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Report:Groundwater-quality Monitoring—March 29-31,2006, 7500 West Eleventh Street, Tracy, CA Page 3 samples from all 15 wells that had been installed at the site by that time (The San Joaquin w t,... Company 2001c, 2001b, 2001a, 2002d, 2002c, 2002b, 2002a, 2003e and 2003d). The scope of the July 2003, October 2003 and January 2004 monitoring rounds (The San Joaquin Company 20044, 2003c, 2003b) included a recovery of samples from a reduced number of monitoring wells at the direction of the SJCEHD. The wells from which those samples were taken are those that were located in the most severely-affected areas of the primary and secondary plumes of contaminated groundwater emanating from the site. r The fourteenth monitoring round was conducted on April 21, 22 and 30, 2004, when groundwater samples were recovered from the 15 pre-existing wells and the seven new groundwater-quality monitoring wells installed in that month (The San Joaquin Company 2004c). (Note: As is discussed in Section 1.3.3 below, five floating product monitoring wells, the locations of which are shown on Figure 2, were installed in April 2004, but groundwater samples were not recovered from them until March 2005.) The fifteenth and sixteenth monitoring rounds included all 22 groundwater-quality monitoring wells, and were conducted on July 26-27 and October 27-28, 2004, respectively (The San Joaquin Company 2004a,b). The seventeenth quarterly monitoring round was conducted at the 7500 West Eleventh Street site over the period February 24-March 16, 2005. That round, performed in conjunction with a formal evaluation of the progress of natural attenuation at the Navarra Site included recovery of samples of groundwater from each of the 22 groundwater- quality monitoring wells and the five floating product monitoring wells then present on the site (The San Joaquin Company 2006d). The eighteenth, nineteenth and twentieth quarterly monitoring rounds, which were conducted in June, October and December 2005, included sampling from a reduced number of monitoring wells (The San Joaquin Company 2006a,b,c). This report documents the twenty-first quarterly monitoring round, which was conducted on March 29-31, 2006. 1.3.3 Monitoring of LNAPL This report also includes records from a program of monitoring the thickness of light non-aqueous phase liquid (LNAPL), present as floating product, in an array of five floating product monitoring wells and three selected groundwater-quality monitoring wells. e When groundwater was first recovered from groundwater-quality monitoring well MW-7 on May 11, 2000, the sample contained moderate concentrations of Total Petroleum Hydrocarbons quantified as diesel (TPHd) and Total Petroleum Hydrocarbons quantified as gasoline (TPHg), with commensurate concentrations of benzene, toluene, ethyl a benzene and total xylene isomers (the BTEX compounds) and 22 gg/L of methyl-tertiary butyl ether (MTBE), a fuel oxygenate. Thereafter, the concentration of components of fuel hydrocarbons followed an increasing trend until the analytes of concern exceeded v their maximum solubility in water(see Table 2). However, as early as the sampling round sic