UNDERSTANDING THE EVOLUTION OF GROUNDWATER CONTAMINANT PLUME CHEMISTRY EMANATING FROM RESIDUAL CONTAMINANT SOURCES, AN EXAMPLE FROM A LONG-TERM CRUDE OIL SPILL

Understanding the evolution of plumes emanating from residual contaminant sources, such as crude oil spills or leaking underground storage tanks, requires an understanding of how source compositions change over time and how those changes relate to changes in plume behavior. A pipeline transporting crude oil near Bemidji, MN, which broke in 1979, provided an ideal site to look at long-term trends in chemical composition of oil and water. Our recent work documented the long-term degradative state of hydrocarbons in the residual oil relative to the original pipeline oil. Loss of 25-85% of the measured hydrocarbons occurred after 30 years; the C_6-30 n-alkanes, toluene and o-xylene were the most depleted hydrocarbons. Extent of hydrocarbon depletion degradation in the oil varied depending on hydrogeologic framework (such as local topography and accessibility to recharge water and electron acceptors). There was a linear relationship between concentrations of benzene and naphthalene in the residual oil and that measured in water samples collected below the oil. This relationship indicates the hydrocarbon concentrations in oil and water are at steady state. Water associated with oil near to the spill had different chemical composition compared to water associated with oil downgradient from the spill. Water associated with more degraded regions of the oil, near the spill site, had, on average, higher concentrations of organic acids and ammonium, and lower concentrations of methane compared to water associated with oil that has migrated downgradient from the spill area. These differences indicate a potential shift in biodegradation reactions. Near the spill, methane concentrations and the carbon isotopic composition of inorganic carbon (δ ¹³C of TIC) analyzed over the past 30 years in several wells close to the oil indicate early rapid increases in CH₄ (reaching a maximum concentration of 34.4 mg/L) and enrichment in δ¹³C of TIC (reaching a maximum of 19.7 per mil). These results emphasize that source zone processes are spatially and temporally heterogeneous and should be accounted for in natural attenuation studies.