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exceeding conservative values for benzene. However, the decay rate of 1.Oyr 1 was within the <br />acceptable range for the model.:=.� <br />The results of the BIOSCREEN model for benzene data going back 10 years showed the <br />plume of groundwater contaminated by benzene exceeding -LO µg/L is 70 feet downgradient <br />of the source area:, This coincides with observed data. -_The input parameters used in the <br />calibration of theImodel were used again in'BIOSCR'EEN to run a 20 -year and 50 -year <br />simulation of the fate and transport given the present day residual concentrations and <br />estimated dissolved mass. The simulation time was based on the trend analysis for benzene in <br />source are wells. The 2009 concentration- in - well -`PZ -l' was used- as thesource' area <br />concentration and the isoconcentration contours were used for the source width. The <br />BIOSCREEN results for the fate and transport of the residual contamination at the Site using <br />a 20 -year simulation indicate that the plume of groundwater contaminated by benzene <br />exceeding 1.0 µg/L would,be,approximately 100 feet downgradient of the source area in 20 <br />year,The 5 -year simulation indicates that using''first order decay rate of I:0 yr_'; the'plu`me` <br />exceeding 1.0 µg/L would only exntendybetween �50 and 100,feet downgradient. If no <br />degradation occurs, the concen[rat oat the �propertyboundary m 50 years would be <br />approximately 160 µg/L. _ ... <br />The applicable input parameters were also used in the ASTM RBCA model. The results of <br />the R-26 equation indicate that the plume would migrate approximately 70,feet before <br />-.:..?., is <br />reaching [he.l .ffµg/L--MCL. The input and results of the modeling is presented along_ ­;rt <br />wh the <br />source of the input pa parameters in Appendix C. <br />PSC entered site-specific data for natural attenuation parameters in BIOSCREEN. These <br />included delta values for dissolved oxygen, sulfate, and nitrate in background wells compared <br />to source area wells and the observed concentrations for ferrous iron and methane for source <br />area wells. The data used was collected prior to AS/SVE. The results for biodegradation as <br />an instantaneous reaction indicated that the plume would have completely biodegraded from <br />1999 through 2009. Based on the observed results for wells, this modeled rate of <br />biodegradation from an instantaneous reaction has not occurred. Therefore, the instantaneous <br />reaction results for the projected plume in 20 or 50 years are not valid. `" <br />