Evaluation of biostimulation using CENSUS qPCR quantification of Dehalococcoides and vinyl chloride reductase genes

Overview

The study site was a shallow aquifer impacted by chlorinated solvents. Historical groundwater monitoring indicated an accumulation of cis-dichloroethene (DCE). A baseline sampling event including CENSUS qPCR quantification of Dehalococcoides and vinyl chloride reductase genes was conducted to assess the potential for complete reductive dechlorination of TCE to ethene under existing site conditions and evaluate the need for electron donor injection.

Baseline Evaluation

  • Dehalococcoides and vinyl chloride reductase genes were detected in baseline samples but at concentrations below the threshold concentration of 1.00E+04 Dehalococcoides cells/mL recommended by Lu et al. (2006) for generally effective rates of reductive dechlorination.
  • Groundwater total organic carbon (TOC) and volatile fatty acid (VFA) concentrations were low suggesting Dehalococcoides populations and therefore reductive dechlorination were limited by a lack of suitable electron donors.

Remedy Performance Monitoring

A commercial electron donor was injected in the source area to stimulate growth of halorespiring bacteria and promote complete reductive dechlorination. CENSUS® qPCR quantification of Dehalococcoides and functional genes (tceA, bvcA, vcrA) was performed to monitor remedy performance.

  • During the baseline sampling event, Dehalococcoides and vinyl chloride reductase genes were detected but at concentrations below 1.00E+04 cells/mL suggesting that monitored natural attenuation (MNA) would not meet remediation goals in an acceptable timeframe.
  • Following electron donor addition, a substantial lag period was observed prior to a rebound in Dehalococcoides concentrations and the onset of enhanced reductive dechlorination.
  • Ultimately, the Dehalococcoides population increased to 106 – 107 cells/bd with a corresponding decrease in DCE.
  • Vinyl chloride concentrations temporarily increased due to the reductive dechlorination of DCE.
  • As indicated by the high number of Dehalococcoides spp. and vinyl chloride reductase genes however, microorganisms capable of reductive dechlorination of vinyl chloride were present.
  • Vinyl chloride concentrations decreased after the initial spike with a corresponding increase in ethene.

Questions answered by qPCR analysis

  • Is MNA possible?

Yes but probably not within an acceptable time frame due to low Dehalococcoides concentrations under baseline conditions.

  • After electron donor injection, why was there a decrease in the Dehalococcoides population and such a long lag period?

While not shown above, qPCR was also performed to quantify methanogens and sulfate reducing bacteria both of which compete with halorespiring bacteria for available electron donor. After electron donor injection, the methanogen population increased by over two orders of magnitude to over 1.00E+07 methanogen cells/mL. An initial lag period following an electron donor injection is not uncommon but can reduce stakeholder confidence in the selected remedy. In this case, qPCR analysis during performance monitoring indicated the temporary stimulation of competing microorganisms and eventually a rebound in the Dehalocococides population.

  • Was electron donor injection successful?

Yes. Dehalococcoides populations increased by over three orders of magnitude with concurrent production of daughter products.

  • Is complete reductive dechlorination to ethene likely? Will the observed spike in vinyl chloride concentration decrease?

Yes. While continued monitoring is recommended, Dehalococcoides populations remained much greater than the 1.00E+04 cells/mL threshold and vinyl chloride reductase genes were detected confirming the potential for complete reductive dechlorination.