Abstract

Surface and interfacial tensions are key parameters affecting nonaqueous-phase liquid (NAPL) movement and redistribution in the subsurface after spill events. In this study, the impact of major additive components on surface and interfacial tensions for organic mixtures and wastewater was investigated. Organic mixture and wastewater compositions were based on CCl4 mixtures released at the U.S. Department of Energy's Hanford site, where CCl4 was discharged simultaneously with dibutyl butyl phosphonate, tributyl phosphate, dibutyl phosphate, and a machining lard oil. A considerable amount of wastewater consisting primarily of nitrates and metal salts was also discharged. The measured tension values revealed that the addition of these additive components caused a significant lowering of the interfacial tension with water or wastewater and the surface tension of the wastewater phase in equilibrium with the organic mixtures, compared with pure CCl4, but had minimal effect on the surface tension of the NAPL itself. These results led to large differences in spreading coefficients for several mixtures, where the additives caused both a higher (more spreading) initial spreading coefficient and a lower (less spreading) equilibrium spreading coefficient. This indicates that if these mixtures migrate into uncontaminated areas, they will tend to spread quickly but will form a higher residual NAPL saturation after equilibrium than pure CCl4. With time, CCl4 probably volatilizes more rapidly than other components in the originally disposed mixtures and the lard oil and phosphates would become more concentrated in the remaining NAPL, resulting in a lower interfacial tension for the mixture. These results show that the behavior of organic chemical mixtures should be accounted for in flow and transport models.

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