QuantArray®-Chlor during Performance Monitoring

Background

The study site is a portion of a chemical manufacturing facility impacted by 1,1,2-trichloroethane (TCA), 1,2-dichloroethane (DCA), trichloroethene (TCE), and chloroform. During site assessment, QuantArray analysis revealed that key halorespiring bacteria including Dehalococcoides, Dehalobacter and Dehalogenimonas spp. were present. However, Dehalococcoides populations were below the 104 cells/mL recommended for effective reductive dechlorination. Therefore, biostimulation through electron donor addition was selected as the treatment option. See case study QuantArray-Chlor during Site Assessment and Remedy Selection for details.

QuantArray-Chlor Results and Interpretation

QuantArray-Chlor was routinely performed to provide a direct and timely line of evidence to assess the impact of electron donor addition on halorespiring bacterial populations. The figure below compares baseline populations (gray bars) at OW3 to concentrations of targeted halorespiring bacteria and functional genes detected 6 months after injection (light blue bars) and 12 months after injection (dark blue bars). Similar results were obtained for observation wells OSW5 and OSW5D.

(1)        Concentrations of Dehalococcoides and vinyl chloride reductase genes increased dramatically to concentrations greater than 106 cells/mL indicating the potential for complete reductive dechlorination of chlorinated ethenes including vinyl chloride.

(2)        Substantial increases in Dehalobacter and Dehalogenimonas populations were also evident demonstrating that electron donor addition also stimulated growth of these groups of halorespiring bacteria.

(3)        Dichloroethane reductase (DCAR) genes which had been below detection limits in baseline samples were detected 12 months after electron donor addition confirming growth of halorespiring bacteria capable of dechlorinating DCA to ethene.

(4)        Likewise, chloroform reductase genes were detected in post-injection groundwater samples indicating the potential for reductive dechlorination of chloroform to dichloromethane.

(5)        Finally, concentrations of Dehalobacter strains capable of degradation of dichloromethane also increased in response to electron donor injection.

 

Questions and Answers – Biostimulation Performance Monitoring

  • Did electron donor promote growth of halorespiring bacteria capable of biodegradation of each site contaminant?

Yes. Populations of Dehalococcoides, Dehalobacter, and Dehalogenimonas spp. increased substantially after electron donor addition. Furthermore, increases in concentrations of functional genes responsible for dechlorination of TCE, DCE, vinyl chloride, DCA, and chloroform were clearly evident.

  • Are they present at sufficient concentrations for effective biodegradation?

Probably. Dehalococcoides concentrations were substantially greater than the 104 cells/mL threshold recommended for generally effective reductive dechlorination. Correlations between degradation rates and concentrations of other halorespiring bacteria have yet to be established. However, the Microbial Insights Database can provide clients with additional insights needed to strengthen interpretation by comparing QuantArray results for specific gene targets against the results observed for samples submitted from other sites around the world. For example, Dehalobacter populations increased by over two orders of magnitude reaching 1.01E+04 cells/mL and an estimated percentile of 83% after electron donor addition. In other words, the concentration of Dehalobacter spp. detected at OW5 twelve months after electron donor injection was greater than the concentration detected in 83% of all other groundwater samples where Dehalobacter spp. were detected.

  • Will vinyl chloride be degraded?

With high concentrations of Dehalococcoides and vinyl chloride reductase genes reductive dechlorination of vinyl chloride to ethene is likely.

  • Is biostimulation an effective remedy?

Yes. Electron donor addition stimulated dramatic growth of key halorespiring bacteria capable of reductive dechlorination of biodegradation of the mixture of chlorinated contaminants present at the site.