Bio-Trap® In Situ Microcosm Study to Evaluate Bioremediation Strategies at a Chlorinated Solvent Site

Overview

The study site was a shallow aquifer impacted by trichloroethene (TCE). Reductive dechlorination of TCE to cis-DCE was occurring based on historical groundwater monitoring but vinyl chloride and ethene concentrations remained low.

Questions

  • Is monitored natural attenuation (MNA) feasible?
  • Will electron donor injection (biosimulation) promote complete reductive dechlorination of TCE to ethene?

In Situ Microcosm (ISM) Study

In situ microcosm studies were performed in selected monitoring wells to evaluate MNA and biostimulation as site management strategies. Each assembly consisted of two ISM units:

  • A Control unit containing no additional amendments which simulated MNA conditions
  • A BioStim unit amended with a commercial electron donor

All units contained passive samplers (COC, GEO, and Bio-Trap) for characterization of the impacts of each treatment approach on subsurface chemistry, geochemistry, and microbial populations. After a 90 day deployment period, Bio-Trap ISMs were recovered from the monitoring well for qPCR quantification of Dehalococcoides and vinyl chloride reductase genes, contaminant concentrations, daughter product formation and geochemical parameters.

Targeted Microbial Populations (qPCR Results)

  • MNA Unit
    • Dehalococcoides and vinyl chloride reductase genes were detected under MNA conditions indicating the potential for complete reductive dechlorination.
    • However, the Dehalococoides population was relatively low.
  • BioStim Unit
    • The Dehalococcoides population detected in the BioStim unit was over one order of magnitude greater than detected in the MNA unit suggesting that electron donor addition would stimulate growth of halorespiring bacteria.
    • Likewise, vinyl chloride reductase gene copies were approximately one order of magnitude greater in the BioStim unit indicating the potential for complete reductive dechlorination to ethene.

Contaminant Concentrations

  • MNA Unit
    • TCE and cis-DCE concentrations were similar to those detected during the most recent groundwater sampling event.
    • Consistent with a low Dehalococcoides population, continued reduction of DCE to vinyl chloride and ethene was minimal.
  • BioStim Unit
    • Substantially increased production of DCE and the detection of vinyl chloride and ethene at low concentrations combined with a greater Dehaloccoides population suggested that biostimulation enhanced reductive dechlorination.

Answer

  • Is monitored natural attenuation (MNA) feasible?

Not within a reasonable timeframe. Dehalococcoides were detected but at a concentration well below the threshold value proposed by Lu et al. (2006) for acceptable rates of reductive dechlorination.

  • Will electron donor injection (biosimulation) promote complete reductive dechlorination of TCE to ethene?

Yes. Biostimulation resulted in the growth of Dehalococcoides, increases in vinyl chloride reductase genes, and increased production of daughter products including ethene indicating that electron donor addition enhanced reductive dechlorination relative to MNA conditions.

 

Lu, X., J. T. Wilson and D. H. Kampbell (2006). “Relationship between Dehalococcoides DNA in ground water and rates of reductive dechlorination at field scale.” Water Research 40(16): 3131-3140.