QuantArray®-Chlor during Site Assessment and Remedy Selection

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. Daughter products including 1,2-dichloroethene (DCE), vinyl chloride, and ethene have been routinely detected suggesting that reductive dechlorination has occurred to some extent historically. With a complex mixture of chlorinated compounds, the possibility of inhibition, as well as the potential for multiple and converging biodegradation pathways, QuantArray®-Chlor was integrated into groundwater monitoring to evaluate biodegradation processes under existing site conditions, potential remediation options, and ultimately performance monitoring (QuantArray-Chlor during Performance Monitoring).

QuantArray-Chlor Results and Interpretation

The Microbial Population figure for observation well OW3 shown below presents the concentrations of target poulations compared to typically observed concentrations to give an overview of the potential biodegradation pathways.

(1)        Halorespiring bacteria known to be capable of reductive dechlorination of TCE, TCA, and DCA were detected at moderate concentrations at OW3 under existing site conditions which was consistent with historical groundwater monitoring.

(2)        Although present at low concentrations, halorespiring bacteria and functional genes responsible for anaerobic biodegradation of DCE, vinyl chloride, and chloroform were also detected further indicating the potential for reductive dechlorination of contaminants of concern at the site.

(3)        One gene (a ring hydroxylating toluene monooxygenase RDEG) was detected at a moderate concentration in the OW3 groundwater sample.

(4)        However, concentrations of most functional genes encoding enzymes capable of co-oxidation of chlorinated ethenes under aerobic conditions were below the laboratory detection limits indicating limited potential for cometabolism under existing subsurface conditions.

Overall, the QuantArray results for OW3, OW5, and OW5D indicated the potential for anaerobic biodegradation rather than aerobic cometabolism. The concentrations of specific target microorganisms and functional genes responsible for reductive dechlorination of chlorinated compounds were then examined more closely to evaluate potential site management strategies.

(5)        Populations of Dehalobacter spp. on the order of 102 to 103 cells/mL and Dehalogenimonas spp. of 104 cells/mL indicated the potential for biodegradation of TCA and DCA. Biodegradation of TCA and DCA by these halorespiring bacteria proceeds by dichloroelimination leading to the production of vinyl chloride and ethene, respectively.

(6)        Multiple bacterial groups capable of reductive dechlorination of TCE to cis-DCE including Dehalobacter, Desulfuromonas, Dehalococcoides, and Desulfitobacterium spp. were also detected but complete dechlorination of DCE and vinyl chloride to ethene was unlikely.

(7)        Dehalococcoides (DHC) and vinyl chloride reductase genes (BVC and VCR) were detected, Dehalococcoides concentrations were well below the 104 cells/mL recommended by Lu et al. (2006) for generally effective rates of reductive dechlorination. Therefore, vinyl chloride produced from dichloroelimination of TCA by Dehalobacter and Dehalogenimonas spp. may accumulate.

(8)        Chloroform reductase genes were below the laboratory detection limit but Dehalobacter strains capable of fermentation of dichloromethane were detected at OW5S and OW5D.

 

Questions and Answers – Site Assessment and Remedy Selection

  • What biodegradation pathways (anaerobic and aerobic) are likely to be occurring?

A diverse population of halorespiring bacteria including moderate concentrations of Dehalogenimonas spp. was detected in groundwater samples from each monitoring well suggesting anaerobic biodegradation was the predominant mechanism. Conversely, concentrations of many of the functional genes involved in aerobic cometabolism of chlorinated ethenes (e.g. methane and ethene monooxygenases) were below detection limits indicating limited potential for aerobic co-oxidation under existing conditions.

  • The site is impacted by a mixture of chlorinated ethanes, ethenes, and methanes – Are halorespiring populations capable of reductive dechlorination of each of these contaminants of concern present?

Yes. Dehalogenimonas spp. detected at moderate concentrations and Dehalobacter spp. which were also present utilize chlorinated ethanes as growth supporting electron acceptors converting 1,1,2-TCA and 1,2-DCA to vinyl chloride and ethene, respectively. Dehalococcoides and vinyl chloride reductase genes (BVC and VCR) were detected indicating the potential for complete reductive dechlorination of TCE, DCE and vinyl chloride but concentrations were low under existing site conditions.

  • Are they present at sufficient concentrations for effective biodegradation?

No. Dehalococcoides populations were well below the threshold of 104 cells/mL proposed by Lu et al. (2006) for generally effective rates of reductive dechlorination. Although Dehalobacter spp. were present, chloroform reductase gene copies were below detection limits under existing site conditions suggesting reductive dechlorination of chloroform would be limited.

  • Will vinyl chloride accumulate?

Probably. The moderate concentrations of Dehalogenimonas spp. suggested that dichloroelimination of 1,1,2-TCA is leading to the production of vinyl chloride observed in routine groundwater monitoring. Although Dehalococcoides spp. capable of reductive dechlorination of vinyl chloride are present, concentrations were probably too low and vinyl chloride is likely to accumulate.

  • Is MNA feasible? Is biostimulation with electron donor addition needed? Bioaugmentation?

MNA is not a feasible site management strategy – aerobic cometabolism was highly unlikely and the low concentrations of Dehalococcoides suggested that vinyl chloride from anaerobic biodegradation of TCA would accumulate. However, since Dehalococcoides, vinyl chloride reductase genes and other halorespiring bacteria were detected at least at low concentrations, bioaugmentation was not needed. Biostimulation through electron donor addition was selected as the remedial action at the site. See the case study QuantArray-Chlor during Performance Monitoring to see how QuantArray results were used to evaluate the effectiveness of the subsequent electron donor injections.