Biodegradation Rates of Petroleum in Contaminated Aquifers: A Comparison of Radiocarbon Correction Methods for Determining Source Zone Natural Attenuation

Abstract

Recently, a novel approach using surficial CO2 efflux surveys has been used to delineate hydrocarbon source zones in contaminated aquifers and provide estimates of hydrocarbon biodegradation rates. This approach requires accurately distinguishing between petroleum–derived CO2 respiration and background natural soil respiration. Radiocarbon can be used to distinguish 14C–depleted CO2 of hydrocarbon sources from 14C–enriched CO2 of natural soil respiration to quantify the contribution of each source to total CO2 efflux. In this study radiocarbon was sampled from soil gas monitoring wells, soil gas probes, static chambers and dynamic closed chambers during CO2 efflux studies. Isotopic corrections for quantifying contribution of petroleum–derived CO2 efflux to total soil respiration using these methods are compared at a historical pipeline rupture site in Bemidji, MN and a de–commissioned oil refinery. Results confirmed that survey CO2 efflux measurements can be used to delineate subsurface contaminant source zones and the study showed that radiocarbon obtained from static chambers and soil probe methods give large CO2 sample yields that provide representative contaminant CO2 efflux measurements.