According to current regulations in the USA, at least two lines of evidence are required to support selection of monitored natural attenuation (MNA) as a site remedy (e.g., USEPA, 1998; 2012). The first line of evidence requires analysis of historical data that demonstrate a clear and meaningful trend of decreasing contaminant concentrations over time. The second line of evidence is required to “demonstrate indirectly the type(s) of natural attenuation processes active at the site, and the rate at which such processes will reduce contaminant concentrations to required levels.”
For many years, degradation of chlorinated hydrocarbons via MNA primarily focused on biological dechlorination… More recently, abiotic reductive dechlorination has attracted considerable attention….
For many years, degradation of CVOCs via MNA primarily focused on biological dechlorination mechanisms. More recently, based on numerous laboratory and field studies, abiotic reductive dechlorination mediated by certain minerals has attracted considerable attention as a MNA degradation mechanism for CVOCs. These findings were summarized in a USEPA report titled “Identification and Characterization Methods for Reactive Minerals Responsible for Natural Attenuation of Chlorinated Organic Compounds in Ground Water” (USEPA, 2009). Since that publication, significant progress has been made in understanding abiotic degradation of CVOCs which has resulted in regulatory recognition of these processes as part of MNA (e.g., He et al., 2015).
The new understanding of MNA degradation mechanisms for CVOCs was reflected in the development of a Quantitative Framework and Management Expectation Tool for the Selection of Bioremediation Approaches (MNA, Biostimulation and/or Bioaugmentation) at Chlorinated Solvent Sites (Lebrón et al., 2015). This ESTCP project was designed to provide an update to the existing MNA evaluation framework that incorporates the recent research results. This allowed for the development of BioPIC, an open source software tool that uses a hierarchical set of questions to identify the MNA degradation mechanism (i.e., the second line of MNA evidence) for degradation of chlorinated ethenes at a subject site.
BioPIC requires input of a set of biogeochemical parameters, including standard geochemical analytes, as well as microbial tests (Dehalococcoides 16S rRNA gene and reductive dehalogenase gene abundances) and solids analyses (magnetic susceptibility) to evaluate the impacts of microbial and abiotic processes on the observed MNA degradation rates of chlorinated ethenes at a given site. All microbial and solids analyses required by BioPIC are provided under SiREMNA™. It should be noted that BioPIC is limited to chlorinated ethenes and the abiotic mechanisms are limited to degradation by magnetite (detected by magnetic susceptibility measurements).
Degradation by iron sulfides, which have been shown to be much more reactive than magnetite is considered for trichloroethene (TCE) only. SiREMNA™ includes additional analytical tools that can be used to evaluate the second line of MNA evidence at sites impacted by other CVOCs and considers abiotic degradation by other reactive minerals, including iron sulfides. The relevant SiREMNA™analytical package can be developed based on site-specific conditions.
ESTCP (Environmental Security Technology Certification Program) 2015. Quick BioPIC User Guide.
He, Y.T., J.T. Wilson, C. Su, and R.T. Wilkin. 2015. Review of abiotic degradation of chlorinated solvents by reactive iron minerals in aquifers. Groundwater Monitoring and Remediation 35, no. 3: 57–75.