Image: Temperature monitoring system comprised of a) thermocouples, b) a vertical array of thermocouples, and c) installation of array with direct-push methods. (Graphics courtesy of Colorado State University)
The most cost-effective remedial solutions are those that take maximum advantage of natural source zone depletion (NSZD) and natural attenuation, the remediation processes that are courtesy of Mother Nature. Measuring the heat generated by microbes breaking down petroleum-based contaminants can provide accurate, continuous monitoring of naturally occurring contaminant mass loss, without the inconvenience and expense of repeated site visits by technicians. Those were two key themes from a recent project that TRC experts, along with specialists from Colorado State University (CSU), presented at Battelle’s Fourth International Symposium on Bioremediation and Sustainable Environmental Technologies, Insights from Continuous Monitoring of LNAPL Natural Source Zone Depletion Rates for Over Two Years.
Our work involved a 15-acre site with contamination from fuel hydrocarbons present as light non-aqueous phase liquids (LNAPL). To measure the NSZD rates of the LNAPL, we initially performed multiple carbon dioxide flux measurement events. This revealed significant variability in how rapidly LNAPLs were depleting in different areas.
To better track both the short-term and long-term variability, in May 2014 we installed a thermal monitoring system (developed by CSU researchers), the first in the country for continuous tracking of NSZD rates. The temperature signal being tracked was from oxidation of methane, the byproduct of methanogenesis, the dominant process of intrinsic bioremediation in the LNAPL-impacted soils. The system involved installing strings of multi-depth thermocouples vertically through the LNAPL zone at multiple locations. Through a wireless connection to a data logger, heat data collected by the thermocouples were downloaded remotely each day. Temperature data were assessed within the framework of an energy balance, which generated estimates of NSZD rates and the volume of LNAPL being eliminated from the site. This approach has proven reliable, convenient, and cost-effective. This approach documented that reductions of LNAPL through NSZD were much higher than mass removal rates being achieved through the operation of an LNAPL recovery system that had been installed at the site. In fact, based on the NSZD rates documented at the site, the state regulatory agency approved the decommissioning of the LNAPL recovery system and approved the combination of NSZD of the LNAPL zone and monitored natural attenuation (MNA) of the associated aqueous phase plume as the remedial strategy for the site.
Since our system went into service, about a dozen more of these systems have been installed around the United States. As we described in our presentation at Battelle, our experience is that this approach to continuous monitoring of LNAPL NSZD produces more informative data than alternative approaches and costs less than the other alternatives when long term tracking of the NSZD rate is required. Because demonstrating NSZD is far less costly than engineered remediation systems, TRC believes this biogenic heat monitoring approach will be increasingly used in LNAPL release management.
I'd like to thank my TRC colleague Eric D. Emerson, and Dr. Tom Sale and Kayvan Karimi Askarani of Colorado State University, for their collaboration on this project.