Stable carbon isotope evidence of biodegradation zonation in a shallow jet-fuel contaminated aquifer

delta(13)C values in dissolved inorganic carbon (DIC) ranged from -28 to +11.9 parts per thousand 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 delta(13)C values representative of the aerobic and anaerobic microbial biodegradation of C-13-depleted jet fuel (delta(13)C similar to -27 parts per thousand). The delta(13)C 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 delta(13)C values were measured, and reflected biodegradation of delta(13)C-depleted jet fuel under aerobic (delta(13)C DIC similar to -26 parts per thousand) or sulfate-reducing (delta(13)C DIC similar to -18 parts per thousand) conditions; the deeper part of the aquifer isolated from recharge was methanogenic and had higher delta(13)C DIC values. Conversely, when rainfall was absent and dissolved oxygen and sulfate concentrations were low in the aquifer, higher DIC delta(13)C values were measured in both shallow and deep contaminated groundwater (delta(13)C DIC up to +11.9 parts per thousand) where H-2 concentrations indicated that the predominant terminal electron-accepting process was methanogenesis. The highest delta(13)C values (+2.6 to +11.9 parts per thousand) were from contaminated groundwater that contained no dissolved oxygen and little sulfate, CH4 concentrations up to 1985 mu mol/L, and acetate concentrations exceeding 12 000 mu mol/L. These results suggest that stable carbon isotopes in DIC can be used to indicate the zonation of C-13-depleted hydrocarbon biodegradation processes under the influence of hydrologically controlled electron-acceptor availability.