LONG-TERM TRENDS IN VADOSE ZONE GAS CONCENTRATIONS AND FLUXES AS INDICATORS OF CHANGING SOURCE ZONE CRUDE OIL COMPOSITION AND DEGRADATION RATES

Evolution of dissolved and gaseous plumes emanating from residual petroleum contaminant sources requires an understanding of how source compositions vary over time and how those transformations relate to changes in plume behavior. The National Crude Oil Spill Fate and Natural Attenuation Research Site near Bemidji, MN, USA has provided an ideal place to look at long-term changes in chemical composition of oil and corresponding trends in vadose zone gas concentrations and surficial gas fluxes. The site was contaminated in 1979 when a pipeline rupture released ~1.7 million liters of light crude oil onto sandy glacial outwash sediments. Approximately 25 percent of the oil remained in the subsurface after initial clean-up efforts. Previous research at the site showed that by 2010, C₆-C₃₀ n-alkanes were 95 percent depleted from the oil near the pipeline rupture site, whereas the C₆-C₃₀ n-alkanes were only about 50 percent depleted from the oil 25 m downgradient of the rupture site. Where the oil is mostly depleted of n-alkanes, vadose zone methane concentrations directly above the oil declined substantially from 2004 to 2018, whereas at locations where n-alkanes are less depleted, vadose methane concentrations showed only modest declines from 2004 to 2018. Methane is oxidized to CO₂ prior to reaching land surface at the site, so measurements of surficial CO₂ efflux can be used as a proxy for the overall rate of subsurface oil degradation. Site-averaged contaminant-related soil respiration (CSR) calculated from surficial CO₂ efflux measurements showed a declining trend between 2009 and 2018. Taken together these results suggest that methanogenic oil degradation rates slow after the easily degraded components of the oil (n-alkanes) are depleted. Long-term repeated measurements of soil methane concentrations and surficial soil CO₂ efflux are methods that can be used to observe these long-term changes.