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Report: Groundwater-quality Monitoring—March 29-31,2006, 7500 West Eleventh Street, Tracy, CA Page 15 <br /> 4.0 ON-GOING NATURAL ATTENUATION OF ANALYTES OF CONCERN <br /> As noted in Section 1.3.4, an extensive program of field testing, laboratory analysis and <br /> geochemical and engineering studies conducted in February and March 2005 <br /> demonstrated that natural attenuation is aggressively reducing contaminant <br /> concentrations in the subsurface at the Navarra Site (The San Joaquin Company Inc. <br /> 2006d). <br /> By examination of the relationship between time and contaminant concentrations that is <br /> compiled in Table 2, which now includes all groundwater chemistry data gathered <br /> through March 31, 2006, it is evident that concentrations of analytes of concern in the <br /> groundwater beneath the site are continuing to oscillate around a declining trend. This is <br /> illustrated in Figures 10-13 for the case of groundwater in Monitoring Well MW-3. Those <br /> Figures show plots of the relationship between the normal logarithm of concentrations of <br /> TPHg, TPHd, BTEX (i.e., the sum of the concentrations of each of the BTEX <br /> compounds) and MTBE versus time in that well. In each case, the regression line through <br /> the data has a negative slope, which is indicative of on-going degradation of the <br /> contaminants by natural attenuation. <br /> LINN!-, <br /> SJC <br />