Sensitivity of organic contaminant transport and persistence models to Henry's Law constants: Case of polychlorinated biphenyls

Abstract

Volatilization can be an important process influencing the transport and persistence of trace organic compounds in the environment. Henry's Law constants (H) describe the equilibrium distribution between vapor and aqueous phases, and thus are important for prediction of biogeochemical dynamics of organic chemicals subject to significant volatilization. Variability in reported H values can be appreciable, particularly for compounds such as polychlorinated biphenyls (PCBs) which possess low vapor pressures and solubilities. Recently reported Henry's constants for a number of PCB congeners were compiled to assess the magnitude of this variability. Coefficients of variation were generally 30 to 70% and tended to increase with increasing chlorine substitution and decreasing magnitude of Henry's constant. The influence of this variability on predicted transport and persistence of PCBs in soil and aquatic systems was evaluated using the behavior assessment and two-layer models, respectively. The 2,4′, 2,2′, 5,5′, and 2,2′,4,4′,5,5′ congeners were selected for this analysis on the basis of their high content in Arochlor fluids and range in hydrophobicity, solubility, and reported H values. Results from the two-layer model indicate that both the liquid and gas boundary layers contribute to the overall resistance to mass transfer. Selection of H had a moderate influence on predicted transport and persistence for the idealized lotic aquatic system modeled. Decreasing H and increasing relative contributions of gas phase resistance to overall resistance to mass transport resulted in longer predicted congener persistence. Selection of H also influenced predicted volatilization rates from soil. The volatilization flux rate was found to be controlled by impedance in the soil body for all but very short times, where boundary layer effects tended to limit volatilization. Evaporation, previously demonstrated to enhance volatilization for some chemicals, did not have a significant effect on predicted PCB vapor losses from soil.