In order to gain regulatory approval for source zone natural attenuation (SZNA) at hydrocarbon-contaminated sites, knowledge regarding the extent of the contamination, its tendency to spread, and its longevity is required. However, reliable quantification of biodegradation rates, an important component of SZNA, remains a challenge. If the rate of CO(2) gas generation associated with contaminant degradation can be determined, it may be used as a proxy for the overall rate of subsurface biodegradation. Here, the CO(2)-efflux at the ground surface is measured using a dynamic closed chamber (DCC) method to evaluate whether this technique can be used to assess the areal extent of the contaminant source zone and the depth-integrated rate of contaminant mineralization. To this end, a field test was conducted at the Bemidji, MN, crude oil spill site. Results indicate that at the Bemidji site the CO(2)-efflux method is able to both delineate the source zone and distinguish between the rates of natural soil respiration and contaminant mineralization. The average CO(2)-efflux associated with contaminant degradation in the source zone is estimated at 2.6 μmol m(-2) s(-1), corresponding to a total petroleum hydrocarbon mineralization rate (expressed as C(10)H(22)) of 3.3 g m(-2) day(-1).

Naturally occurring biodegradation of hydrocarbon compounds may offer a sustainable management option at contaminated sites. However, a sound understanding of contaminant mass loss rates is required to enable estimation of source zone longevity, serving to alleviate public concerns and inform decision makers. Under some conditions, surficial CO2 efflux measurements can be useful to delineate petroleum hydrocarbon containing source zones, and to provide estimates of depth‐integrated vadose zone hydrocarbon degradation rates. However, the accuracy of degradation rate estimates is limited by our ability to separate CO2 effluxes associated with contaminant decomposition from those attributable to naturally occurring soil respiration. To understand CO2 sources and transport processes within the vadose zone, this work combines measurement of surficial CO2 effluxes with detailed analysis of soil gas composition– including the radiocarbon and stable isotopic composition of CO2. Quantitative reactive transport modeling allows further evaluation of controls on CO2 generation and fate, and CH4 generation and oxidation. Results confirm that, in the source zone at the Bemidji site, the majority of CO2 originates from degradation of the oil body. In addition, radiocarbon in CO2 proves particularly useful in determining the contribution of contaminant degradation to the measured CO2 efflux.

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Core Ideas Contaminant mass loss rates vary seasonally. Vadose zone temperatures in the source zone correlate with rates of contaminant respiration. Natural soil respiration and gas transport seasonality affect mass loss estimates. Understanding seasonal changes in natural attenuation processes is critical for evaluating source‐zone longevity and informing management decisions. The seasonal variations of natural attenuation were investigated through measurements of surficial CO2 effluxes, shallow soil CO2 radiocarbon contents, subsurface gas concentrations, soil temperature, and volumetric water contents during a 2‐yr period. Surficial CO2 effluxes varied seasonally, with peak values of total soil respiration (TSR) occurring in the late spring and summer. Efflux and radiocarbon data indicated that the fractional contributions of natural soil respiration (NSR) and contaminant soil respiration (CSR) to TSR varied seasonally. The NSR dominated in the spring and summer, and CSR dominated in the fall and winter. Subsurface gas concentrations also varied seasonally, with peak values of CO2 and CH4 occurring in the fall and winter. Vadose zone temperatures and subsurface CO2 concentrations revealed a correlation between contaminant respiration and temperature. A time lag of 5 to 7 mo between peak subsurface CO2 concentrations and peak surface efflux is consistent with travel‐time estimates for subsurface gas migration. Periods of frozen soils coincided with depressed surface CO2 effluxes and elevated CO2 concentrations, pointing to the temporary presence of an ice layer that inhibited gas transport. Quantitative reactive transport simulations demonstrated aspects of the conceptual model developed from field measurements. Overall, results indicated that source‐zone natural attenuation (SZNA) rates and gas transport processes varied seasonally and that the average annual SZNA rate estimated from periodic surface efflux measurements is 60% lower than rates determined from measurements during the summer.