Remediation. 2020;30:55–62. wileyonlinelibrary.com/journal/rem © 2020 Wiley Periodicals, Inc. | 55 DOI: 10.1002/rem.21640 RESEARCH ARTICLE Carbon isotope forensics for methane source identification Richard Coffin 1 | James Mueller 2 1 Department of Physical and Environmental Sciences, Texas A&M University, Corpus Christi, Texas 2 Provectus Environmental Products Inc, Freeport, Illinois Correspondence Richard Coffin, Department of Physical and Environmental Sciences, Texas A&M University, Corpus Christi, NRC‐3506, 6300 Ocean Drive, Corpus Christi, TX 78414. Email: richard.coffin@tamucc.edu Abstract Methane (CH 4 ) in ecosystems originates from ancient petroleum formed deep within the earth and/or via microbial fermentation of organic carbon and subsequent reduction of carbon dioxide (CO 2 ). Given the complexity of different ecosystems, origins of CH 4 present can be difficult to determine. This issue was realized in a situation where an antimethanogenic in situ chemical reduction (ISCR) remedial amendment containing organic carbon plus zero‐valent iron was applied to treat chlorinated solvents in groundwater at a former dry cleaner facility. The technology rapidly and effectively reduced the concentration of tetrachloroethene in groundwater thus meeting project goals without the stoichiometric accumulation of catabolites such as trichloroethene (TCE), cis‐1,2‐dichloroethene, or vinyl chloride and without excessive methanogenesis (e.g., <2 mg/L) in the treated area. However, approximately 9 months after treatment, increased levels of CH 4 (from 5 to 10 mg/L) were observed downgradient from the treated area. The applied remedial amendment contained approximately 60% (weight basis) fermentation organic carbon and was therefore a potential source of this CH 4 . However, there was <500 mg/L total organic carbon in groundwater emanating from the upgradient treatment area which was unlikely sufficient to produce that much CH 4 . Moreover, the soil gas also contained benzene, toluene, ethylbenzene, and xylenes and other gasoline constituents. These data suggested that the presence of three gasoline/ diesel underground storage tanks that were previously closed in place with no active remediation performed could be the source of elevated CH 4 . Thirdly, there were sewer lines, utilities, multiple gasoline stations, and industrial activities in the immediate area. With an initial assumption that CH 4 source(s) could include the ISCR amendment over stimulation of production, gasoline sourced CH 4 from nearby leaking lines, or sewage from local fractured pipes, carbon isotope analyses—radiocarbon (Δ 14 C) and stable car- bon ( δ 13 C) —were coupled with CH 4 and CO 2 concentration data from groundwater samples to determine the origin of respired carbon. The δ 13 C range for carbon sources respired in the process would be approximately -26.5‰ to -33.0‰ for the ISCR amendment and total petroleum hydrocarbons (TPH) residuals, respectively. Δ 14 C is approximately 0‰ and -999‰ for the ISCR amendment (young carbon) and TPH (old carbon), respectively. The isotopic signature of respired gasses confirmed that elevated CH 4 downgradient of the treated area originated primarily from sewer gasses (or fer- mentation of liquids released from sewer lines). This study provides an overview of the capability to apply carbon isotope geochemistry to confirmation of remedial protocols and sources of anthropogenic carbon pools that conclusively identify the origin of CH 4 in a complex ecosystem undergoing a remedial action.