Radiocarbon Assessment of Aerobic Petroleum Bioremediation in the Vadose Zone and Groundwater at an AS/SVE Site

Abstract

Radiocarbon ( 14 C) measurement from vadose zone air and groundwater is an alternative technique to quantitatively estimate aerobic microbial CO 2 production from petroleum mineralization. The objectives of this study were (1) to demonstrate the effectiveness of radiocarbon in providing direct quantified evidence of in situ aerobic petroleum hydrocarbon biodegradation from vadose zone CO 2 and groundwater DIC at a gasoline-contaminated site being remediated by an air sparging/soil vapor extraction (AS/ SVE) system and (2) to quantify the microbial contribution to vadose zone CO 2 and DIC production. Vadose zone CO 2 and groundwater DIC were collected from soil vapor probes, SVE wells, AS/SVE stack exhaust, and groundwater monitoring wells. Analogous gas and groundwater samples were collected from a geologically similar uncontaminated site. Vadose zone CO 2 and DIC extracted from groundwater were concentrated using NaOH traps and sent to analytical laboratories for conventional 14 C analysis using β counters. 14 C values ranged from 15.9 to 47.7 percent modern carbon (PMC) and demonstrated isotopic depletion from aerobic microbial mineralization of the petroleum. Aerobic biodegradation was calculated to account for 59-87% of the CO 2 produced. 14 C analysis clearly illustrated biodegradation from one time point and even in samples of low CO 2 content after CO 2 was concentrated.

Measuring Carbon-based Contaminant Mineralization Using Combined CO₂ Flux and Radiocarbon Analyses

“…It is thus possible to estimate the proportion of the entire CO 2 pool (respired carbon) attributable to the contaminant. Using solely this proportion, fossil-hydrocarbon or industrial chemical oxidation at field sites has been confirmed [7][8][9][10][11][12][13] . This proportion of contaminant derived CO 2 can then be coupled with total CO 2 mineralization rate (all CO 2 collected per unit time and volume) to determine intrinsic contaminant mineralization rate.…”

“…Because very distinct isotopic signatures are available in carbon-based contaminants, carbon isotopes have been recently applied to understanding contaminant attenuation processes at field sites [5][6][7][8][9][10][11][12][13] . Stable carbon isotopes can be used to determine if a source is attenuating based on Rayleigh distillation kinetics (c.f.…”

“…The technique described here relies on the observation that any CO 2 generated from a fossil derived contaminant will be devoid of 14 C while CO 2 generated from microorganisms degrading NOM will contain easily-measurable amounts of 14 C. Measuring 14 CO 2 also allows one to directly link full contaminant degradation (i.e., mineralization) to a harmless end product. 14 CO 2 analysis has been used to follow fossil fuel-derived contaminant degradation products [7][8][9][10][11][12][13] . This is due to the analytical resolution between end members (fossil and contemporary) which is roughly 1,100 parts per thousand (‰).…”

Radiocarbon and Stable Carbon Isotope Analysis to Confirm Petroleum Natural Attenuation in the Vadose Zone

“…With a half-life of approximately 5700 years, petroleum sources are radiocarbon-free and thus provide a definitive end-member when analyzed against photosyntheticbased carbon sources, which contain modern CO 2 from the atmosphere. The percentage modern carbon (pMC) of living photosynthetic biomass is approxiamtely 110% because of nuclear bomb tests, which peaked before 1965 (Aelion et al, 1997). In contrast, petroleum carbon and CO 2 derived from its biodegradation have no 14 C and are thus 0 pMC.…”

“…Radiocarbon analysis was able to confirm petroleum biodegradation. Radiocarbon was also used to survey groundwater contaminant degradation at a gasolinecontaminated site undergoing remediation with an air sparging/ soil vapor extraction system (Aelion et al, 1997). The CO 2 radiocarbon signature in the soil gas and groundwater-dissolved inorganic carbon suggested aerobic petroleum biodegradation produced 59-87% of the total CO 2 .…”