Environ. Sci. Technol. 2001, 35, 4643-4647
Effect of Redox Conditions on MTBE Biodegradation in Surface Water Sediments PAUL M. BRADLEY,* FRANCIS H. CHAPELLE, AND JAMES E. LANDMEYER U.S. Geological Survey, 720 Gracern Road, Suite 129, Columbia, South Carolina 29210-7651
Microbial degradation of methyl tert-butyl ether (MTBE) was observed in surface water-sediment microcosms under anaerobic conditions. The efficiency and products of anaerobic MTBE biodegradation were dependent on the predominant terminal electron-accepting conditions. In the presence of substantial methanogenic activity, MTBE biodegradation was nominal and involved reduction of MTBE to the toxic product, tert-butyl alcohol (TBA). In the absence of significant methanogenic activity, accumulation of [14C]TBA generally decreased, and mineralization of [U-14C]MTBE to 14CO2 generally increased as the oxidative potential of the predominant terminal electron acceptor increased in the order of SO4, Fe(III), Mn(IV) < NO3 < O2. Microbial mineralization of MTBE to CO2 under Mn(IV)or SO4-reducing conditions has not been reported previously. The results of this study indicate that microorganisms inhabiting the sediments of streams and lakes can degrade MTBE effectively under a range of anaerobic terminal electron-accepting conditions. Thus, anaerobic bed sediment microbial processes may provide a significant environmental sink for MTBE in surface water systems throughout the United States.
Introduction MTBE (methyl tert-butyl ether) is a fuel oxygenate that has received national attention due to its widespread contamination of surface water and groundwater drinking supplies (1-9). The primary oxygenate in domestic gasoline, MTBE can contaminate the environment via numerous point (10, 11) and nonpoint (1, 2, 4, 5, 12-15) mechanisms. MTBE is the second most frequently detected contaminant in drinking water wells in the United States (3, 6-8) and an increasingly reported contaminant in surface water systems (1, 2, 4, 5, 9). Consequently, in 2000 the U.S. EPA announced its intention to “reduce or eliminate” the use of MTBE in domestic fuels over the next 3 yr (16). In anticipation of these actions, attention is now focused on identifying environmental sinks for MTBE in an effort to understand the long-term fate of MTBE contamination. The potential for aerobic microbial degradation of MTBE is wellrecognized (10, 17-26) but rarely realized under in situ conditions because of natural (10, 17, 27) or contaminantassociated (11, 22, 28) limitations on oxygen transport in surface water and groundwater systems. Historically, MTBE has been considered recalcitrant under anoxic conditions * Corresponding author telephone: (803)750-6125; fax: (803)7506181; e-mail:
[email protected]. 10.1021/es010794x Not subject to U.S. Copyright. Publ. 2001 Am. Chem. Soc. Published on Web 10/31/2001
(11, 18, 29-31) with a limited potential for microbial transformation to TBA (tert-butyl alcohol) under methanogenic conditions (29). Transformation to TBA is problematic because TBA also is toxic (32) and resistant to anaerobic biodegradation (30, 33-35). In contrast, the results of a new study (36) indicate that microorganisms indigenous to the bed sediments of surface water systems are capable of effective degradation of MTBE to CO2 under denitrifying conditions without the accumulation of TBA. The apparent ability of indigenous microorganisms to catalyze anaerobic mineralization of MTBE to CO2 under relatively oxidized, denitrifying conditions (36) but not under strongly reduced, methanogenic conditions (10, 28, 29, 36) indicates that the extent and efficiency of anaerobic MTBE oxidation are dependent on the oxidative potential of the available terminal electron acceptors. To test this hypothesis, the potential for microbial degradation of MTBE to CO2 was examined in stream- and lake-bed sediments under a range of terminal electron-accepting conditions.
Methods Chemicals. MTBE mineralization was investigated using uniformly labeled [U-14C]MTBE (New England Nuclear, Boston, MA) as described previously (10, 17, 36). The radiochemical composition of the [U-14C]MTBE stock (10.1 mCi/mmol specific activity) was evaluated in our lab by direct injection radiometric detection high-performance liquid chromatography (HPLC/RD) and direct injection radiometric detection gas chromatography (GC/RD) and found to be 96.8 ( 0.4% as [14C]MTBE and 3.8 ( 0.5% as [14C]TBA. The presence of TBA as a trace contaminant in commercially available MTBE is not uncommon (36) and must be considered when evaluating the significance of TBA as an intermediate in MTBE biodegradation. Authentic [U-14C]TBA (Moravek Biochemicals, Brea, CA), H14CO3- (Sigma Biochemicals, St. Louis, MO), and 14CH4 (New England Nuclear, Boston, MA) were used as radiolabeled standards for calibration and methods development. All had radiochemical purities >98%. Study Sites. Because of spatial and temporal variation in the hydrologic and geochemical character of shallow stream systems, the sediments of these systems are often characterized by highly dynamic microbial communities capable of rapidly responding to and exploiting changes in the geochemical environment. This dynamic metabolic capability makes stream-bed sediments a useful hydrologic system for examining the effect of redox conditions on the potential for microbial degradation of MTBE in the environment. The ability of surface water microorganisms to degrade MTBE was examined in surface water sediments from Charleston (Oasis), SC; from Naval Air Station (NAS) Cecil Field, Jacksonville, FL; from Picatinny Arsenal, NJ. At the Oasis site (10, 16), MTBE-contaminated groundwater discharges to a shallow freshwater stream containing poorly sorted sandy bed sediments. Oasis sediments contained significant concentrations of dissolved CH4 (100 ( 15 µM), detectable concentrations of SO4 (
