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I <br />' for the gas phase (cm3 air CM-3 soil), and a,9 is the gas-diffusion coefficient in soil <br /> (cm3 soil air cm-1 sod d-'), defined by, <br /> where Gb,g is the gas diffusion coefficient in free air (cm2 d-i) and f is a tortuousity <br /> term (-) The dependency of fa on air-filled porosity, e(cm3 air cm-3 soil), was <br />' determined for each soil from gas diffusion experiments with a trace gas <br /> If the air-filled porosity of the soil, c(cm3 soil air cm-3 soil), and the sod water <br />' content, #(cm3 sod water cm-3 soil),are assumed constant and solute diffusion is <br /> negligible, effective diffusion of a sorbing chemical such as naphthalene in the soil <br /> can be described by, <br /> k <br /> D <br /> where Fis the chemical flux (pg cm-2 soil d-t), CGoe.r is the total chemical <br />' concentration (dig cm-3 soil), z is soil depth (cm soil), D6ff is the effective diffusion <br /> coefficient (cm2 soil d-1), Rg is the ratio of total/gas-phase concentration at <br /> equilibrium (Cinlei,equdibnum/gas,equildmium)hereafter referred to as the retardation factor <br /> for the gas phase (cm3 air cm-3 soil), and a,9 is the gas-diffusion coefficient in soil <br />' (cm3 soil air cm-1 soil d-1) <br /> le The gas diffusion coeffient, Db,Q, for naphthalene (Table 2)and f(c) can be used to <br /> I <br /> calculate Db g for naphthalene at a given sand used in Eq-J2] If instantaneous and <br /> reversible equilibrium between the soil phases is assumed,where Henry's <br /> constant, KH (cm3 soil water am-3 soil air), describes the distribution between the <br />' liquid and gas phase, and Ko (cm3 soil water g-i soil) is the linear adsorption <br /> coefficient, the retardation factor for the gas-phase becomes, <br /> KD 0 <br />' RS zl KI-1 [4] <br /> whereilb is the sod bulk density (g sol[ cm-3 sod) Following Olesen et al (2001), <br /> Eq f41 and U were used to estimate the apparent linear sorption coefficient <br />' (Ko,.pp) observed in the effective naphthalene diffusion experiment. Equation M is <br /> only an approximation of R91 since sorption typically will be nonlinear and time- <br /> dependent(Gamst et al , 2001) <br /> where Gb,g is the gas diffusion coefficient in free air (cm2 d-') and f is a tortuousity <br />' term (-) The dependency of f on air-filled porosity, , (cm3 air cm-3 sod), was <br /> n determined for each soil from gas diffusion experiments with a trace gas and in the <br /> UST excavation area (Figure A) layer concentration at—13, to—18' increases <br /> approximately 2 orders of magnitude in the 30 ' distance from the east to west <br /> inverted depth of the excavation <br /> I <br /> • <br /> I <br /> z <br />