Stable Carbon Isotope Evidence of Biodegradation Zonation in a Shallow Jet-Fuel Contaminated Aquifer

Abstract

δ13C values in dissolved inorganic carbon (DIC) ranged from −28 to +11.9‰ in a sandy, noncarbonate shallow aquifer contaminated with jet-fuel petroleum hydrocarbons. This range was observed over a 4-year study in shallow and deep monitoring wells and comprised δ13C values representative of the aerobic and anaerobic microbial biodegradation of 13C-depleted jet fuel (δ13C ∼ −27‰). The δ13C DIC values were found to be influenced by the extent of rainwater infiltration of dissolved oxygen or sulfate or, conversely, by the absence of recharge, lack of dissolved oxygen or sulfate input to the aquifer, and the ensuing methanogenic conditions. After some recharge events delivered dissolved oxygen or sulfate to the shallow part of the aquifer, low to medium DIC δ13C values were measured, and reflected biodegradation of 13C-depleted jet fuel under aerobic (δ13C DIC ∼ −26‰) or sulfate-reducing (δ13C DIC ∼ −18‰) conditions; the deeper part of the aquifer isolated from recharge was methanogenic and had higher δ13C DIC values. Conversely, when rainfall was absent and dissolved oxygen and sulfate concentrations were low in the aquifer, higher DIC δ13C values were measured in both shallow and deep contaminated groundwater (δ13C DIC up to +11.9‰) where H2 concentrations indicated that the predominant terminal electron-accepting process was methanogenesis. The highest δ13C values (+2.6 to +11.9‰) were from contaminated groundwater that contained no dissolved oxygen and little sulfate, CH4 concentrations up to 1985 μmol/L, and acetate concentrations exceeding 12 000 μmol/L. These results suggest that stable carbon isotopes in DIC can be used to indicate the zonation of 13C-depleted hydrocarbon biodegradation processes under the influence of hydrologically controlled electron-acceptor availability.