SITE INFORMATION AND CORRESPONDENCE_FILE 2

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v A S S O C I A T E S I N C • A hydraulic conductivity value of 0.0012 cm/s (4x10' feet per second) was selected. This value falls within the range of values that are typical of silty sands (Freeze and Cherry, 1979). Subsequent to the calibration of the flow model, the transport model was calibrated by adjusting the concentration of recharge flux until the longitudinal extents of the two sets of TPHg isocontours were in approximate agreement. Groundwater hydrocarbon concentrations predicted from the flow and transport model and those from laboratory analytical data collected during the groundwater monitoring events may not exhibit exact agreement. However, the model does appear to predict the general spread and extent of the hydrocarbon plume and the approximate hydrocarbon concentrations in the wells over time. • The mass balance calculations of hydrocarbons present in water beneath the site were calculated based on areas estimated from isocontours produced from November 2003 laboratory analytical data. Volumes were calculated by multiplying these areas by a depth of 15 feet. This calculation would appear to be a conservative estimation since the contaminants are primarily short carbon chain hydrocarbons that would tend to be concentrated near the water surface. However, there appears to be over 2,600 kilograms of hydrocarbons remaining in the soil beneath the site. The MODFLOW model utilized the recharge package to simulate the loading of the hydrocarbon plume due to leaching of hydrocarbons sorbed in the soil to groundwater. Additionally, the MTRCTI chemical reaction package was utilized in the MT3D transport model to simulate biodegradation of hydrocarbons and the interaction of aqueous-phase and solid-phase concentrations. Discussion of No Further Action Required Reporting Criteria • Criterion 3: Contaminants remaining in the vadose zone cannot migrate in soil vapor 01- leach at concentrations that would cause groundwater to exceed water quality objectives The EHD indicated that the soil model does not account for rising or falling groundwater elevation of for contaminant flux from submerged impacted soil. The model was roughly calibrated to field data while utilizing the MODFLOW recharge package and the MT313 chemical reaction package, therefore, the model does account for flux from vadose zone contaminant flux as well as submerged impacted soils. • Criterion 4: There are no existing water supply wells, sur face waters of other receptors threatened by the remaining contaminants in soil or groundwater. The EHD indicated that the main threat posed by the plume, should it expand under natural conditions, would be to additional volumes of groundwater currently not impacted by petroleum hydrocarbons. It is apparent from the groundwater receptor survey that the plume does not threaten water quality in any wells in the adjacent area. EHD's opinion that "the main threat posed by the plume, should it expand under natural conditions, would be to additional volumes of groundwater currently not impacted by petroleum hydrocarbons" extends criterion 4 beyond env ironmenta1127142\reponse.doc 3