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Evaluation of Natural Attenuation: 7500 West Eleventh Street, Tracy, CA_ Page 48 When those evaluations have been made, well sampling interval recommendations are expressed as Quarterly, Semi-annually, Annually or Biennially for each analyte of concern in each well (United States Naval Facilities Engineering Service Center 2000). The MAROS protocol employs some modifications to the CES methodology. Instead of using a distribution-free version of the coefficient of variation to assess statistically rate of change in concentration of analytes of concern with time, MAROS uses Mann-Kendall statistical procedures similar to those described in Section 8.2.2.2 above. In addition, MAROS substitutes preliminary remediation goals (PRGs) promulgated by Region IX of f the US-EPA (United States Environmental Protection Agency Region IX 2004) for the MCL criteria originally used by LLNL. The groundwater-monitoring program at the 7500 West Eleventh Street Site has extended from May 2000 thru March 2005. For each well, all of the data for each of the analytes of primary concern were used to make an initial recommendation for optimal sampling frequency. To conservatively account for any sampling frequency requirements based on the recent data obtained from the site, the results of the analyses of the critical analytes of concern for the last five sampling rounds, extending over the period January 2004 to March 2005, were considered. The results of the MAROS sampling frequency optimization are presented in the final table in Appendix A. Recommendations are made for sampling frequencies at each of the monitoring wells in the primary plume of contaminated groundwater emanating from the 7500 West Eleventh Street property and individual recommendations are made for each of the BTEX compounds and MTBE. The results show that, with three exceptions, to maintain statistically-reliable groundwater-quality data, it is necessary to recover samples from the monitoring wells only once every two years (biennially) or once per year -' (annually). The exceptions are monitoring wells MW-3, MW-7 and MW-13, for which an optimal sampling frequency of quarterly is recommended. The quarterly sampling recommendation for MW-13 is based on the relatively-high amplitude of the variations in the concentrations of benzene that have been observed in samples of groundwater recovered from that well since its relatively-recent installation in April 2004. Similarly, the highly-variable concentrations of analytes of concern in MW-7, which are related to ti>>3 the occasional appearance of thin layers of LNAPL in that well since 2004, that have been periodically purged, leads to the recommendation of quarterly sampling from that r well. In the case of Monitoring Well MW-3, based on the total data record over the history of sampling from that well, an annual sampling frequency was recommended; however, as can be observed by examining Table 2, over the previous year (i.e., the "recent" groundwater-quality data), there have been relatively high amplitude variations in the concentration of benzene recovered from that well. Those concentrations have been related to fluctuations in the depth to groundwater that have caused the groundwater to be periodically exposed to significantly different concentrations of benzene adhered to soil in the smeared zone around that well. i sic