Surface and interfacial properties of nonaqueous-phase liquid mixtures released to the subsurface at the hanford site

Surface and interfacial tensions are key parameters aff ecting nonaqueous-phase liquid (NAPL) movement and redistribu tion in the subsurface atier 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 composi- tions were based on CCl ₄ mixtures released at the U.S. Department of Energy's Hanford site, where CCl ₄ 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 signifi cant 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 CCl ₄, but had minimal eff ect on the surface tension of the NAPL itself. These results led to large diff erences in spreading coeffi cients for several mixtures, where the additives caused both a higher (more spreading) initial spreading coeffi cient and a lower (less spreading) equilibrium spreading coeffi cient. This indicates that if these mixtures migrate into uncontaminated areas, they will tend to spread quickly but will form a higher residual NAPL saturation atier equilibrium than pure CCl ₄. With time, CCl ₄ 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 fl ow and transport models.