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Mr James L Barton 1 December 2, 2004 Page 8 Iso-concentration plots of benzene and TPHg over the period studied are included in the Attachments The plots indicate that the central mass of the plume is centered in an area generally under Robinhood Drive between MW-11 and MW-3 extending southwards to MW-2 ' The portion of the plume does not currently have any sparge wells within the area Monitoring well MW-2 is located in an area surrounded with sparge wells but MW-3 lies about 25 feet north of the nearest sparge well (VEW-12B) The nearest sparge wells to MW-11 are more than 20 feet away (MW-19D and AS-26) which is greater than their expected zone of influence, thus, the contaminant mass is not likely being impacted in the vicinity of MW-11 System Ob ectives Performance and Closure Criteria The objective of the remediation systems is to reduce dissolved concentrations of the CDCs measured in groundwater to below levels that are deemed safe to human health and the ' environment ENSR will continue to communicate with the RWQCB to ensure that remediation goals and objectives are met Site Visit Summary ENSR visits the site weekly since taking over remediation activities on behalf of Unocal in November of 2003 In general, ENSR found the remediation systems to be well maintained and operating as intended The design seems to be appropriate given what was known about the site at the time of construction The observations and recommendations provided in this report are not intended to imply a deficiency in the work of either the designers or operators, but are offered as constructive suggestions in the best Interest of both Unocal and the public These recommendations have the benefit of the operational data unavailable to the original designers System Performance Aside from the initial overhauls conducted prior to restarting the AS and SVE systems, neither system has required significant upkeep to maintain operation The air sparge system has operated for a total of 14,820 hours since operations began in October 1999 The AS system has operated 1,480 hours since ENSR restarted the system on June 14, 2004, approximately 50 percent of available uptime The majority of downtime is attributed to the AS system having been operated in a "pulse" mode---one week on/one week off—until the SVE system was restarted in September The SVE system has operated for a total of 6 919 hours since nrneratinng began in October 1999 and 507 hours since ENSR restarted the system on September 28, 2004, approximately 68 percent of available uptime The majority of SVE system downtime is attributed to monthly and quarterly groundwater sampling during which the system must be shut down to allow groundwater to equilibrate prior to sampling It is difficult to evaluate the performance of the SVE system, as it has only operated for two months since being restarted, thereby limiting the amount of data that Is available However, since being restarted the system has performed similarly to past performance levels Samples 14Dcsacramentolproiects\Proiects10694015098 293TinallReports098 RSE Results Reportdoc