Stable and Radio Isotope Analysis Identified Sources of Methane During an In-Situ Remedial Action

Methane can originate thermogenically from regions of petroleum formation within the earth and/or via microbial fermentation of fermentable carbon and subsequent microbial reduction of carbon dioxide. Hence, the origin of methane is not always clear. Such was the situation where Provect-IR® antimethanogenic ISCR reagent was applied to treat chlorinated solvents in groundwater at a former dry cleaner facility. The technology rapidly and effectively reduced the concentration of PCE in groundwater thus meeting the project goals without the accumulation of catabolites (TCE, DCE or VC) and without excessive methanogenesis (e.g., <2 mg/L) in the treated area. However, about 9 months post-treatment, methane (5 to <10 mg/L) was observed downgradient of the treated areas. The source of the methane was potentially a result of the remedial action. Alternatively, the methane was not a direct result of the remedial action, noting: i) the area of interest was directly associated with gasoline/Diesel USTs (removed but no active remediation was performed), ii) the soil gas contained CH4 along with BTEX and other gasoline COIs, and iii) there was <500 ppm TOC in groundwater emanating from the upgradient treatment area (unlikely to produce that much CH4). In addition, there were sewer lines, utilities, multiple gasoline stations, and operation industrial activities in the immediate area.

Carbon isotope analyses - radiocarbon (Delta¹⁴C) and stable carbon (del¹³C) – where coupled with methane (CH₄) and carbon dioxide (CO₂) data from groundwater samples to determine the origin of the respired carbon. The del¹³C range for carbon sources respired in the process would be approximately -26.5‰ and 33.0‰ for Provect-IR and TPH residuals, respectively. Delta¹⁴C is approximately 0‰ and -999‰ for Provect-IR (young carbon) and TPH (old carbon), respectively. The data confirmed that the methane originated primarily from sewer gasses with some contribution from the remedial amendment proximal to its application area.