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

Abstract

Radiocarbon (14C) measurement from vadose zone air and groundwater is an alternative technique to quantitatively estimate aerobic microbial CO2 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 CO2 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 CO2 and DIC production. Vadose zone CO2 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 CO2 and DIC extracted from groundwater were concentrated using NaOH traps and sent to analytical laboratories for conventional 14C analysis using β counters. 14C 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 CO2 produced. 14C analysis clearly illustrated biodegradation from one time point and even in samples of low CO2 content after CO2 was concentrated.