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1 <br /> 1 <br /> EIIu% t® <br /> Mr James L Barton <br /> 1 December 2, 2004 <br /> Page 7 <br /> 1 Area Volume Average Concentration & Mass Calculations <br /> SurferO can calculate the planar area that exceeds a specified level of the input grid file In this <br /> 1 application, the specified level is the concentration above which we wish to calculate the plume <br /> size For example, in Figure A-4, the plume is delineated to a benzene concentration of 5 pg/I <br /> This represents a concentration slightly above the cleanup level of 1 pg/I The area of the <br />' plume calculated by Surfer® would be defined as the planar area within the 5 pg/I iso- <br /> concentration contour line <br /> The average concentration within the plume is determined using the volume function in Surfer® <br />' The volume function is most often used in calculating cut and fill volumes for excavating protects <br /> but can be used with the planar area to calculate an average concentration within the plume <br /> (Ricker) <br /> The mass of contaminant within the plume was found by multiplying the plume volume times an <br /> assumed average porosity of 0 3 times the average concentration as calculated above The <br /> average porosity is based on the porosity of sand which is the predominant sod beneath the site <br /> (Fetter) The plume volume is determined by multiplying the planar area by the assumed <br /> thickness of the aquifer zone, 20 feet for zone B <br /> 1• Contaminant Plume Evaluation <br /> Table 1 summarizes the plume calculations since the fourth quarter of 1998 Plume <br /> I assessments were calculated using groundwater analytical data <br /> Figure 4 is a plot of the changes in plume area versus time since prior to the start of <br /> remediation The plume area has generally been decreasing since remediation began except <br /> for a period of rebound at the end of 2003 This was a period where the remediation system <br /> was beginning to experience significant downtime prior to a system overhaul in the spring of <br /> 2004 The benzene plume area has decreased from 4 7 acres just prior to remediation to <br /> 2 71 acres in the latest quarter The area of the TPHg plume has decreased from 4 85 acres to <br /> 2 68 acres in the latest quarter <br /> While the plume area has been gradually decreasing, the average contaminant concentration <br /> has had more variability but still exhibits a decreasing trend The average concentration of <br /> benzene has decreased from 1,249 pg/I in the fourth quarter of 1998 to 747 dig/I in the fourth <br /> quarter of 2004 (40 percent reduction) TPHg has decreased from an average concentration of <br /> 7,700 pg/I in the fourth quarter of 1998 to 3,702 fag/I in the current quarter (52 percent <br /> reduction) Figure 5 depicts the changes in average concentration within the plume over time <br /> Similar to the changes in plume area and averse concentration,, the contaminant mass has <br /> also been decreasing since remediation began Initially there was a steep decline in the mass <br /> at the start of remediation but then the mass leveled off until the end of 2003 when there was a <br /> . large spike in the mass This corresponded to the system operating issues just prior to system <br /> overhaul Figure 6 is a plot of the contaminant mass changes since remediation began <br /> k1DcsacrarnentolprojectslProyectsl0694015098 29IFinahReports15098 RSE Results Report doc <br />