Anticipation nationwide risks to drinking water : predicting local scale contamination of community supply wells by gasoline additives

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2001."June 2001."Includes bibliographical references.Only ten years after the increased addition of methyl-tert-butyl-ether (MTBE) to U.S. gasolines, nationwide MTBE contamination of thousands of drinking water supply wells has been widely documented, reflecting enormous environmental and economic costs. Due to its abundance in gasoline, high aqueous solubility, and slow degradation rate in aquifers, MTBE has migrated in significant quantities from subsurface gasoline spills to a substantial number of community and private drinking water wells in a short period of time. For the purposes of this project, it was hypothesized that the tendency for gasoline additives to contaminate subsurface drinking water resources could be accurately predicted a priori using a generalized transport model. A screening method was developed to predict both the migration times of gasoline constituents from a leaking underground fuel tank (LUFF) to a community drinking water supply well and expected contaminant levels in the well. A review of literature revealed that U.S. municipal drinking water supplies are typically found in shallow sand and gravel aquifers. A subsurface transport model was parameterized based on the proximity of community supply wells to LUFTs (1000 in); probable characteristics of sand and gravel aquifers; typical pumping rates of community supply wells (80 to 400 gal/min); and reasonable gasoline spill volumes from LUFTs (100 to 1000 gal). The transport model was tailored to individual solutes based on their estimated abundances in gasoline, gasoline-water partition coefficients (Kgw), and estimated organic matter-water partition coefficients (Kom). Transport calculations were conducted for 17 polar and four nonpolar compounds currently proposed for or found in contemporary U.S. gasolines, including MTBE, ethanol, and methanol. Subsurface degradation processes were not considered. The transport model predicted MTBE concentrations of 40 to 500 ppb in municipal wells, which compared favorably with observed well concentrations at a significant proportion of sites in the U.S. The transport model therefore captured the order of magnitude of observed MTBE contamination of municipal wells without any use of adjustable or "fitted" parameters. Subsurface transport calculations of gasoline constituents required prior knowledge or estimation of their gasoline-water partition coefficient and organic matterwater partition coefficients. In anticipation of the need to conduct transport calculations for novel or previously unstudied compounds, a review of methods for calculating or predicting solute partition coefficients in gasoline-water, organic matter-water, and octanol-water systems was conducted. Additionally, a new linear solvation energy relationship (LSER) was developed for estimating gasoline-water partition coefficients of organic compounds, having an estimated standard error of 0.22 log Kgw units.by J. Samuel Arey.S.M

Universal Solvation Model Based on Conductor-Like Screening Model

ABSTRACT: Atomic surface tensions are parameterized for use with solvation models in which the electrostatic part of the calculation is based on the conductor-like screening model (COSMO) and the semiempirical molecular orbital methods AM1, PM3, and MNDO/d. The convergence of the calculated polarization free energies with respect to the numerical parameters of the electrostatic calculations is first examined. The accuracy and precision of the calculated values are improved significantly by adjusting two parameters that control the segmentation of the solvent-accessible surface that is used for the calculations. The accuracy of COSMO calculations is further improved by adopting an optimized set of empirical electrostatic atomic radii. Finally, the electrostatic calculation is combined with SM5-type atomic surface tension functionals that are used to compute the nonelectrostatic portions of the solvation free energy. All parameterizations are carried out using rigid (R) gas-phase geometries; this combination (SM5-type surface tensions, COSMO electrostatics, and rigid geometries) is called SM5CR. Six air–water and 76 water–solven