Seasonal Variations in CO2 Efflux, Vadose Zone Gas Concentrations, and Natural Attenuation Rates at a Crude Oil Spill Site

Abstract:

Accurate estimates of hydrocarbon source zone natural attenuation (SZNA) rates are important for managing contaminated sites but are difficult to measure. Moreover, SZNA rates may vary seasonally in response to climatic conditions. Previous research at a crude oil spill site near Bemidji, Minnesota, USA showed that SZNA rates in the summer can be estimated by subtracting background soil CO₂ efflux from the total soil CO₂ efflux above the contaminated source. In this study, seasonal variations in surficial CO₂ efflux were evaluated with measurements of gas concentrations (including ¹⁴CO₂), temperature, and volumetric water content in the vadose zone at the site during a 2-year period. Soil CO₂ effluxes in the source zone were consistently greater than background CO₂ effluxes, and the magnitude and areal extent of the increased efflux varied seasonally. In the source zone, the ¹⁴CO₂ and the CO₂ efflux data showed a larger proportion of soil CO₂ was derived from SZNA in fall and winter (October – February) compared to the summer (June – August). Surficial CO₂ effluxes and vadose zone CO₂ and CH₄ concentrations in the source (2 – 7 meters below land surface) were positively correlated with soil temperature, indicating seasonal variability in SZNA rates. However, peak surficial CO₂ effluxes did not correspond with periods of highest CO₂ or CH₄ concentrations at the 2 – 7 meter depth, demonstrating the effects of physical attributes (such as soil depth, frost, and volumetric water content) on gas transport. Overall, results showed that SZNA rates, background soil respiration rates, and gas transport varied seasonally, and that biological and physical factors are important to consider for accurately estimating SZNA rates.