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Surface and Interfacial Properties of Nonaqueous-Phase Liquid Mixtures Released to the Subsurface at the Hanford Site

^a Dep. of Civil and Environmental Engineering, Environmental Engineering and Science, Univ. of Illinois, 205 N. Mathews Ave., Urbana, IL 61801
^b Hydrology Group, Energy and Environment Directorate, Pacific Northwest National Laboratory, P.O. Box 999, MS K9-33, Richland, WA 99354

Correspondence: ^* Corresponding author (hyoon3{at}illinois.edu).

Received for publication 25 June 2008. 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.

Abbreviations: DBBP, dibutyl butyl phosphonate • DBP, dibutyl phosphate • DDA, dodecylamine • DNAPL, dense nonaqueous-phase liquid • LO, machining lard oil • NAPL, nonaqueous-phase liquid • TBP, tributyl phosphate