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� v <br /> a <br /> A S S O C I A T E S I N C <br /> upgradient direction TPHg was detected in MW7 once, in January 2002, at a <br /> ' concentration of 170 ug/1 The only BTEX compound detected in MW7 was a detection of <br /> toluene at a concentration of 0 84 ug/l and xylenes at a concentration of 1 5 ug/1 in January <br /> 2002 This was the same sampling event during which TPHg and BTEX concentrations <br /> ' reached a peak in well MW1 MTBE has never been detected in MW7 <br /> 4 2 3 Mass Balance Calculations - Groundwater <br /> ' Prior groundwater monitoring reports indicate that a total of 2,709,959 gallons of hydrocarbon <br /> impacted groundwater have been extracted and treated since April 2002 Mass balance <br /> ' calculations from the prior reports indicate that a total of 5 89 pounds of benzene and 146 7 <br /> pounds of TPHg have been extracted from groundwater beneath the site <br /> ' Using groundwater analytical results presented in Table 2, the most recent data available, ATC <br /> estimated the lateral extent of dissolved TPHg concentrations beneath the site as depicted on <br /> Figure 7 The total mass of benzene remaining in the groundwater was not calculated because <br /> ' benzene was not detected in groundwater samples collected during the most recent sampling <br /> event The isoconcentration shown on Figure 7 was used by ATC to perform mass balance <br /> calculations to estimate the mass of TPHg impacted groundwater The total mass of TPHg <br /> remaining in the groundwater is estimated to be approximately 2 59 pounds Mass balance- <br /> calculations are included in Appendix C <br /> ' 5.0 MODEL OF GROUNDWATER PLUME <br /> ' A flow model was created using the modular finite-difference ground-water flow model <br /> (MODFLOW) developed by the U S Geological Survey (USGS) Once a flow model was <br /> created, a solute transport model was generated using the MT31) model MT313 is a <br /> comprehensive three-dimensional solute transport model for simulation of advection, <br /> dispersion and chemical reactions of contaminants in groundwater MT31) was first developed <br /> by Chunmiao Zheng in 1990 at S S Papadopulos & Associates, Inc with partial support from <br /> ' the U S Environmental Protection Agency (USEPA) MT31) has been widely accepted by <br /> practitioners and researchers and has been applied to numerous field-scale modeling studies in <br /> the United States and throughout the world <br /> 1 51 Groundwater flow model using MODFLOW <br /> The groundwater flow model was generated by setting up a 370-foot by 400-foot grid with a cell <br /> size of 10-feet A single unconfined layer was specified with a thickness of 25 feet The <br /> hydraulic gradient was assumed to be towards the west based on laboratory analytical data from <br /> groundwater samples and the plume migration The cell values for initial hydraulic heads were <br /> set to —12 5-feet along the western boundary of the model and to —7 9 feet along the eastern <br /> boundary The corresponding IBOUND values for these cells were set to —1 (constant head) <br /> These input parameters produced a hydrologic gradient of approximately 0 0125 ft/ft (12 5 <br /> w U71421reponslclosure doe 16 <br /> 11 <br /> I� <br />