2003 Progress Report: Beyond MTBE: Evaluating the Future Threats to Drinking Water Supplies from Chemicals in Our GasolineEPA Grant Number: R829023
Title: Beyond MTBE: Evaluating the Future Threats to Drinking Water Supplies from Chemicals in Our Gasoline
Investigators: Gschwend, Philip M.
Institution: Massachusetts Institute of Technology
EPA Project Officer: Packard, Benjamin H
Project Period: September 1, 2001 through August 31, 2004 (Extended to August 31, 2005)
Project Period Covered by this Report: September 1, 2002 through August 31, 2003
Project Amount: $239,524
RFA: Exploratory Research to Anticipate Future Environmental Issues (2000) RFA Text | Recipients Lists
Research Category: Economics and Decision Sciences , Ecological Indicators/Assessment/Restoration , Water , Ecosystems
The objective of this research project is to develop a methodology suited to evaluating the potential for new (and old) fuel additives to contaminate drinking water.
Continuing modifications of fuels such as gasoline should include evaluations of the proposed constituents for their potential to damage environmental resources such as subsurface water supplies. Consequently, we developed a screening model to estimate well water concentrations and transport times for gasoline components migrating from underground fuel tank releases to typical at-risk community water supply wells (Arey and Gschwend, 2005). Representative fuel release volumes and hydrogeologic characteristics were used to parameterize the transport calculation. Subsurface degradation processes were neglected in the model in order to make risk-conservative assessments. The model was tailored to individual compounds based on their abundances in gasoline, gasoline-water partition coefficients (Kgw), and organic matter-water partition coefficients (Kom). Transport calculations were conducted for 20 polar and 4 nonpolar compounds found in gasoline, including MTBE and other ether oxygenates, ethanol, methanol, and some aromatic hydrocarbons. With no calibration, the screening model successfully captured the reported magnitude of methyl t-butyl ether (MTBE) contamination of at-risk community supply wells. Such screening indicates that other oxygenates would cause similar widespread problems unless they were biodegradable. Analysis of field parameter variability concluded that community supply well contamination estimates had order-of-magnitude reliability. This indicated that such pre-manufacturing analyses may reasonably prevent widespread environmental problems and/or inspire focused investigations into chemical properties (e.g., biodegradability) before industrial adoption of new fuel formulations.
In addition, we need to be able to estimate fuel-water partition coefficients needed for screening models as described above (Arey and Gschwend, in press, 2005). To estimate the partition coefficients that quantify organic solutes distributing between immiscible phases that consist of liquid mixtures, we evaluated the efficacy of combining linear solvation energy relationships (LSERs) developed for pure 1:1 systems via application of linear solvent strength theory. In this way, existing LSERs could be extended to treat solute partitioning from gasoline, diesel fuel, and similar mixtures into contacting aqueous mixtures. Unlike other approaches, this method allows prediction of liquid-liquid partition coefficients in a variety of fuel-water systems for a broad range of dilute solutes. When applied to 37 polar and nonpolar solutes partitioning between an aqueous mixture and 12 different fuel-like mixtures (many including oxygenates), the estimated model error was a factor of approximately 2.5 in the partition coefficient. This is considerably more accurate than application of Raoult’s law for the same set of systems. Regulators and scientists could use this method to estimate fuel-water partition coefficients of novel additives in future fuel formulations and thereby provide key inputs for environmental transport assessments of these compounds.
We are working to develop a model and employ molecular orbital calculations to estimate the LSER solute polarity parameter, 2H. We believe this polarity parameter is related to two more fundamental physical quantities: a measured polarizability term and a calculated solute-solvent electrostatic interaction term. The proposed method would allow computational estimates of 2H values for novel (untested) solutes and will hopefully lead to further advances in the physical interpretation and modeling of LSERs.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
|Other project views:||All 4 publications||3 publications in selected types||All 3 journal articles|
||Arey JS, Gschwend PM. Estimating partition coefficients for fuel-water systems: developing linear solvation energy relationships using linear solvent strength theory to handle mixtures. Environmental Science & Technology 2005;39(8):2702-2710.||
||Arey JS, Gschwend PM. A physical-chemical screening model for anticipating widespread contamination of community water supply wells by gasoline constituents. Journal of Contaminant Hydrology 2005;76(1-2):109-138.||