Gasoline leaking from underground storage tanks: impact on drinking water quality

Of the 1.4 million underground storage tanks (USTs) now in use for storing gasoline, it is estimated that 10-35% are leaking or fail to pass a tightness test. In order to assess the potential for pollution of drinking water due to gasoline leaking from USTs, it is necessary to understand the complex composition of gasoline, the movement of individual gasoline components through the soil matrix to the water table and their transport via groundwater movement to receptor wells. The primary factors that contribute to the environmental movement of gasoline components are water solubility, soil-sorption, biodegradation, and rainfall. Gasoline is a complex mixture of at least 150 hydrocarbons with about 60-70% paraffins and 25-30% aromatics. Of these, the aromatics are of greatest concern due to their water solubility and resistance to biodegradation. Benzene, toluene, and xylenes are of particular concern. Another chemical of concern is methyl tertbutyl ether (MTBE), a hydrocarbon now being added in large quantities in many brands as an octane-enhancer. MTBE has been found in many wells as the first chemical arriving at receptor wells following a gasoline leak or spill. Models are available for predicting concentrations at receptor wells; however, site specific input, e.g., soil type, biodegradation,more » and rainfall, are necessary for the meaningful and reliable application of the models. Movement of chemicals through the environment is a slow process; often it may take several years for a chemical to move from a leaking tank to the nearest receptor well.« less