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A S S O C I A T E S I N C <br /> • ATC chose to construct the model using a simplified single-layer model with an unconfined <br /> aquifer. The lithology beneath the site consists of primarily silty sand with lesser amounts of <br /> clean sand and clay which appears to present a single unconfined aquifer to a depth of at least <br /> 25 feet. <br /> • Historic groundwater elevation data shows that the hydrologic gradient has varied over a <br /> wide range of directions over time. A west-southwest hydrologic gradient was chosen for the <br /> model based on historical hydrocarbon plume definition. Isocontour maps of groundwater <br /> hydrocarbon plumes from groundwater monitoring events clearly show that hydrocarbon <br /> plume migration is towards the west-southwest despite the appearance of varying <br /> groundwater flow directions. Additionally, the calibration of the plume extents with field <br /> data results in model data that appears to accurately show the direction of the plume <br /> migration. <br /> • In June 2001, Advanced GeoEnvironmental, Inc. (AGE) conducted an aquifer pump test at <br /> the site. Groundwater was extracted from wells VE6 and MW4 using an electric submersible <br /> pump. Groundwater depths were monitored in these wells and in surrounding monitoring <br /> wells. Hydraulic conductivities were calculated ranging from 0.015 to 0.020 centimeters per <br /> second (cards). However, ATC chose a hydraulic conductivity value of 0.0012 cm/s for the <br /> MODFLOW groundwater flow model. This value was selected'by manually calibrating the <br /> flow model by comparing the extents of the hydrocarbon plume predicted by the model with <br /> the isocontour maps produced during quarterly groundwater monitoring events. This <br /> hydraulic conductivity value falls within the range of a silty sand (Freeze & Cherry, 1979). <br /> This lithology is consistent with data presented in soil boring logs and soil cross sections <br /> produced for the site. The discrepancy between the hydraulic conductivity values calculated <br /> from the AGE pump test data and those obtained from the flow model may be due to the <br /> presence of backfill material placed in the former UST excavations that may be coarser <br /> grained than the surrounding native soils, therefore having a higher hydraulic conductivity. <br /> • The EHD correspondence stated that data from MW-1 indicates that remediation has been <br /> the dominant mechanism for total petroleum hydrocarbons as gasoline (TPHg) and benzene <br /> concentration reductions. The flow and transport model was calibrated by comparing total <br /> petroleum hydrocarbons, modified for TPHg isocontours generated from January 2002 <br /> groundwater laboratory data and TPHg isocontours generated by the model and adjusting the <br /> horizontal hydraulic conductivity of the soil until the positions of the two sets of isocontours <br /> were in approximate agreement. Therefore the model takes the effects of the SVE system <br /> into consideration but does not take the effects of the GWE system into consideration. This <br /> takes into account the rebound of the hydrocarbon concentrations prior to the shutdown of <br /> the SVE system. It should also be noted that laboratory analytical data from MW-1 indicates <br /> that-TPHg and benzene concentrations had stabilized after an initial rebound. Laboratory <br /> analytical data from MW-1 shows significant decreases in TPHg and benzene concentrations <br /> subsequent to the startup of the GWE system. However, data from MW-3, MW-4, and MW- <br /> 5 show significant decreases prior the startup of the GWE system. Thus it appears that <br /> natural attenuation is a significant factor at the site. <br /> env ironmenta1\271421reponse.doc 2 <br />