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Evaluation of Natural Attenuation_ 7500 West Eleventh Street, Tracy, C4. Page 44 distribution of monitoring wells within the well array as site characterization work is extended over greater ares around the release site. Accordingly, it is necessary to perform Mann-Kendall statistical analysis of the trend of the zeroth moment with time and to evaluate the statistical confidence with which the trend can be determined. Appendix A includes the results of the zeroth moment analyses for each of the BTEX compounds and MTBE (in the MAROS software, the alternate name, tert-butyl ethyl ether, is used for MTBE) for the 7500 West Eleventh Street site. -+ For each of the BTEX compounds, the Mann-Kendall statistics show that the zeroth moment trends are all declining and the trends have very high statistical confidence - in no case less than 98%. This is strong evidence that the primary plume at the site is stable and that the mass the mass of contaminants within the groundwater is decreasing. It is recognized that the zeroth moment analysis data for MTBE is classified as having "no trend" due to the low statistical confidence (56.4%) of the Mann-Kendall trend line; however, this is of little practical concern because of the concentrations of MTBE in the J plume are all at trace levels and the inability to compute a reliable trend in the data is related to the frequent reoccurrence of undetectable concentrations intermixed with very low positive detections of that analyte in the few wells where it has been, at any time, detected. Note: The MAROS software is also capable of computing first and second moment trends from the geochemical data, but, due to the sensitivity of such computations to changes with time in the number of :-. •.� wells in a monitoring array and the affect of the remediation by removal of contaminated groundwater from the source area that was conducted in 1998 on the distribution of concentrations of analytes of concern with distance down-gradient along the axis of the plume, such statistics are not useful for the purpose of evaluating the plume at the Navarra Site. 8.2.2.3 Well Redundancy Analysis The MAROS software can examine the statistical time history of contaminant ;_ concentrations in individual wells together with the contaminant mass distribution history of a plume to evaluate whether any of the wells in the well field can be eliminated without significantly degrading the statistical quality of the monitoring program. Details of the statistical procedures that the protocol applies for this purpose are presented in the software User's Guide (United States Air Force 2004). The methodology was developed based on Delaunay triangulation, which is the triangulation of a point set that has the property that no point in the set falls within the interior of the circumcircle of any triangle in a triangulation. (Okabe et al 1992, Watson 1994). Delaunay triangles are formed by lines joining all well locations in the well array. Spatial analyses are then made to 1 determine the relative importance of each well. The relative importance of a well is measured by its slope factor. The slope factor of a well location is defined as the - standardized difference between the logarithmic scales of its measured analyte concentrations and the estimated concentrations at the well locations if the well had not been present and concentrations at that point had been estimated from concentrations in sic