Impacts of Back Diffusion and Biodegradation on MTBE/TBA Plumes and Impacts of Spills of Ethanol-Blended Biofuels on Groundwater: Development of Models for Evaluating Field Experiments and Their Implications

• Main Author: Rasa, Ehsan
• Format: Dissertation
• Language: English
• Subjects: engineering, environmental; hydrology
• ISBN: 9781267662811; 1267662816

Fate and transport of bromide tracer, gasoline oxygenates including methyl tert-butyl ether (MTBE), tert-butyl alcohol (TBA), ethanol, and selected petroleum hydrocarbons (benzene, toluene, and o-xylene [BToX]) are studied and simulated using data from field experiments at Site 60, Vandenberg Air Force Base (VAFB), CA.
Conservative tracer experiments can provide information useful for characterizing various subsurface transport properties. High resolution data from a 410-day controlled field experiment at VAFB are used to construct and calibrate a groundwater flow and transport model. The effectiveness of three different types of transport observations (conservative tracer breakthrough curves, first temporal moments of breakthrough curves, and tracer mass discharge through control planes combined with hydraulic head observations) are evaluated for sensitivity analysis and parameter estimation of the site-specific flow and transport model. Results showed the advantages of using temporal moment of conservative tracer and mass discharge as observations for inverse modeling. First temporal moments provided feedback to the model and remained sensitive within a wider range of parameter values than did the other approaches and even when the observed and simulated breakthrough curves did not overlap or the peak concentrations were not similar.
Following a gasoline spill in late 1994 at VAFB, several excavations of the spill area were performed in 1995, 2007, and 2008. However, an MTBE/TBA plume persisted for over 15 years within 200 feet of the original spill source. Available monitoring data and published details about the source are used to develop a two-dimensional reactive transport model of MTBE and its potential metabolite TBA. The analyses suggested that MTBE diffused from the thin anaerobic aquifer into the adjacent anaerobic silts and transformed to TBA in both aquifer and silt layers. At later times TBA was the dominant solute, diffusing back out of the silts into the aquifer, and sustaining plume concentrations much longer than would have been the case in the absence of such diffusive exchange. Aerobic degradation of MTBE or TBA at the water table in the overlying silt layer is shown by the simulations to limit the mass available for back diffusion to the aquifer.
Decreased biodegradation rates of BToX resulting from co-release of ethanol lead to longer plumes than in the absence of ethanol as co-contaminant, and thus potentially greater risks may be posed by gasohol spills. In a previous field study at VAFB, two plumes were created by re-injecting extracted groundwater spiked with BToX in one side, and BT oX plus ethanol in another side for 283 days. A field-calibrated reactive transport model is developed in this study to evaluate the impact of ethanol on fate and transport of BToX species. The model was developed by stepwise inclusion of various reaction processes, as suggested by comparison of model simulations to field data, and drawing on literature for insight into reaction rates and constraints. This iterative process illustrates the limitations of models if not calibrated to or at least founded on field data. Best fits to the experimental data were achieved by incorporating the processes of ethanol fermentation, anaerobic methane oxidation, three dimensional mixing, and microbial growth. The final simulations matched temporal histories for electron donors, acceptors, and products quite well. In particular, the simulations matched the observations that the benzene plume in the With-Ethanol Lane was about 4.5 times longer than in the No-Ethanol Lane due to the slower natural attenuation rate of benzene in the presence of ethanol. This model, which incorporates more reaction processes than models previously applied to investigate ethanol impacts on BTEX fate, can be used to better understand the rate limiting processes, evaluate impacts under different geochemical conditions, and recommend potential remediation techniques.