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MTBE IS A LITTLE BIT OK?
Citation:
Weaver, J W. AND M. C. Small. MTBE IS A LITTLE BIT OK? Presented at National Groundwater Association Petroleum Hydrocarbons Conference, Atlanta, GA, November 6-8, 2002.
Impact/Purpose:
To assist decision-making by improving approaches to subsurface contaminant transport modeling based on evaluation of field observations and subsequent development of appropriate modeling approaches and tools.
Description:
Methyl tertiary butyl ether (MTBE) has been used as a gasoline additive to serve two major purposes. First, MTBE was used as an octane-enhancer to replace organic lead, beginning in about 1979. Beginning in about 1992, MTBE was also used as a fuel oxygenate additive to meet requirements of the Clean Air Act Amendments (CAAA) of 1990. Generally, the amount of MTBE used for octane enhancement was lower than that required to meet CAAA requirements. An unintended consequence of using MTBE to address air quality issues has been widespread groundwater contamination. The decision to use certain amounts of MTBE or other chemicals as gasoline additives is the outcome of economic, regulatory, policy, political, and scientific considerations. Decision makers ask questions such as "How do ground water impacts change with changing MTBE content? How many wells would be impacted? and What are the associated costs?" These questions are best answered through scientific inquiry, but many different approaches could be developed. Decision criteria include time, money, comprehensiveness, and complexity of the approach. Because results must be communicated to a non-technical audience, there is a trade-off between the complexity of the approach and the ability to convince economists, lawyers and policy makers that the results make sense.
The questions on MTBE content posed above were investigated using transport models, a known release scenario and varying gasoline compositions. A set of simulations was performed that assumed 3% (octane enhancement) and 11% (CAAA) MTBE in gasoline. The results were that ground water concentrations would be reduced in proportion to the reduction of MTBE in the fuel. Plume lengths, though, would not be proportionately reduced. One implication of these results was that the concentrations would be reduced, but the number of impacted wells would remain similar. Because simulations included emplacement of the gasoline, dissolution from contact with flowing ground water and transient transport in the aquifer, a common sense explanation of the results was difficult to construct. A simpler model was then used for the purpose of explaining to policy makers why the plume length reductions were less than proportionate to the reduction of the amount of MTBE. The model was simple enough (one-dimensional, steady state, constant source concentration) so that the effect of each term of the transport equation on plume length could be easily shown. The weight of evidence from using multiple models, direct explanations from the transport equation, and field observation, should provide a sufficient basis for policy makers to understand scientifically how gasoline composition affects ground water impacts.