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Evaluation ofNaturat Attenuation: 7500 West Eleventh Street, Tracy, CA. Page 49 VA 8.2.2.5 Summary ofMAROSAnalysis } <l Following is a summary of the principal findings from the MAROS analysis. i> • Statistical analysis by Mann-Kendall techniques of the historical r ; concentrations of the BTEX compounds and MTBE in samples of groundwater recovered from the Monitoring wells that have been installed at the 7500 West Eleventh Street site demonstrates that the plume of contaminated groundwater is stable. This finding is supported by the results of zeroth moment analyses of the geochemical data from each round of groundwater-quality monitoring that has been performed at the site. • The MAROS analysis indicates that, in regard to the array of groundwater-quality monitoring wells at the site that were installed i, in compliance with SJC's site characterization plans,none of those wells are redundant and, as a total array, they have permitted ' statistically reliable characterization and monitoring of the principal plume of contaminated groundwater affecting the site. With respect to the floating product monitoring wells installed at 1` the direction of the SJCEHD, the MAROS analysis flags Monitoring Well MWFP-1 as being unnecessary for proper monitoring of the plume and several of the others are of less than significant statistical importance. However, SJC recognizes that Monitoring Well MWFP-3 serves a useful purpose in that it acts as a sentinel for down-gradient migration of LNAPL that is periodically seen floating on the surface of groundwater in groundwater-quality Monitoring Well MW-7, which is approximately 20 ft. up-gradient from Monitoring Well MWFP-3. • The MAROS optimum frequency analysis has demonstrated that, .:. ; with three exceptions, samples do not have to be recovered from any wells at the Navarra site more frequently than once per year to provide for a statistically reliable and cost-effective groundwater- r:::r; quality monitoring program. In fact, samples need only be a recovered from 8 of the 27 wells that have been installed in the contaminated aquifer once every two years to achieve effective monitoring. The exceptions are recommendations to, for the present, continue quarterly sampling of groundwater in Monitoring Wells MW-3. MW-7 and MW-13. Those recommendations are q based on relatively high amplitude variations in concentrations of analytes of concern around the trend lines of plots of concentration versus time that have been observed in groundwater recovered i from Monitoring Wells MW-3 and MW-7 and on the limited geochemical data available for groundwater in Monitoring Well f = MW-13, which was not installed until April 30, 2004. sic