Gas partitioning of dissolved volatile organic compounds in the vadose zone: Principles, temperature effects and literature review

Enthalpy and entropy of volatilization from dilute aqueous solutions for 26 volatile organic compounds (VOCs) have been determined using Henry's Law values reported in published literature. Based on the linearity of van't Hoff plots, for the temperature ranges common in soils, the differences in heat capacities of volatilization for reactants and products are very small for the VOCs studied. When volatile solutes such as VOCs are present in soil water, soil-gas concentration often nearly is in equilibrium with the dissolved solute. Setchinow salting coefficients are linearly related to dissolved partial molar volumes for halogenated aliphatic compounds. Based in part on approximations from this linear relationship, equilibrium deviations from Henry's Law behavior for dilute VOC concentrations due to capillary tension or the presence of ionic solutes are small for common soil conditions. Since gas/water partitioning of VOCs if temperature-sensitive and since annual soil moisture and temperature patterns vary geographically in documented fashion, geographically specific temporal patterns in soil-gas VOC concentrations are predictable in vadose zones containing dissolved VOCs. A U.S. map depicting these general soil-moisture and temperature patterns is provided. Gas concentrations in vadose zones containing dissolved VOCs tend to increase with increasing temperature and decreasing moisture content due to equilibrium partitioning effects. Diagrams useful for understanding the results of soil-gas surveys and the efficacy of various remediation options are provided. The effect of bubbles in VOC water-sample vials on aqueous concentrations is shown to be very small. The effect of head-space volume of soil samples on estimated soil-gas concentrations can be large.