ARCHIVED REPORTS_XR0010993 - Laserfiche WebLink

Home Browse Search

SECTION 5 CONCLUSIONS AND RECOMMENDATIONS i 5-1,5-2,and 5-3. TPH-gas plume volumes for each isoconcentration interval were calculated based on areas and thicknesses derived from the plume maps and cross-section maps. The mass of the dissolved fraction of TPH-gas beneath the site was calculated by multiplying the volume of soil within each TPH-gas isoconcentration interval,with the soil porosity (conservatively assumed to be 40%) and the TPH-gas dissolved concentration for that interval.Table.5.1 presents the volumes and mass calculations for the dissolved and sorbed portions of the TPH-gas plume. The total mass of dissolved TPH-gas is estimated to , p be approximately 2400 ounds. � In an effort to estimate the mass of TPH-gas that is sorbed to soil,a simplified,worst-case calculation was performed. A lumped,TPH-gas sorption distribution coefficient was derived using the following linear sorption isotherm equation and soil and groundwater samples collected within boring CPT-4: _ C., Kd � C" _ where: C.,=mass of solute sorbed per unit weight of solid (mg/Kg) C. =concentration of solute in solution (mg/L) Kd= distribution coefficient This Ka relationship is only valid for solutes of individual compounds.Gasoline is comprised of hundreds of hydrocarbon compounds,and the sorption behavior of gasoline is quite complex. Therefore,a single distribution coefficient for gasoline is not technically valid. A mass estimate using this distribution coefficient is only an approximation. The sorbed (Cs)TPH-gas concentration was assumed to be 510 mg/Kg,based on the soil sample collected from CPT-4 (92-93 ft bgs). This concentration reflects a conservative estimate of the true sorbed concentration,since this result is comprised of a combination of the total dissolved phase,vapor phase,and sorbed phase of TPH-gas within the sample. The dissolved (Cm)TPH-gas concentration was assumed to be 160 mg/L,based on the groundwater sample CPT-4(97 ft bgs). Based on data from these soil and groundwater samples,the lumped TPH-gas distribution coefficient,Kd,is estimated to be 3.2 L/Kg.Using a Kd of 3.2 L/Kg,and the soil volumes and soil mass presented in Table 5-1,the calculated mass of sorbed TPH-gas is approximately 36,500 pounds. This relates to a mass of sorbed TPH-gas that is j approximately 15 times that of the dissolve portion. In order to check the validity of a Kd value of 3.2 L/Kg,a lumped TPH-gas retardation factor was derived using the following equation: 5.2