Laserfiche WebLink
Report:Groundwater-quality Monitoring—March 29-31 2006, 7500 West Eleventh Street, Tracy, CA Page 19 <br /> p p <br /> The employs MAROS protocol s some modifications to the CES methodology. Instead of <br /> using a distribution-free version of the coefficient of variation to assess statistically rate <br /> of change in concentration of analytes of concern with time, MAROS uses Mann-Kendall <br /> statistical procedures. In addition, MAROS substitutes preliminary remediation goals <br /> (PRGs) promulgated by Region IX of the US-EPA (United States Environmental <br /> Protection Agency Region IX 2004) for the MCL criteria originally used by LLNL. <br /> The groundwater-monitoring program at the 7500 West Eleventh Street Site has extended <br /> from May 2000 thru March 2006. For each well, all of the data for each of the analytes of <br /> primary concern were used to mare an initial recommendation for optimal sampling <br /> frequency. To conservatively account for any more rigorous sampling frequency <br /> requirements called for by unusual variations in the concentrations of analytes of concern <br /> in groundwater in samples recovered from any of the wells on the site, the data obtained <br /> from the last six sampling rounds, extending over the period January 2004 to March <br /> 2006, were considered separately from the total compilation of available data. <br /> The results of the MAROS sampling frequency optimization are presented in the Table <br /> headed MAROS Sampling Frequency Optimization Results in Appendix A. <br /> Recommendations are made for sampling frequencies at each of the monitoring wells in <br /> the primary plume of contaminated groundwater emanating from the 7500 West Eleventh <br /> Street property and individual recommendations are made for each of the BTEX <br /> compounds. For toluene, ethylbenzene and total xylene isomers, a statistically-efficient <br /> groundwater monitoring program calls for samples to be recovered from more than 50% <br /> ! of the wells at a frequency no greater than once every two years (biennially) and at a <br /> frequency of no more than once per year in the remainder of the wells (annually). In the <br /> case of benzene, the recommended sampling frequency is similar to that of the other <br /> BTEX compounds, with two minor exceptions. <br /> In the case of benzene in Monitoring Well MWFP-4, the recommended frequency for <br /> sampling is semi-annual; however, examination of Table 2 shows that that frequency is <br /> due to the combination of its very low assigned cleanup goal of 0.5 µg/L and to a <br /> statistical perturbation caused by the appearance of a trace of benzene at a concentration <br /> of 5.6 µg/L that appeared in the sample recovered from that well on March 31, 2006. The <br /> samples from that well had previously been free of detectable concentrations of benzene. <br /> A statistical call for quarterly monitoring of groundwater in Monitoring Well MW-7 was <br /> similarly generated by temporal variations in the concentration of benzene over the recent <br /> series of groundwater-quality monitoring rounds. As is noted in the optimization results <br /> in Appendix A, the required sampling frequency for that well based on the complete <br /> database is once per year. In SJC's opinion, these slight statistical variations are not of <br /> practical significance and the design of a future groundwater monitoring program at the <br /> Navarra Site should be based on the annual frequency recommended by the MAROS <br /> analysis for all of the BTEX compounds in the 19 other wells. <br /> r <br /> sic <br />