Monitored natural attenuation (MNA) is a potential alternative for management of large, diffuse perchlorate plumes in a cost-effective manner. Natural attenuation is defined by the USEPA as the “biodegradation, diffusion, dilution, sorption, volatilization, and/or chemical and biochemical stabilization of contaminants to effectively reduce contaminant toxicity, mobility or volume to levels that are protective of human health and the environment”. The term MNA refers to the reliance on natural attenuation processes, within the context of a carefully controlled and monitored site cleanup, to achieve site-specific remedial goals. The overall goal of this project was to evaluate the potential for MNA of perchlorate and identify conditions for use of MNA as a remedial technology. More specifically:
- Demonstrate to regulatory agencies through field study that perchlorate MNA can be an effective method for managing impacts of perchlorate released to the environment; and
- Provide DoD managers with the tools needed to evaluate whether MNA may be appropriate for management of perchlorate–impacted groundwater on their site(s).
A variety of DoD sites that had known perchlorate releases and where it was suspected that natural attenuation was occurring were identified. Samples from seven sites were subjected to laboratory testing and microcosm studies to estimate potential bioactivity on perchlorate. The use of enzymatic activity assays and isotopic ratio measurements as indicators of perchlorate biodegradation was evaluated in laboratory incubations. Further, the rate and extent of MNA was assessed at two field sites: (1) near Building 1419 at Indian Head Naval Surface Warfare Center (NSWC), Indian Head, MD; and (2) at the TCE/Perchlorate Solid Waste Management Unit (SMWU) field site at a manufacturing facility in Maryland. From these studies, a protocol for monitoring the natural attenuation of perchlorate was developed.
To assess the demonstration sites, the project used the tiered approach developed and described in the Perchlorate MNA Protocol prepared during this project. The Protocol guides the end-user through the process of developing multiple lines of evidence to support perchlorate MNA. It includes the following steps: (1) Tier 1 - Plume Stability and Geometry, (2) Tier 2 - Biogeochemical Parameters and Biological Indicators, and (3) Tier 3 - Biodegradation Rates.
At the Indian Head site, trends in groundwater flow, biogeochemical parameters, microbial populations, and perchlorate concentrations indicated that perchlorate attenuates mostly as a result of non-biological mechanisms near the presumed source and areas downgradient from the source, but prior to discharge to Mattawoman Creek (a large tributary of the Potomac River). As contaminated groundwater moved away from the source area toward the discharge zone along the creek bank, perchlorate was shown to biologically degrade in the intertidal, organic-rich Littoral Zone. Low oxidation-reduction potential, elevated total organic carbon, reduced competition with nitrate, pH>5.5, and the presence of perchlorate-reducing bacteria provided conditions conducive to biodegradation. Biodegradation rates were calculated by several methods and were generally reproducible, providing supporting lines of evidence for natural bioattenuation.
At the Maryland manufacturing site, the perchlorate in a commingled trichloroethene (TCE)/perchlorate plume on the east side of the manufacturing facility has attenuated slowly over time. There was some evidence that perchlorate decreased in several source area wells, but TCE appeared to have remained largely unchanged for over 3,400 ft from the source. The apparent decrease in perchlorate was likely a result of the combination of abiotic attenuation processes, an ongoing pump-and-treat system in the area, and enhanced anaerobic reductive dechlorination from a bioremediation pilot study conducted years ago. There was little change in perchlorate in the mid-plume area, but as the plume approaches its end at Little Elk Creek, the intermediate and shallow aquifers merge and contaminated groundwater migrates vertically until it discharges to the creek. The conditions within the riparian buffer alongside the creek were not optimal for biodegradation of perchlorate, but were nonetheless more conducive to biodegradation than the areas downgradient of the source and throughout the mid-plume. Consequently, sufficient biodegradation of perchlorate was observed to keep it from entering the creek, while TCE was transformed minimally throughout the same area and was reported both in and just beyond the creek. Perchlorate biodegradation rates were calculated; however, bioattenuation timeframes were measured in decades.
A potential issue that may occur with this technology is the need for more detailed site characterization to demonstrate attenuation, which may result in more complex and costly up-front investigation. MNA of perchlorate may require institutional controls to ensure long-term protection, thus the long-term performance monitoring will generally be more expensive and last for a longer time period. In addition, the potential exists for continued contaminant migration and/or cross-media transfer of contaminants, as well as a re-evaluation of MNA due to changing site conditions over time. Finally, public acceptance for this technology may be more difficult and costly to obtain.
The laboratory and field demonstrations performed as part of this project demonstrated the potential for using MNA as a groundwater remedy for perchlorate, and thus effectively provide DoD managers and industry professionals with the tools needed to promote acceptance of this technology among regulatory agencies.