Effects of Volatilization on Carbon and Hydrogen Isotope Ratios of MTBE

Contaminant attenuation studies utilizing CSIA (compound-specific isotope analysis) routinely assume that isotope effects (IEs) result only from degradation. Experimental results on MTBE behavior in diffusive volatilization and dynamic vapor extraction show measurable changes in the isotope ratios of the MTBE remaining in the aqueous or nonaqueous phase liquid (NAPL) matrix. A conceptual model for interpretation of those IEs is proposed, based on the physics of liquid−air partitioning. Normal or inverse IEs were observed for different volatilization scenarios. The range of carbon enrichment factors (ε) was from +0.7‰ (gasoline vapor extraction) to −1‰ (diffusive volatilization of MTBE from gasoline), the range of hydrogen ε was from +7‰ (gasoline vapor extraction) to −12‰ (air sparging of aqueous MTBE). The observed IEs are lower than those associated with MTBE degradation. However, under a realistic scenario for MTBE vapor removal, their magnitude is within the detection limits of CSIA. The potential for interference of those IEs is primarily in confusing the interpretation of samples with a small extent of fractionation and where only carbon CSIA data are available. The IEs resulting from volatilization and biodegradation, respectively, can be separated by combined carbon and hydrogen 2D-CSIA.