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I ' Partitioning of Compounds Between Water and Subsurface Solids ' • Given sufficient time, equilibrium can be established between concentrations of MTBE in air and water as described by Henrys Law constant, which is temperature dependent If the same units are selected for the air and water I concentrations, then Henry's Law constant is dimensionless However, the Henry's Law constant alone cannot be used to predict the volatility of MTBE from natural water because volatilization also depends on environmental vanables such as ' surface-water turbulence, and to some extent wind velocity Water solubility is probably the most important chemical property affecting the partitioning of organic compounds between water and subsurface solids Many ' organic compounds exhibit water solubilities in the low milligrams-per-liter to micrograms-per-liter range In general, these low solubilities indicate a strong partitioning to the organic carbon associated with the subsurface solids However, ' MTBE is very water soluble compared to the BTEX compounds and other components in gasoline, the solubility of pure liquid MTBE in water is about 50,000 mg/L (milligrams per liter) whereas the next most-soluble component of gasoline is ' benzene, which has a solubility of 1,780 mg/L (Mackay and others, 1992, 1993) Diffusion of HOCs in the vadose zone consists of several interacting processes of ' which three appear to be the dominating and controlling mechanisms; gaseous diffusion, liquid diffusion, and intraparticle and intraorganic matter diffusion (controlling adsorption-desorption reactions) Gas-phase diffusion coefficients are generally considered 103 to 104 larger than solute diffusion coefficients (Petersen I et al ,_1994),and the intraparticle and intraorganic matter diffusion is considered to be orders of magnitude lower (Grathwohi. 1998) Hence, it is important to recognize that the effective diffusion of HOCs in soil is not alone controlled by the ' soil-water content,but also by soil and chemical properties because these strongly affect the partitioning of HOCs between the soil phases and therefore controls the ' velocity at which HOCs move Gaseous diffusion has been shown to be dominant for many volatile HOCs in unsaturated soil because of a significant partitioning into the gaseous phase ' (McArthy and Johnson. 1993, Petersen et al . 1994, Batterman et al . 1996) If the air-filled porosity of the soil, C(cm3 soil air cm-3 soil), and the soil water content, #(cm3 soil water cm-3 soil),are assumed constant and solute diffusion is ' negligible, effective diffusion of a sorbing chemical such as naphthalene in the soil can be described by, fi/rT h l^ 1� .. • � �� f'1 C21 ' where Fis the chemical flux (Ng cm-2 soil&), Ctoiai is the total chemical concentration (jig cm-3 soil), z is soil depth (cm soil), 06 is the effective diffusion ' coefficient (cm2 soil d-1), Rg is the ratio of total/gas-phase concentration at equilibrium (CioW,equiiibnurdGas equdibnum)hereafter referred to as the retardation factor • 1