Isotopic and geochemical analysis of microbial processes in uncontaminated and contaminated ground water and sediment: Innovative application of radiocarbon

Monitoring carbon dioxide (CO₂) or dissolved inorganic carbon (DIC) and methane (CH₄) produced in situ, and their radiocarbon (14C), stable carbon (δ13C) and deuterium (δD) signature provide a novel method to assess aerobic or anaerobic microbial processes. The objective was to determine the capability of carbon (δ13C and 14C) and deuterium (δD) isotopes to detect and quantify in situ anaerobic and aerobic biodegradation processes in the vadose zone and groundwater contaminated with petroleum hydrocarbons (PHCs) and chlorinated hydrocarbons (CHCs).; The objective was met by utilizing δ13C of CO ₂ to quantify aerobic phenanthrene biodegradation in laboratory microcosms and by combining δ13C and 14C measurement of vadose zone CO₂ or groundwater DIC in conjunction with contaminant measurement using gas chromatography (GC)-mass spectrometry to assess biodegradation of PHC and CHC from three field sites. Vadose zone O₂, CO ₂, CH₄, ethane and ethene concentrations were measured using a GC. Geochemical parameters were measured using hand-meters or colorimetric techniques in the field and in the laboratory. Vadose zone CO₂ and groundwater DIC were purified cryogenically for isotopic analysis and sent to analytical laboratories for 14C, δ13C, and δD analysis.; Combined δ13C and CO₂ measurement effectively quantified aerobic phenanthrene mineralization in laboratory microcosms when natural organic matter and phenanthrene δ13Cvalues were separated by only 3.3%. In the field, the anaerobic biodegradation rate of PHC was estimated between 0.017 to 0.055 mg/kg soil/day. Carbon isotopes indicated that fermentation of PHC was the sole source of CH₄ in the vadose zone while 47% of the total CO₂ was from PHC mineralization and 53% from plant root respiration. CHC-derived CO₂ accounted between 8 and 46% of total vadose zone CO₂ and between 0 and 8% of total DIC. 74% of vadose zone CH₄ was derived from CHC biodegradation. The estimated CHC mineralization rate was between 0.005 to 0.010 mg dichloroethylene (DCE)/kg soil/day and 1.1 × 10−4 to 3.4 × 10−5 mg DCE/kg soil/day in the vadose zone and groundwater, respectively. 14C and δ13C monitoring, combined with metabolite and geochemical monitoring elucidated many significant microbial processes occurring during intrinsic biodegradation of subsurface contamination.