Biodegradability of trimethylbenzene isomers under the strictly anaerobic conditions in groundwater contaminated by gasoline

Trimethylbenzene (TMB), as a constituent of gasoline, is often expected to be used as a conservative tracer in anaerobic BTEX-contaminated groundwater site to correct for attenuation due to dispersion, dilution and sorption along a flow path. To evaluate the suitability of using TMB as a tracer and to better understand biodegradability of TMB in contaminated groundwater by gasoline under anaerobic conditions, laboratory microcosms were conducted with mixed nitrate/iron/sulfate electron-acceptor amendments, using aquifer materials collected from Canadian Forces Base (CFB), Borden, Ontario, Canada. The results showed that under denitrifying conditions, biodegradation of 1,3,5-TMB, 1,2,4-TMB and 1,2,3-TMB were relatively slow and after 204 days of incubation approximately 27, 24, and 16% of the initial concentrations, respectively, were degraded in the microcosms. Under sulfate-reducing conditions, TMB isomers were recalcitrant. In contrast, significant biodegradation of TMB was observed under iron-reducing conditions. 1,3,5-TMB, 1,2,4-TMB and 1,2,3-TMB were degraded to 44, 47, and 24% of initial concentrations with first-order biodegradation rate constants of 0.003, 0.006 and 0.013 d⁻¹, respectively. This study indicates that TMB biodegradation is insignificant under nitrate and sulfate-reducing conditions but significant under iron-reducing conditions. Therefore, the use of TMB as a tracer for interpreting removal of other biodegradable gasoline constituents such as BTEX requires caution, especially in the presence of iron-reducing conditions.