The overarching objective of this project was to develop and validate a framework that can be used to make bioremediation decisions based on site-specific physical and biogeochemical characteristics and constraints. This framework represents an update to the U.S. Environmental Protection Agency’s (EPA) Technical Protocol for Evaluating the Natural Attenuation of Chlorinated Solvents in Ground Water.

Technology Description

The quantitative framework developed is a systematic approach that uses the relationships between specific biogeochemical parameters and degradation rates to deduce major degradation pathways and determine the best bioremediation approach at sites impacted with chlorinated ethenes. The major element of this demonstration was to quantify the relationship(s) between selected, measurable biogeochemical screening parameters and both biotic and abiotic degradation rates. Evaluating these relationships enabled the development of the quantitative framework. In turn, the quantitative framework enabled the development of BioPIC, a software tool that guides users (e.g., remedial project managers [RPMs]) through a hierarchical set of questions to ultimately identify the optimal pathway for remediating chlorinated ethenes at a particular site. BioPIC is an easy-to-use decision tool that informs RPMs about relevant biogeochemical parameters and their impacts on degradation pathways and rates at a given site.

A number of measurable parameters, such as the concentrations of volatile organic compounds (VOCs); alternate electron acceptors (e.g., oxygen, sulfate); reduced products (e.g., ferrous iron {Fe[II]}, methane [CH4]); Dehalococcoides(Dhc) 16S rRNA gene and reductive dehalogenase (RDase) gene abundances; and magnetic susceptibility, affect the detoxification of chlorinated ethenes. The relationships between each parameter and the degradation rates were determined and used to develop the decision matrix and BioPIC.

Demonstration Results

The quantitative framework was developed by compiling available data from multiple sites with different biogeochemical backgrounds across the United States. For those sites where sufficient hydrogeologic, geochemical, and microbial data were available, degradation rates for different chlorinated ethenes were calculated using BIOCHLOR. The calculated degradation rates for the chlorinated ethenes tetrachloroethene (PCE), trichloroethene (TCE), cis-1,2-dichloroethene (cDCE), and vinyl chloride (VC) were plotted against multiple measurable parameters, as discussed in the Final Report.

This analysis revealed that the following parameters correlated well with the degradation rates of TCE, cDCE, and VC:

  • Dhc abundance for TCE, cDCE, and VC.
  • Mass magnetic susceptibility as a surrogate to magnetite abundance.
  • Ferrous sulfide (FeS).
  • Methane (CH4).
  • Fe(II).

Correlations between the following parameters were also identified:

  • VcrA + bvcA gene copies per liter and Dhc copies per liter for cDCE and VC.
  • A ratio of Dhc to total bacterial 16S rRNA genes exceeding 0.0005 correlates with ethene formation.
  • A ratio of vcrA + bvcA genes to total bacterial 16S rRNA genes exceeding 0.0005 correlates with ethene formation.
  • A ratio of Dhc to vcrA + bvcA near unity correlates with ethene formation.

These ratios are useful normalized parameters for predicting detoxification. Validated quantitative real-time polymerase chain reaction (qPCR) assays to obtain this information are commercially available.

No correlations were observed between dissolved oxygen concentrations and reductive dechlorination rates, proving that dissolved oxygen data are problematic and unreliable for determining anoxia and the potential for anaerobic degradation activity. Although dissolved oxygen is known to be inhibitory to strict anaerobes, such as those that perform reductive dechlorination, difficulties in sample collection and analysis negate the use of this parameter alone to deduce anoxic conditions and therefore conclude that anaerobic microbial reductive dechlorination is a major pathway. The measurement of Fe(II) and CH4 concentrations are more reliable parameters to determine the availability or, perhaps more importantly, the lack of dissolved oxygen to predict oxidative versus reductive degradation processes.

In summary, this project identified a short list of parameters that are measurable, quantifiable, and useful for deducing degradation pathways. Further, these parameters can be used to estimate site-specific degradation rates and thereby help users decide what treatment options will best meet the remedial action objectives.

Implementation Issues

The decision framework and the management expectation tool (BioPIC) are based on the current scientific understanding of the processes contributing to the detoxification of chlorinated ethenes. Although process understanding has significantly improved over the past decade, knowledge gaps remain.

In some cases, the investigator may not want to expend the resources to fully implement the decision framework. For example, when the investigator has worked through the decision framework and will not be able to proceed without magnetic susceptibility data, the RPM may not want to expend the resources to collect soil core data, thus negating further use of the tool to deduce degradation pathways. Development of downhole technologies for obtaining magnetic susceptibility data, such as through use of a downhole sonde, would circumvent this potential problem with implementation of the decision framework, including BioPIC. Additionally, the decision framework does not consider the potential for aerobic degradation of TCE.

Also, the decision framework only considers chlorinated ethenes. Separate decision frameworks for the chlorinated ethanes and chlorinated methanes could be developed to round out the toolkit to provide similar decision frameworks.