MTBE biodegradation in aquifers: Natural and engineered systems

The use of compound-specific isotope analysis (CSIA) as a diagnostic toot for MTBE biodegradation in aquifers was tested at a Port Hueneme, CA site. There, a 1500-m long dissolved MTBE plume and associated engineered aerobic flow-through biobarrier have been well-studied allowing the comparison of field-scale CSIA results with a priori knowledge of aerobic MTBE biodegradation. Groundwater samples were collected and analyzed for both C-13 and D in MTBE through the aerobic bioactive treatment. Within this aerobic zone the C-13 enrichment was only about one fourth of what was observed for similar MTBE concentration reductions in laboratory microcosm experiments, and increasing enrichment with decreasing MTBE concentration seen in microcosms was not as clearly evident in field data. The discrepancy between field and lab data may reflect small-scale (<1 m) spatial heterogeneity in MTBE biodegradation activity and the mixing of water from adjacent strata during groundwater sampling. Overall the results suggest that C-13 data alone may produce inconclusive results at sites where MTBE undergoes aerobic biodegradation even with two-dimensional CSIA (C-13 and D). An increase in the confidence of data interpretation may only be possible with data sets larger than those typically collected in practice.; A laboratory microcosm activity (rate and extent of biodegradation) assay was used to assess the spatial distribution of MTBE-degrading activity in the vicinity of the Port Hueneme biobarrier after two years of operation. Generally, the activity was lowest at the upgradient locations and it increased moving downgradient through the treatment zones. A strong correlation between microbial activity and in-situ dissolved oxygen (DO) concentration was observed, and the MTBE degradation was most rapid where the in-situ DO was >20 mg/L.; Large scale laboratory experiments (1.22x1.22x0.05 m tanks and 1.22 m long 15.2 cm-diameter columns) were conducted to study the spatial redistribution of MTBE-biodegradation activity through time after bioaugmentation of a mixed MTBE-degrading culture.