Quantification of In Situ Biodegradation Rates of Groundwater Contaminants Using Laboratory-Derived Isotopic Enrichment Factors

Compound specific isotope analysis (CSIA) can be used to attribute concentration decreases to degradative versus non-degradative attenuation processes. This approach is based on the fact that during biodegradation, molecules containing the light isotopes react at a slightly faster rate than those containing heavier isotopes, resulting in the progressive isotopic enrichment of the heavy isotope in the remaining contaminant. While biodegradation of petroleum hydrocarbons and fuel oxygenates involve substantial isotopic fractionation, non-degradative attenuation processes typically result in small to negligible isotopic fractionation. Early studies applying CSIA mainly focused on identifying the occurrence of in situ biodegradation through isotopic enrichment in ¹³C of the contaminant relative to the source zone. However more recently, isotopic fractionation has been used as a novel and independent means to calculate biodegradation rates.

The isotopic fractionation of many organic contaminants can be described by the Rayleigh isotopic enrichment model, which describes the enrichment in the heavier isotope as biodegradation proceeds by an enrichment factor (e). Values of e measured in laboratory experiments are used to quantify in situ biodegradation. Given that the range of e reported for biodegradation of MTBE is fairly large (–9.2 to –15.6‰), comparison of biodegradation rates using the full range of isotopic enrichment factors to rates using concentration-based approaches is important to verify that stable isotopes can provide a robust means of quantifying MTBE biodegradation. The ethanol release studies at Vandenberg Air Force Base provided an excellent opportunity to compare biodegradation rate constants. The MTBE biodegradation rates of 12.0 to 20.3 y^-1 calculated using carbon isotopic values showed good agreement with the rate constant of 16.8 y^-1 calculated using MTBE mass-discharge. Our results suggest that the range in e reported in the literature for MTBE is not so large that it results in a significant difference in the biodegradation rate estimates.