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------------ Report: Groundwater-quality Monitoring—January 20,2003: 7500 West Eleventh Street, Tracy, CA. Page 11 r concern. It was at a concentration of 5.7 4g/L, which is a slight decrease from the concentration of that fuel oxygenate, measured at 9.8 µglL in the sample recovered from that well on October 25, 2002. Samples recovered from Monitoring Well MW-3, which is close to the location on the 7500 West Street site where the fuel hydrocarbons were released, but where the groundwater quality has benefited from the remediation work performed when underground storage tanks and piping were removed from the site, contained generally similar concentrations of diesel and fuel oxygenates as had been the case on October 25 Yg , F:. 2002, but the sample recovered on January 20, 2003 contained no detectable concentration of total petroleum hydrocarbons quantified as gasoline (TPHg). Similarly, ` the sample recovered from Monitoring Well MW-4 on January 20, 2003 contained no detectable concentration of TPHg, although it contained diesel and some fuel oxygenates at concentrations similar to those that have been detected in samples of groundwater recovered from that well during the last several rounds of groundwater-quality monitoring. When groundwater in Well MW-7 was first analyzed in May 2000, it was affected by relatively moderate concentrations of diesel- and gasoline-range petroleum hydrocarbons (at concentrations of 1,300 µg/L and 1,400 µg/L, respectively) and MTBE, at 22 gg/L. However, over the next two years, the concentrations of both diesel and gasoline in samples recovered from that well exhibited a significant upward trend. As the concentration of those analytes increased, MTBE was no longer detected. That is almost certainly due to its presence being obscured in the chromatograms rather than by its actual absence. By April 11, 2002, it was evident that there was floating product in Well MW-7 and it was measured to have an apparent thickness of 0.3 ft. Since that time, the apparent thickness of floating product in that well has fluctuated in the expected manner in response to the rise and fall of the p groundwater table. On January 20, 2003, the apparent thickness of floating product in Monitoring Well MW-7 was 0.32 ft. An increase in the apparent thickiiess of floating product in a well that is perforated both above and below the maximum elevation reached by groundwater is commonly seen in association with a falling water table. Conversely, as was the case in the period from 4 April to July 2002, when the water table rises the apparent thickness of floating product in the well decreases. i When the water table is depressed, the well casing and its surrounding permeable filter pack in the annular space between the casing and the wall of the well boring serve as a 1 sump that receives hydrocarbons or other light, non-aqueous phase liquids (LNAPL) that seep from soil above the water table. This increases the apparent thickness of floating f product in the well casing. When the groundwater table rises, the floating product is pushed upward in the well casing and filter pack and permeates back into the formation; thus, the apparent thickness of floating product in the well is reduced when the water table is rising. 'i i sic