounds of PCE was removed from the subsurface by the ERH remediation, which equaled a 99.9999% reduction in known contamination. The impacts that were orders of magnitude above applicable SLs were cleaned up to below SLs or even non-detect. A one-year equilibration timeframe was implemented to allow the subsurface to cool to ambient conditions and a two-year confirmation sampling regime (including sampling from the monitoring well network a total of eight quarters) confirmed that the contamination had been remediated. The Site has obtained closure from the regulatory agency.
There are a plethora of different remediation approaches one could choose from to cleanup a contaminated Site. ERH is one such alternative that should be evaluated. It’s proven effective, regulators are becoming more familiar and comfortable with the approach, and in many cases reduces or eliminates the long-term stewardship needs (and in turn, reduces long-term costs).
ERH was utilized at a Site in central Indiana and after six months of operation, PCE impacts were reduced 99.9999%. Further, a short-term confirmation sampling program (eight quarters total) provided enough evidence that the ERH was extremely effective in remediating the Site. Since that time the regulatory agency has provided closure for the Site.