Estimating biodegradation rates, based on concentration changes, is difficult due to the uncertainty in distinguishing concentration changes due exclusively to degradation, from those due to non-degradative processes such as, sorption, dilution or dispersion.  Alternatively, compound specific isotope analysis (CSIA) can directly measure biodegradation of monoaromatic and chlorinated hydrocarbons in groundwater by monitoring isotopic fractionation of the remaining contaminant as biodegradation proceeds.  Provided that isotopic fractionation shows a strong fit to a Rayleigh isotopic model under a variety of experimental conditions, CSIA can be used to quantify biodegradation rates in the field.  This presentation will demonstrate the potential and advantages in using carbon isotope analysis to quantify anaerobic benzene biodegradation rates in the field.  

Characterization of carbon isotopic fractionation during anaerobic benzene biodegradation using enrichment cultures under nitrate-, sulfate- and methanogenic reducing conditions produced robust and reproducible enrichment factors within each culture (-2.2 ± 0.4‰, -3.6 ± 0.3‰ and -1.9 ± 0.1‰, respectively) and a narrow range of enrichment factors between the three different cultures.  10-fold higher initial biomass showed no major influence on the enrichment factors.  Significantly, two different nitrate-reducing cultures, originating from different contaminated sites, showed no variability in enrichment factors (-2.2 ± 0.4‰ and -2.3 ± 0.3‰).  In addition, past studies showed that non-degradative processes do not cause significant carbon isotopic fractionation outside of typical analytical uncertainty (±0.5‰).  Given the narrow range of benzene enrichment factors and the ability of CSIA to differentiate between concentration decreases due to degradative and non-degradative processes, isotope-derived rate constants can provide a more conservative and precise estimate than concentration-derived rate constants. Furthermore, estimating biodegradation rates using stable isotope analysis can be particularly advantageous at sites where the breakdown products of the contaminant of study are unknown or non-exclusive.