Incidental trichloroethene (TCE) releases at a former electronics manufacturer impacted shallow 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 shallow bedrock. Dissolved TCE was detected 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 detection limits. A proof-of-concept biostimulation study was initiated utilizing ERDenhancedâ„¢, a carbon/carbohydrate-based biostimulant formulated with a proprietary blend of inorganic macro-micro nutrients to stimulate biotic reductive dechlorination and increase the residence time of its beneficial effect by leveraging endogenous decay. Bioaugmentation was not performed given the baseline detection of ethene. Results from the three-year study demonstrated: 1) an 80-99.9%Reduction in TCE concentrations relative to baseline concentrations up to approximately 30% of TCE’s aqueous solubility limit; 2) in two of three performance wells, up to 99%Reduction in cis-1,2-DCE concentrations, with no change at a third well despite a 99.9%Reduction in TCE; 3) consistent indicator parameters of a robust biotic dechlorination pathway; 4) increased ethene concentrations following biostimulation demonstrating reaction completion; and 5) the presence of Dehalococcoides mccartyi and the dehalogenase genes tceA, bvcA, and vcrA at biostimulated performance wells indicating dechlorinators are present expressing these benefical genes facilitating the TCE→ethene completion pathway. Following the period of active biostimulation, ongoing monitoring of passive biostimulation by endogenous decay demonstrated a positive effect residence time of at least three years. Importantly, no observed TCE rebound was noted in the well network during the period monitoring endogenous decay, including the well with the baseline concentration of 550 mg/L. In fact, TCE concentrations at that well during this period generally ranged from approximately 0.07 to 2.6 mg/L1, with a geometric mean of 0.8 mg/L, which represents a 99.9% Reduction from baseline concentrations. In addition to the extended effect residence time, while many ERD additives achieve a one order-of magnitude (90%Reduction) reduction in DNAPL constituents, reductions on the order of two orders-of-magnitude, as achieved using ERDenhancedâ„¢ during this proof-of-concept, are far less common. The authors hypothesize that the up to 99%Reduction in TCE concentrations observed are the result of combined microbial behaviors induced by the introduction of the additive. An expanded injection-based pilot study is currently underway as part of the collection of pre-design data for a full scale biostimulation enhanced reductive dechlorination remedy.
Primary Author / Conference Presenter:
Kent Armstrong
TerraStryke Products LLC
Andover, New Hampshire, USA

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