When organic contaminants such as benzene, TCE or MTBE are degraded, the ratio of the stable isotopes of carbon in the organic contaminants will often change in a predictable fashion. In the last ten years, advances in analytical chemistry have made it possible to measure these changes, even when the organic contaminants are present in water at low concentrations. A determination of the ratio of the stable isotopes can reveal whether the contaminant has been degraded. If the mechanism of degradation is understood, and laboratory studies are available that define the relationship between the extent of degradation of the contaminant and the shift in the ratio of isotopes, it is often possible to estimate the extent of biodegradation of the contaminant.
This webinar will briefly review the theory behind isotopic effects, it will explain the units used to characterize the ratio of isotopes, and it will discuss the simple mathematics that can relate the shift in the ratio to the extent of degradation. Then the webinar will illustrate an approach to estimate rate constants for natural biodegradation of contaminants in ground water. The isotope analysis will be used to estimate the extent of natural biodegradation of MTBE at a gasoline spill site. The extent of biodegradation will be combined with the hydrological parameters at the site to estimate rate constants for biodegradation.
The webinar will conclude with a number of cautions and warnings. Heterogeneity in flow paths in the aquifer and proximity to NAPL or other source of contamination to ground water can substantially confuse the interpretation of stable isotope data. Both these conditions cause the isotope analysis to underestimate the extent of degradation. Heterogeneity in the rate of biodegradation can produce substantial errors in the forecasts of plume behavior. The webinar will provide recommendations to deal with the effects of heterogeneity in rates of biodegradation.
U.S. EPA has released A Guide for Assessing Biodegradation and Source Identification of Organic Ground Water Contaminants using Compound Specific Isotope Analysis (CSIA) [EPA 600/R-08/148 | December 2008 | www.epa.gov/ada]. The Guide provides recommendations for sample collection, sample preservation, and sample analysis; recommendations on QA/QC issues; details on calculations; and a catalogue of expected initial values for the ratios of 13C to 12C in organic compounds such as TCE and PCE. The Guide also illustrates in detail the process to use isotope ratio data to estimate rate constants for degradation of organic compounds in ground water.