UNDERSTANDING THE SEASONAL VARIABILITY IN UNSATURATED ZONE GAS COMPOSITION AND FLUXES ABOVE A SUBSURFACE CRUDE OIL SPILL

Estimated rates of hydrocarbon natural attenuation are important for managing contaminated sites, but are difficult to reliably measure under field conditions. This research examines the seasonal variability in surficial soil CO₂ fluxes and unsaturated zone gas composition and above an area of crude oil in contact with groundwater at a spill site near Bemidji, Minnesota, USA. Previous research showed that reasonable estimates of hydrocarbon natural attenuation rates in the summer can be achieved by subtracting the “natural” background soil CO₂ flux (respiration rate) from the soil CO₂ flux above the contaminated source zone. This simple method was applied to CO₂ flux measurements made at the Bemidji spill site on a monthly basis over a two-year period. Estimated rates of hydrocarbon natural attenuation were highly variable by month. To better understand the source of the soil CO₂ flux, samples of unsaturated zone gas were collected from a series of multi-level unsaturated zone wells in summer and winter and analyzed for ¹⁴CO₂ isotopic content. The ¹⁴C data showed a decrease in percent modern carbon from summer to winter, indicating that a larger proportion of carbon in soil gas was derived from hydrocarbon degradation in winter compared to summer. This change in soil gas composition suggests that background CO₂ fluxes and contaminated source zone CO₂ fluxes vary at different rates throughout the year. The seasonal variability in CO₂ flux from each source, background respiration and hydrocarbon respiration, are important for interpreting snapshot measurements of soil CO₂ flux and estimating longer-term natural attenuation rates and contaminant mass losses.