| Until recently, the chemical of greatest concern at the 140,000 petroleum leaking underground storage tank (LUST) sites in the United States was benzene. Benzene is classified as a known human carcinogen with a federal maximum contaminant level (MCL) of 5 parts per billion (ppb) in drinking water. However, benzene tends to dissolve slowly, move slowly, and degrade fairly quickly in the environment, minimizing the potential for human exposure.; Now the main chemical of concern at many LUST sites is methyl tertiary butyl ether (MTBE). Added to gasoline up to 14% by volume to comply with 1990 Clean Air Act requirements, MTBE tends to dissolve quickly, migrate with ground water, and degrade slowly, maximizing the potential for human exposure. EPA set a consumer acceptability advisory level in water of 20--40 ppb based on taste and odor, and a preliminary remediation goal of 13 ppb based on carcinogenicity, Analysis of UST leak detection requirements shows a high potential for ongoing, undetected releases of petroleum containing MTBE. Reducing MTBE content in gasoline reduces the exposure duration and concentration, but not the potential for exposure.; In the absence of a nationwide MTBE ban, the most effective risk management approach remains prevention of leaks. In addition, properties of materials stored in USTs should be examined. Compounds that have low solubility, low mobility, and readily degrade (e.g. petroleum compounds) should be favored. Compounds that are more soluble, highly mobile, and degrade slowly (e.g. MTBE) should be avoided. Ground water protection should be considered when locating new USTs. Existing USTs located in environmentally sensitive areas should be targeted for more intensive oversight.; Managing the risks of exposure to MTBE contamination in ground water at LUST sites requires cleanup of the highest priority sites within a risk-based framework that includes long-term site tracking with appropriate use of engineering and institutional controls. The SiteRank model is presented for ranking sources of ground water contamination and water well vulnerability. The LUSTimpact model is presented for more quantitative evaluation of potential exposure risks associated with an MTBE source. A unique analytical solution is developed for evaluating contaminant transport to a pumping well. |
