What Are Your Data Telling You?

A Budget and Schedule Streamlining Review & Summary of Anaerobic Reductive Dechlorination of Chlorinated Hydrocarbons

Several previous articles discuss simplifying and streamlining environmental data acquisition and evaluation.

Mindful consideration of the information covered in the aforementioned articles can increase project margins and provide tools for avoiding some common pitfalls, such as:

  • Misinterpretation of nondetect data as an indication that a site is not contaminated when samples have been diluted such that quantitation limits (QLs) are too high to support the conclusion. This can occur because
    • Matrices are not amenable to the analyses.
    • Contamination concentrations require large dilutions.
    • Analysts are reluctant to analyze samples at lesser dilutions due to not understanding project purposes, inexperience, to avoid instrument maintenance, or to protect expensive and delicate instrumentation.
  • Failure to plan for an approach to achieve site closure when it is not possible to achieve detection limits (DLs) less than regulatory levels
    • Because regulatory levels are usually established using toxicological studies, not instrumental analyses, current methodologies may not be able to achieve these values.
    • Identifying the approach for addressing this issue during the project planning phase, and securing client and regulatory approval of the approach when the plan is finalized, is more efficient than securing approval after samples have been analyzed.
    • Additionally, planning for this issue allows for identification of more sensitive methodologies if they exist and performance of a cost/benefit analysis for the use of such methods with cooperation of clients and regulators.
    • Equally as important, it provides a tool to avoid misinterpretation of such data as an indication that the site is contaminated.
  • Attempting to remediate to levels less than native background.
    • Sometimes Federal regulatory levels are less than native levels (ex: arsenic in the West) and it is crucial to perform site assessments against background data (or perform background studies) in such cases.
  • Long-term monitoring of common contaminants that have been misidentified as contaminants of concern (COC). In such cases, assessment of historical data is necessary and several sampling rounds may be required to identify and gain approval for eliminating these parameters.

These pitfalls are easily avoided with proper assessment of site data and data requirements. However, site data can provide far more information. This may require analysis of parameters that are not COCs so careful planning is important, because analysis for parameters that do not provide relevant information wastes time and money and does not support the protection of human health and the environment.

Natural attenuation is one example of a remediation process where analysis for non-COCs can provide valuable information. Natural attenuation occurs when naturally occurring processes reduce contamination in soil and groundwater. These processes occur in situ and include dilution, dispersion, volatilization and other natural processes. Monitored natural attenuation (MNA) is an approved remedy at some sites and involves collection of data to assess and document the efficacy of the attenuation process.

When MNA is in place, non-COC data can be used to confirm that conditions are amenable to attenuation and the presence of breakdown products. Aerobic or anaerobic conditions (or one and then the other), specific pH values, and the presence of specific metals and/or microbes may be needed. Natural attenuation may be enhanced through forcing a site to anaerobic conditions, introducing bacteria and/or feeding native bacteria, and/or temporarily altering the pH. In each case, data must be collected to determine if the desired conditions have been achieved, and subsequent data must be collected to determine if attenuation was enhanced.

Contaminants that may undergo natural attenuation include chlorinated solvents, certain metals, radionuclides, and oil & gas-related aromatic hydrocarbons such as benzene, toluene, ethylbenzene, and xylenes (BTEX). For this article, the MNA process to be further considered is anaerobic reductive dechlorination (ARD) of chlorinated hydrocarbons, specifically ARD of the volatile organic compounds (VOCs) tetrachloroethene (PERC or PCE) and trichloroethene (TCE).

The primary or initial contaminant for this process may be either PCE or TCE. These contaminants are present in the environment due to past use as degreasers, dry-cleaning agents, and through use in manufacturing processes. The ARD process breaks down PCE to TCE, and TCE subsequently breaks down to cis-1,2,-dichloroethene (DCE) and trans-1,2-DCE. 1,1-DCE may also be produced. The DCE isomers break down to vinyl chloride, and vinyl chloride breaks down to ethene. If all PCE and/or TCE breaks down to ethene, remediation is complete, because ethene is not an environmental risk.

To assess if ARD is occurring, break down products are included in the analytical data set. If TCE (and/or PCE) concentrations are decreasing and vinyl chloride concentrations are increasing, the process is successfully progressing. Regulatory criteria for vinyl chloride are stringent and the concentrations and may exceed the action limit; however, additional actions are not needed to address the exceedances unless the process stalls at this stage or  if vinyl chloride is also present because manufacturing at the site included polyvinyl chloride (PVC) production. .

Because MNA can be unacceptably time-consuming, often taking many decades to progress, various enhancement processes, as previously noted, have been implemented. The ARD process may be bio-attenuated with the microorganisms Dehalococcoides (abbreviated as DHC or DHE), which can be naturally occurring or inoculated into a site. Even when naturally occurring, the native population may not be great enough to speed ARD to the desired extent. In such cases, the microorganisms may be fed with vegetable oil or molasses. The table below shows some of the analyses that may be used at an ARD site.

ARD Table

Additional information about ARD and various analytical parameters can be found at the EPA Clu-In website, here. The information at the link provides more detailed and extensive information as well as links to additional resources. However, in the experience of the authors, if results show that ARD is occurring, assessing other parameters at the frequency indicated at the Clu-In link is not necessary. Some of the tabulated analyses listed are high-cost specialty analyses that may not add value for your specific project. If you are unsure of the frequency and type of analyses needed at your site, we invite you to contact Oak Services, LLC for a consultation.

 References & Resources

2007, Agency for Toxic Substances & Disease Registry (ATSDR), https://www.atsdr.cdc.gov/csem/csem.asp?csem=15&po=5, November.

2014, ATSD, https://www.atsdr.cdc.gov/phs/phs.asp?id=263&tid=48, October.

1999, United States Environmental Protection Agency (USEPA) https://www.epa.gov/sites/production/files/2014-02/documents/d9200.4-17.pdf, April.

2012, USEPA, https://clu-in.org/techfocus/default.focus/sec/bioremediation/cat/Anaerobic_Bioremediation_(Direct), -.

2001, Unites States Geological Survey (USGS), Natural Attenuation Strategy for Groundwater Cleanup Focuses on Demonstrating Cause and Effect, January.

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