Using Stable Isotope Ratios to Evaluate Natural Biodegradation of MTBE at Fuel Release Sites in California

摘要

In recent studies, researchers have reported biodegradation of MTBE under a host of redox conditions, ranging from methanogenic to aerobic groundwater environments. Unfortunately, these success stories have not been as well documented at most fuel-release sites, where definitive evidence for the natural attenuation of MTBE by aerobic and anaerobic pathways is often lacking. Probably the most significant complicating factor for demonstrating MTBE biodegradation is the lack of a unique degradation product. Much attention has focused recently on using the stable isotope ratios of carbon (13C/12C) and hydrogen (D/H) as a tool for evaluating MTBE biodegradation in-situ. By convention, these ratios are expressed in permil notation (‰), where 13C/12C ratios are expressed as a permil difference, or δ13C, relative to the ratio of a carbon standard with δ13C=0 ‰. This technique is promising because changes in the δ13C of MTBE occur during extensive biodegradation but the δ13C value is unchanged by other processes affecting concentration such as dispersion and sampling artifacts that cause dilution. It has also been shown that MTBE from different gasoline suppliers has a relatively consistent δ13C value, ranging from –28 to –32 ‰. As MTBE is degraded, the δ13C value increase as the concentration decreases. Therefore, δ13C values greater than –28 ‰ can be considered to be evidence for biodegradation. Thus, it has been suggested that isotope fractionation in MTBE samples from existing monitoring wells at fuel-release sites would provide stronger evidence for biodegradation in-situ and would be considerably less costly and time consuming than the current state-of-the-art efforts such as microcosm studies and mass-flux calculations. This study evaluates the utility of stable carbon isotope ratios as a tool for monitoring biodegradation in-situ. First, aerobic microcosm studies were completed for comparison with previous studies completed by others. Second, the δ13C values of MTBE were measured from samples collected at locations from a highly controlled in-situ field experiment where the aerobic biodegradation of MTBE was known to be occurring. Third, groundwater samples were collected from monitoring wells at fuelrelease sites over a range of geochemical conditions, from methanogenic to weakly aerobic. The δ13C values of these samples were measured and compared with known source values for MTBE. The aerobic microcosm results were consistent with the findings of others, where δ13C values increased in a small but predictable manner as MTBE was consumed by aerobic microbes. The δ13C values from samples collected within the aerobic zone of the highly controlled field experiment increased by approximately 1 ‰ relative to initial undegraded MTBE. This enrichment in δ13C is small but consistent with the enrichment trend from microcosm studies. Because initial MTBE concentrations entering the in-situ treatment system were low (on the order of 200 μg/L), and aerobic degradation was rapid, there was only a short path along which biodegradation occurred and MTBE could be detected, and only one sampling location within that zone that could offer insight into the isotopic shift. Groundwater samples collected from the fuel release sites ranged in MTBE concentration from less than 100 μg/L to greater than 100,000 μg/L, where δ13C values ranged from - 31.6 to –29.3 ‰. These values are within the range reported for undegraded MTBE, and are not considered to provide evidence for biodegradation at the sites sampled. However, given the difficulties in detecting an isotope shift in the well-monitored field experiment where biodegradation was certainly occurring, it is not surprising that the results from the fuel release sites are inconclusive. These results suggest that the application of stable isotope ratios for evaluating natural aerobic biodegradation at fuel-release sites may be complicated by the difficulties in locating and monitoring the zone of active