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Evaluation of Natural Attenuation: 7500 West eleventh Street, Tracy, CA. Page 3 number of wells sampled for the July 2003, October 2003 and January 2004 monitoring rounds (The San Joaquin Company 2004e, 2003c, 2003b) were reduced at the direction of the San Joaquin County Environmental Health Division (SJCEHD) to the sampling of 1: seven, ten and seven of the 15 groundwater-quality monitoring wells, respectively. The fourteenth monitoring round was conducted on April 21, 22 and 30, 2004, when f: 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 2004d). The fifteenth and sixteenth monitoring rounds included all 22 groundwater- quality monitoring wells and were conducted on Jul 26-27 and October 27-28 2004 q Y g Y , respectively(The San Joaquin Company 2004b, c). This report documents the seventeenth quarterly monitoring round, which was conducted at the 7500 West Eleventh Street site over the period February 24-March 16, 2005. That round also included sampling of all 22 groundwater-quality monitoring wells extant at the =:1 Navarra Site at that time and was conducted in conjunction with an evaluation of natural attenuation of chemicals of concern in groundwater at the site. 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. 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 benzene and total xylene isomers (the BTEX compounds) and 22 p.g/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 their maximum solubility in water(see Table 2). However, as early as the sampling round 1 performed on November 28, 2000, no MTBE was detected in the samples recovered from Monitoring Well MW-7. It is believed that this does not indicate that, by that time, the groundwater in the vicinity of that well was free of MTBE. Rather, it indicates that the high concentrations of other components of fuel hydrocarbons were such as to raise the method detection limit (MDL) of the MTBE analysis to a level greater than the concentration of that analyte actually present in that groundwater sample. By April 11, 2002, it was clear that floating product was present in the surface of the groundwater in Monitoring Well MW-7. It continued to be present at various apparent thicknesses up to 0.58 ft. until an LNAPL-purging program in that well was initiated on f November 8, 2003. As part of the further extension of the site characterization program in April 2004, five floating product monitoring wells were installed at the locations shown on Figure 2 to investigate the down-gradient extent to which LNAPL may have migrated sic ;� i