Ground‐Water Contaminant Plume Differentiation and Source Determination Using BTEX Concentration Ratios

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Abstract
Concentration ratios of benzene, toluene, ethylbenzene, and xylenes (BTEX) in ground water can be used for ground‐water contaminant plume differentiation and source determination. Computer modeling utilizing BTEX soil‐water partitioning coefficients and biodegradation rates shows that hydraulic dispersion, retardation, and biodegradation do not significantly modify the BTEX concentration ratios in ground water, particularly those of ethylbenzene and xylenes. Therefore, the BTEX concentration ratios are similar in the contaminant plumes that are derived from a common source or sources of similar BTEX compositions. In the vadose zone, the BTEX concentration ratios in downward‐migrating contaminant free‐product remain essentially unchanged because of the dynamic nature of soil adsorption and restricted natural soil ventilation. The only significant change in the BTEX concentration ratios occurs in the partitioning between ground water and contaminant free‐product as the result of differential BTEX solubilities in water. In the partitioning, benzene/toluene and toluene/ethylbenzene concentration ratios of ground water are 3.6 and 3.3 times greater respectively than the ratios at the source, while the ratios of ethylbenzene and xylenes remain unchanged.The geochemical methods were successfully applied and tested at two sites where multiple ground‐water contaminant plumes from different sources were superimposed. The conclusions of geochemical analysis are consistent with the site‐specific hydraulic characteristics and facility operation histories.