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1 <br /> E11at® <br /> • Mr James L Barton <br /> December 2, 2004 <br /> Page 6 <br />' fully operational since September 2004 Two quarterly groundwater monitoring and sampling <br /> events have been conducted since the restart of the SVE system <br /> Methodolo <br /> The analysts conducted herein is based on a method developed for analyzing contaminant <br />' plume stability at a chemical facility in Tennessee The methodology was developed as part of a <br /> USEPA RCRA Showcase Pilot study (Ricker, 2000) <br /> Plume behavior is assumed to be represented by the primary CDCs during the period between <br />' fourth quarter 1998 and fourth quarter 2004 For this analysis, the primary indicators selected <br /> were benzene and TPHg because they are the driving constituents leading the cleanup of the <br /> site The water table is currently approximately 36 feet below ground surface (bgs) Monitoring <br /> wells are screened at various depths across the site and some wells screened at differing <br /> depths are clustered together <br />' For this analysis, wells screened within the same zone were evaluated together No analysis of <br /> zone A was performed because concentrations of benzene and TPHg did not exceed laboratory <br /> detection limits in any of the monitoring wells in the most recent quarterly monitoring events <br />'0 There are currently three wells screened in zone C On November 16, 2004 well MW-31 was <br /> installed and screened in zone C but no groundwater data currently exists for MW-31 At least <br /> three points are needed to construct an iso-concentration plot, therefore, no iso-concentration <br /> plots were created for one C There are four wells screened in zone D and three wells have <br /> concentrations exceeding laboratory detection limits On November 17, 2004 well MW-32 was <br /> installed and screened in zone D but no groundwater data currently exists for MW-32, therefore, <br /> a plume analysis would be subject to much uncertainty No analysis of zone D was performed <br /> zone B is the only zone that has an adequate distribution of wells for an analysis <br /> For each indicator, iso-concentration maps were developed for aquifer zone B The iso- <br /> concentration maps were delineated to concentrations (5 pg/I for benzene and 60 pg/I for TPHg) <br /> dust above the laboratory detection limits This was done because for the wells where <br /> concentrations did not exceed detection limits, the detection limit was used as the input value If <br />' the maps were delineated to the cleanup levels, the outermost iso-concentration contour would <br /> merely"connect the dots" between all the wells that did not exceed the detection limit <br /> Iso-concentration maps were generated using Surfer' by Golden Software, Inc Surfer uses <br /> the kriging gridding method with the default impar vnringram Pimire A-4 in Appendix A shovrs <br /> the resulting concentration plume of benzene generated using Surfer' for pre-remediation <br /> conditions Figure B-16 in Appendix B shows the resulting concentration plume of benzene for <br /> current conditions These two sets of data were compared to observe the changes in plume <br /> area over time Overall plume average concentrations and volumes were also calculated based <br /> on an aquifer zone thickness of 20 feet <br /> llDcsacramentolprojects4Projects10694015098 293TinalkReportsl5098 R5E Results Reportdoc <br />