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Report:Groundwater-quality Monitoring—March 29-31,2006, 7500 West Eleventh Street, Tracy, CA Page 1$ The results of the plume stability analysis are presented in the MAROS Mann-Kendall Statistics Summary presented in Appendix A. Examination of that Table shows that, none of the BTEX compounds, which, due to their carcinogenic and toxicological properties are the analytes of greatest concern at the Navarra Site, has a trend vs. time that is classified as "increasing," nor even "probably increasing." These results demonstrate that the primary plume emanating from the 7500 West Eleventh Street Site is stable, as SJC has previously concluded, based on conventional plume stability observations (The San Joaquin Company 2006d). 5.3 Optimal Sampling Frequency Y Unnecessarily frequent sampling from arrays of groundwater--quality monitoring wells and sampling from a larger number of wells than is necessary to provide sufficient statistically-reliable data to efficiently monitor groundwater quality at a given site can generate very high project costs without improving understanding of conditions within a plume of contaminated groundwater. To permit design of an efficient groundwater-quality monitoring program, the MAROS ' protocol computes optimal sampling frequencies for each well in the well array for each of the critical analytes of concern. The method employed by the software is based on the Cost-Effecting Sampling (CES) methodology developed by Lawrence Livermore National Laboratory(LLNL) (Ridley, et al 1995). The CES method employs three steps for determining sampling frequencies. In Step 1, Y the rates of change of concentrations of an analyte in recent samples from a given well are examined and the rates are divided into four categories. The lowest rates of change are those that range from zero to 10' µg/L per year. The highest rates category is associated with a change of 30 µg/L per year. The rates of change between those two ; extremes are quantified by variability information, with higher variability requiring higher sampling frequency. Variability is characterized by a distribution-free version of the coefficient of variation, whereby the data range over the time period considered is 1 divided by the mean concentration during that period with 1.0 as the cutoff. Step 2 adjusts the proposed sampling frequency based on the overall trend of the sampling data. _ If the long-term history of change is significantly greater than the recent change, the sampling frequency may be reduced by one level. If this is not the case, the proposed sampling frequency is left unchanged. Finally, in Step 3, sampling frequencies are further adjusted based on risk. Because not all compounds that may be present in groundwater are equally harmful, sampling frequency is reduced by one level if recent maximum concentrations of compounds of high risk (e.g., benzene) are less that one-half of the applicable maximum contaminant level (MCL). When those evaluations have been made well sampling W p g interval recommendations are expressed as Quarterly, Semi-annually, Annually or Biennially for each analyte of Econcern in each well (United States Naval Facilities Engineering Service Center 2000). SJC