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Report: Groundwater-quality Monitoring--March 24-31,2006, 7500 West Eleventh Street, Tracy, CA Page 27 8.0 RECOMMENDATIONS Based on our evaluation of the current environmental condition of the 7500 West Eleventh Street Site, as developed from data gathered over seven years of site characterization and groundwater-quality monitoring, SIC has the following recommendations. 8.1 Groundwater-quality Monitoring Program Guidance provided by the American Society for Testing and Materials Standard Guide for Remediation of Groundwater by Natural Attenuation at Petroleum Release Sites. (American Society for Testing and Materials 2004) calls for annual monitoring of groundwater quality by recovery of groundwater samples from wells located along the plume axis at sites where natural attenuation processes have been demonstrated to be active, which is the case at the 7500 West Eleventh Street Site. If that guidance were = followed, the scope of annual monitoring would involve recovery and analysis of groundwater samples from Monitoring Wells MW-3, MW-13, MW-7, MW-14, MW-16, , MW-18, MW-11 and MW-19. However, in developing recommendations for groundwater-quality monitoring programs, SJC prefers to rely on a thorough well- specific statistical analysis of geochemical data to evaluate the sampling frequencies required to achieve an efficient program that, at the same time, ensures that changes in the concentration of analytes of concern in groundwater can be tracked with a high degree of confidence. As was discussed in Section 5.0 of this report, SJC again analyzed the groundwater— quality monitoring data that has been gathered for the 7500 West Eleventh Street Site using the MAROS protocols (United States Air Force 2004), use of which is advocated by both the United Stares Environmental Protection Agency (US-EPA), and the California Environmental Protection Agency (Cal-EPA) (LJnited States Environmental Protection Agency 2005). That analysis demonstrated that an efficient groundwater- quality monitoring program at the site would require samples to be recovered from fifty f' percent of the wells in the monitoring array on no more than a biennial schedule and, with minor exceptions that are of no practical significance, for samples to be recovered from the remainder of the wells on no more than an annual schedule. Based on those results, SJC recommends that groundwater samples be recovered for analysis from the wells located along the axis of the principal plume of affected groundwater emanating from the 7500 West Eleventh Street on an annual schedule. However, to monitor groundwater quality on the western and eastern peripheries of the primary plume, we also recommend monitoring groundwater quality in Monitoring Wells MW-2, MW-4, MW-5, MW-10 and MW-17 on the same schedule. In addition, to monitor conditions in and around the area of relatively-elevated concentrations of components of fuel hydrocarbons that is centered at Monitoring Wells MWFP-1 and MW-7, annual sampling and analysis of groundwater in Monitoring Wells MWFP-1 through MWFP-5 is also recommended. Samples should also be recovered annually from Monitoring Well MW-12, so that the concentrations of analytes of concern at the area around the origin of the secondary volume of affected groundwater at the site can be monitored. SJC