Comparison of Source Zone Natural Attenuation Rates At Crude Oil and Ethanol-Blended Fuel Release Sites

Understanding differences in source zone natural attenuation (SZNA) rates occurring among field sites impacted by the same contaminant and across field sites impacted by different contaminants is critical for developing management strategies. For example, unique site conditions can favor or inhibit biodegradation, whereas differences between contaminants can lead to variations in biodegradation potential. However, the implications of these effects for real-world release scenarios remain ambiguous. To better understand the implications of these effects, the authors investigated differences in SZNA rates at two crude oil and two denatured fuel-grade ethanol (DFE) spill sites using on-site measurements of surficial gas effluxes. At the crude oil sites, CH4 effluxes were below detection, whereas CH4 effluxes occurred at both DFE sites. Similarly, SZNA rates among sites impacted by the same contaminant were comparable, whereas order of magnitude differences existed between sites impacted by crude oil or DFE. At DFE sites, results also revealed source zone expansion in relation to the initial mass in place, suggesting that extended spatial monitoring may be required to characterize risk potential. Overall, key differences between crude oil and DFE release sites demonstrated the importance of site-specific interactions between hydrogeology and contaminant composition for mediating gas emissions and SZNA rates, and modulating gas transport regimes under field conditions.