**2. BTEX Characteristics and Fate into the Aquifers**

The BTEX forms about the 16% of a typical gasoline blend and is associated with adverse impact on human-health. Although the individual BTEX compounds are widely used as solvents and in manufacturing, gasoline leaks from underground storage tanks and distribution pipelines are the primary contributor of BTEX contamination in ground water [21].

Among the four BTEX compounds, benzene is classified as a carcinogen by the European Union Council Directive 98/83/EC, the U.S. Environmental Protection Agency and the International Agency for Research on Cancer. As a result of this concern, threshold levels in freshwater have been established, namely the Maximum Contamination Limits (MCL) [22,23]. Nevertheless, respective thresholds or permissible levels in groundwaters have not been set, so far. Table 1 presents the BTEX MCLs, expressed in parts per million (ppm, or mg/L) in the EU and several other countries with high recorded oil pollution.


It is well known that many organic contaminants exist in a liquid phase and are not soluble in water. These are the Non-Aqueous Phase Liquids (NAPLs) which include fuels (gasoline, aviation fuel), chlorinated solvents, and polychlorinated biphenyls. In a typical petroleum hydrocarbon contamination incident, such as an underground leaking of a storage tank or pipeline, the escaping organic substance (e.g., gasoline) usually moves downwards through the unsaturated or vadose zone until it reaches the aquifer. There, the mobility of the organic substance (e.g., hydrocarbon) decreases as water saturation increases respectively and accumulation takes place particularly at the interface, above the water-table.

Some important parameters determining the environmental fate of the organic substance are the compound's volatility (gaseous phase), solubility in water (aqueous phase), specific weight determining floating or sinking in the aqueous zone, and affinity to minerals and other organic compounds and surfaces, determining sorption (absorption/adsorption and dissolution) [24]. Furthermore, the degradation rate of the compound depends on the presence of bacteria and fungi species, the environmental conditions (temperature, aquifer minerals, organic matter content), and the availability and concentration of carbon sources available to the microbial consortia. The rate of biodegradation tends to slow down when DO concentrations are less than about 1–2 ppm. Anaerobic biodegradation of benzene appears to be more aquifer specific than that for the other monoaromatic hydrocarbons [25–27].
