Incidental releases of trichloroethene (TCE) at a former electronics manufacturer located in east central Ohio impacted shallow overburden and bedrock groundwater. Site hydrogeology includes 15 to 20 feet of alluvial silty clay or gravelly sand overlying fractured sandstone, with an estimated 10-foot thick interval of dense, non-aqueous phase liquid (DNAPL) trapped in the primary and secondary porosity of the shallow bedrock. Dissolved TCE was detected in bedrock groundwater at concentrations between 55 and 550 milligrams per liter (mg/L) along with the dechlorination byproduct cis-1,2-dichloroethene (cis-1,2-DCE) at up to 15 mg/L; however, vinyl chloride (VC) was not detected above method detection limits ranging between0.25 and 10 mg/L. A pump & treat system is operating to manage plume migration; however, the responsible party opted to segue to a more sustainable enhanced reductive dechlorination (ERD) remedy targeting DNAPL destruction. A proof-of-concept study was initiated in 2012 utilizing ERD-Enhanced™, a carbon-carbohydrate based biostimulant formulated with a proprietary blend of macro-micro inorganic nutrients, to stimulate biotic reductive dechlorination by providing electron donor and enhancing relationships between dechlorinators and fermenters which increase both the rate and effect residence time of TCE dechlorination. The additive appears to enhance biosurfactant production, biofilm development, which assists in the increase of the bulk fraction of dechlorinators. Bioaugmentation was not performed given the baseline detection of ethene at all test wells demonstrated the presence of dechlorinators which express the genes required to complete the TCE dechlorination pathway. Proof-of-concept results from the three-year study demonstrated: 1) an 80-99.9%Reduction in TCE concentrations relative to baseline concentrations which were approximately 30% of TCE’s aqueous solubiliy limit; 2) consistent indicator parameters of a biotic dechlorination pathway; 3) increased ethene concentrations following biostimulation demonstrating reaction completion; and 4) the presence of Dehalococcoides mccartyi and the dehalogenase genes tceA, bvcA, and vcrA at biostimulated well locations indicating dechlorinators are present expressing these benefical genes facilitating the TCE→ethene completion pathway. Based on these encouraging results, a pilot study was initiated in 2015 utilizing two bedrock injection wells with a supporting performance network of eight bedrock wells using the same biostimulant to collect pre-design data for a full-scale remedy. Over the first 20 months, the pilot study injection achieved the following results within the study zone:
1) a 70-99%Reduction in TCE concentrations at each of eight wells with baseline concentrations up to 130 mg/L, approximately 10% of TCE’s aqueous solubility limit, with four of eight wells exhibiting up to 99%Reductions;
2) an up to 95%Reduction in cis-1,2-DCE concentrations at six of eight wells providing no evidence of a “cis-stall” in most of the study zone;
3) VC concentrations either decreased or remained constant at five of eight wells;
4) up to two orders-of-magnitude increases in chloride concentrations at seven of eight wells consistent with an active dechlorination pathway for TCE DNAPL;
5) ethene was detected during 40-80% of post-injection sampling rounds for all eight wells demonstrating dechlorination was progressing to completion, consistent with the molecular diagnostic results from the earlier study;
6) dissolved iron and manganese concentrations increased at all eight wells indicating Redox conditions decreased to at least ferrogenic and manganogenic reduction, respectively; and
7) while pH was significantly depressed at a few wells during certain rounds, values have generally remained around neutrality. Collectively, the performance data suggest that biostimulation with this additive achieved robust dechlorination of TCE DNAPL without the “cis stall” often seen when biostimulation is not coupled with bioaugmentation. Moreover, while many additives achieve a one order- of magnitude (90%Reduction) reduction in DNAPL constituents, reductions on the order of two orders-of-magnitude, typically achieved using ERDenhanced™, are far less common. The authors hypothesize that the up to 99%Reduction in TCE concentrations observed using ERDenhanced™ may be the result of additive optimized efficiencies between multiple bacterial populations to yield faster rates of dechlorination as well as longer effect residence times.

Primary Author / Conference Presenter:
Kent Armstrong
TerraStryke Products, LLC
Andover, New Hampshire, USA

*date & time subject to change