Environ. Sci. Technol. 2002, 36, 4087-4090
TBA Biodegradation in Surface-Water Sediments under Aerobic and Anaerobic Conditions PAUL M. BRADLEY,* JAMES E. LANDMEYER, AND FRANCIS H. CHAPELLE U.S. Geological Survey, 720 Gracern Rd, Suite 129, Columbia, South Carolina 29210-7651
The potential for [U-14C] TBA biodegradation was examined in laboratory microcosms under a range of terminal electron accepting conditions. TBA mineralization to CO2 was substantial in surface-water sediments under oxic, denitrifying, or Mn(IV)-reducing conditions and statistically significant but low under SO4-reducing conditions. Thus, anaerobic TBA biodegradation may be a significant natural attenuation mechanism for TBA in the environment, and stimulation of in situ TBA bioremediation by addition of suitable terminal electron acceptors may be feasible. No degradation of [U-14C] TBA was observed under methanogenic or Fe(III)-reducing conditions.
Introduction Efforts to decrease methyl tert-butyl ether (MTBE) contamination in groundwater, while retaining the positive air quality benefits of reformulated gasoline programs, have focused attention on potential alternative fuel oxygenates. Among these, tert-butyl alcohol (TBA) is unique due to its status as a current fuel oxygenate (1, 2), a common co-contaminant in MTBE contaminated groundwater (1, 2), and a potential product of in situ MTBE degradation (3-10). TBA, a known toxin and a possible carcinogen (11, 12), is not currently an EPA priority groundwater pollutant. However, the recent introduction of drinking water standards for TBA in a number of states suggests that future regulation is likely (ex. see ref 13). Under these circumstances, a detailed assessment of the environmental fate of TBA is appropriate. Although the biodegradability of TBA under oxic conditions is recognized (4, 14, 15), natural (14-16) and contaminant-associated (1, 10, 17) limitations on oxygen transport in surface- and groundwater systems undermine the importance of this process as a natural attenuation mechanism. TBA has long been considered recalcitrant under highly reducing, methanogenic conditions (3, 4, 14, 18-21), but recent findings suggest that anaerobic microbial mineralization of TBA can be substantial under relatively oxidized, denitrifying (3) or humics-amended, Fe(III)-reducing conditions (22). These observations suggest that the anaerobic biodegradability of TBA is sensitive to the available terminal electron accepting processes. To test this hypothesis, the potential for microbial degradation of 14C-TBA to 14CO2 was examined in surface-water sediments under a range of terminal electron accepting conditions. * Corresponding author phone: (803)750-6125; fax: (803)750-6181; e-mail:
[email protected]. 10.1021/es011480c Not subject to U.S. Copyright. Publ. 2002 Am. Chem. Soc. Published on Web 09/04/2002
Methods Chemicals. TBA mineralization was investigated using uniformly labeled [U-14C] TBA (Moravek Biochemicals, Brea, CA) as described previously (14). The radiochemical composition of the [U-14C] TBA stock (5 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 greater than 99% 14C-TBA. H14CO - (Sigma Biochemicals, St. Louis, MO) and 3 14CH (New England Nuclear, Boston, MA) were used as 4 radiolabeled standards for calibration and methods development. Both had radiochemical purities > 98%. Study Sites. The ability of surface-water microorganisms to degrade TBA was examined in surface-water sediments from Laurens, SC and Charleston, SC. The dynamic metabolic character of the microbial communities in shallow stream systems makes stream-bed sediments a useful hydrologic system for examining the effect of various redox conditions on the potential for microbial degradation of TBA in the environment. At the Charleston site, MTBE and TBA contaminated groundwater discharges to a shallow freshwater stream containing poorly sorted sandy bed sediments. The sediments contained 27 ( 1% water and dry materials composed of 88 ( 1% sand (grain size 0.125-2 mm), 11 ( 1% silt clay (grain size < 0.125 mm), and 1 ( 0% organic content (loss on ignition). Maximum dissolved concentrations of total BTEX compounds, MTBE, and TBA of 476 µg/L, 138 µg/L, and 2094 µg/L, respectively, were observed in groundwater monitoring wells 7 m up-gradient of the stream. These stream sediments contained significant concentrations of dissolved CH4 (100 ( 15 µM), detectable concentrations of SO4 (
