Various remediation methods have been used to bring contaminated sites to closure. Thermal methods, pump and treat, bioremediation, and chemical injection, to name a few. What has frustrated environmental consultants is that it seems that each method has its limits, and often when treatment is temporarily stopped, the contamination rebounds, often to pre-remediation levels. Some methods are better at containing plumes, but are not effective in treating the source. Other methods, such as thermal, treat the source but are not particularly effective stopping the flow of the contaminant plume.
On Thursday, February 8, James Cummings of the United States Environmental Protection Agency (EPA) conducted a webinar on the method of Combined Remedies. Mr. Cummings has 30 years experience in site remediation. He has mostly worked on innovative tools for characterization and remediation of recalcitrant sites.
Cummings has found that the best approach to remediating sites is to make temporal and spatial changes to remediation technologies during the remediation process. Often, it is best to use different technologies for the source and the plume.
Different contaminants have different physical properties, and are transported differently in groundwater. Cummings has developed methods that he calls “package deals” – where two chemicals can be injected into the subsurface during the same remediation event. This cuts cost by reducing the number of mobilizations to a site.
Often, sites with low levels of contamination are remediated by monitored natural attenuation (MNA). However, it has been noted that the decrease in contaminant concentrations behaves asymptotically as regulatory or screening levels are approached, drawing out the MNA process and delaying closure. Then, late in the remediation process, some sort of chemical injection technology is used to finally decrease concentrations below the desired levels. Cummings has found that MNA is most suitable when the source is treated early in the remediation process.
At sites contaminated with chlorinated solvents, thermal treatment of the source zone has been shown to work effectively with ISCO – In Situ Chemical Oxidation. That is because the ISCO releases bioavailable dissolved phase carbon, and dechlorinating bacteria thrive best at 30 to 35 degrees Celsius (86 to 95 degrees Fahrenheit). Overall, ISCO is best coupled with bioremediation. Furthermore, microbial evaluation should be conducted throughout the remediation process, not just sporadically after particular injection or thermal events. This does add to the cost of remediation, but evaluating the groundwater for microbial activity at regular intervals has been found to provide data that the consultant can use to alter the remediation regime, and in the long run saves money by facilitating closure.
Cummings gave several examples of combined technologies that have been found to work. One is a blend of permanganate and persulfate. These chemicals are usually injected as liquids, at different remediation events. Cummings has found that these two chemicals can be combined together into a sold paraffin wax matrix, and then inserted into the subsurface with a direct push drill rig. This semi-solid slowly dissolves into the groundwater, and acts as a sustained release – sort of like a time-release medicine capsule for a contaminated site.
Cummings also promoted the use of activated carbon with ISCO, particularly BOS 100 iron-impregnated carbon marketed by RPI. Another product that Cummings has found to be useful is Plume Stop by Regenesis. Plume Stop is colloidal carbon with bio-amendments.
These remediation techniques are not inexpensive. They are a considerable one- or two-time time cost. I have found that some property owners are not willing to pay for these modern remediation techniques, in hopes that MNA or a one-time mobile extraction event will clean up their site. However, it has been found repeatedly that in the long run it is less costly to aggressively remediate the site, rather than wait years and hope that the site will clean up itself.