CRUDE-OIL IN A SHALLOW SAND AND GRAVEL AQUIFER .3. BIOGEOCHEMICAL REACTIONS AND MASS-BALANCE MODELING IN ANOXIC GROUNDWATER

Crude oil floating on the water table in a sand and gravel aquifer pro vides a constant source of hydrocarbons to the groundwater at a site n ear Bemidji, Minnesota. The degradation of hydrocarbons affects the co ncentrations of oxidized and reduced aqueous species in the anoxic par t of the contaminant plume that developed downgradient from the oil bo dy. The concentrations of Fe2+, Mn2+ and CH4, Eh measurements, and the deltaC-13 ratios of the total inorganic C indicate that the plume bec ame more reducing over a 5-a period. However, the size of the contamin ant plume remained stable during this time. Field data coupled with la boratory microcosm experiments indicate that benzene and the alkylbenz enes are degraded in an anoxic environment. In anaerobic microcosm exp eriments conducted under field conditions, almost complete degradation (98%) was observed for benzene in 125 d and for toluene in 45 d. Conc entrations of aqueous Fe2+ and Mn2+ increased in these experiments, in dicating that the primary reactions were hydrocarbon degradation coupl ed with Fe and Mn reduction. Mass transfer calculations on a 40-m flow path in the anoxic zone, downgradient from the oil body, indicated tha t the primary reactions in the anoxic zone are oxidation of organic co mpounds, precipitation of siderite and a ferroan calcite, dissolution of iron oxide and outgassing of CH4 and CO2. The major difference in t he two models presented is the ratio of CO2 and CH4 that outgasses. Bo th models indicate quantitatively that large amounts of Fe are dissolv ed and reprecipitated as ferrous iron in the anoxic zone of the contam inant plume.