Je. Landmeyer et al., STABLE CARBON-ISOTOPE EVIDENCE OF BIODEGRADATION ZONATION IN A SHALLOW JET-FUEL CONTAMINATED AQUIFER, Environmental science & technology, 30(4), 1996, pp. 1120-1128
delta(13)C values in dissolved inorganic carbon (DIC) ranged from -28
to +11.9 parts per thousand in a sandy, noncarbonate shallow aquifer c
ontaminated with jet-fuel petroleum hydrocarbons. This range was obser
ved over a 4-year study in shallow and deep monitoring wells and compr
ised delta(13)C values representative of the aerobic and anaerobic mic
robial 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 i
nfluenced by the extent of rainwater infiltration of dissolved oxygen
or sulfate or, conversely, by the absence of recharge, lack of dissolv
ed oxygen or sulfate input to the aquifer, and the ensuing methanogeni
c conditions. After some recharge events delivered dissolved oxygen or
sulfate to the shallow part of the aquifer, low to medium DIC delta(1
3)C values were measured, and reflected biodegradation of delta(13)C-d
epleted jet fuel under aerobic (delta(13)C DIC similar to -26 parts pe
r thousand) or sulfate-reducing (delta(13)C DIC similar to -18 parts p
er thousand) conditions; the deeper part of the aquifer isolated from
recharge was methanogenic and had higher delta(13)C DIC values. Conver
sely, when rainfall was absent and dissolved oxygen and sulfate concen
trations were low in the aquifer, higher DIC delta(13)C values were me
asured in both shallow and deep contaminated groundwater (delta(13)C D
IC up to +11.9 parts per thousand) where H-2 concentrations indicated
that the predominant terminal electron-accepting process was methanoge
nesis. The highest delta(13)C values (+2.6 to +11.9 parts per thousand
) were from contaminated groundwater that contained no dissolved oxyge
n and little sulfate, CH4 concentrations up to 1985 mu mol/L, and acet
ate concentrations exceeding 12 000 mu mol/L. These results suggest th
at stable carbon isotopes in DIC can be used to indicate the zonation
of C-13-depleted hydrocarbon biodegradation processes under the influe
nce of hydrologically controlled electron-acceptor availability.