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Assessment of MTBE biodegradation in contaminated groundwater using 13C and 14C analysis: Field and laboratory microcosm studies

Thornton, Steven F.; Bottrell, Simon H.; Spence, Keith H.; Pickup, Roger; Spence, Michael J.; Shah, Nadeem; Mallinson, Helen E.H.; Richnow, Hans H.. 2011 Assessment of MTBE biodegradation in contaminated groundwater using 13C and 14C analysis: Field and laboratory microcosm studies. Applied Geochemistry, 26 (5). 828-837. 10.1016/j.apgeochem.2011.02.004

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Abstract/Summary

Radiolabelled assays and compound-specific stable isotope analysis (CSIA) were used to assess methyl tert-butyl ether (MTBE) biodegradation in an unleaded fuel plume in a UK chalk aquifer, both in the field and in laboratory microcosm experiments. The (14)C-MTBE radiorespirometry studies demonstrated widespread potential for aerobic and anaerobic MTBE biodegradation in the aquifer. However, delta(13)C compositions of MTBE in groundwater samples from the plume showed no significant (13)C enrichment that would indicate MTBE biodegradation at the field scale. Carbon isotope enrichment during MTBE biodegradation was assessed in the microcosms when dissolved O(2) was not limiting, compared with low in situ concentrations (2 mg/L) in the aquifer, and in the absence of O(2). The microcosm experiments showed ubiquitous potential for aerobic MTBE biodegradation in the aquifer within hundreds of days. Aerobic MTBE biodegradation in the microcosms produced an enrichment of 7 parts per thousand in the MTBE delta(13)C composition and an isotope enrichment factor (epsilon) of 1.53 parts per thousand when dissolved O(2) was not limiting. However, for the low dissolved O(2) concentration of up to 2 mg/L that characterizes most of the MTBE plume fringe, aerobic MTBE biodegradation produced an enrichment of 0.5-0.7 parts per thousand, corresponding to an epsilon value of -0.22 parts per thousand to -0.24 parts per thousand. No anaerobic MTBE biodegradation occurred under these experimental conditions. These results suggest the existence of a complex MTBE-biodegrading community in the aquifer, which may consist of different aerobic species competing for MTBE and dissolved O(2). Under low O(2) conditions, the lower fractionating species have been shown to govern overall MTBE C-isotope fractionation during biodegradation, confirming the results of previous laboratory experiments mixing pure cultures. This implies that significant aerobic MTBE biodegradation could occur under the low dissolved O(2) concentration that typifies the reactive fringe zone of MTBE plumes, without producing detectable changes in the MTBE delta(13)C composition. This observed insensitivity of C isotope enrichment to MTBE biodegradation could lead to significant underestimation of aerobic MTBE biodegradation at field scale, with an unnecessarily pessimistic performance assessment for natural attenuation. Site-specific C isotope enrichment factors are, therefore, required to reliably quantify MTBE biodegradation, which may limit CSIA as a tool for the in situ assessment of MTBE biodegradation in groundwater using only C isotopes. (C) 2011 Elsevier Ltd. All rights reserved.

Item Type: Publication - Article
Digital Object Identifier (DOI): 10.1016/j.apgeochem.2011.02.004
Programmes: CEH Topics & Objectives 2009 onwards > Biogeochemistry
CEH Sections: Parr
ISSN: 0883-2927
NORA Subject Terms: Biology and Microbiology
Date made live: 27 Feb 2012 16:32
URI: http://nora.nerc.ac.uk/id/eprint/16775

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