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Report:Groundwater-quality Monitoring—October 25,2002: 7500 West Eleventh Street, Tracy, CSI. Page 11 April to July 2002, when the water table rises, the apparent thickness of floating product >� in the well decreases. 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 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 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; 1' thus, the apparent thickness of floating product in the well is reduced when the water - ' table is rising. Around the periphery of the principal plume of affected groundwater, only minor changes are seen in the concentrations of analytes of concern in samples recovered from monitoring wells in those areas when the results of the October 25, 2002 groundwater- ""J' quality monitoring round are compared with the July 29, 2002 round. The groundwater in Monitoring Well MW-5, which is to the west of the principal plume of affected >- groundwater, continued to be free of any detectable analytes of concern except for a trace of MTBE at a concentration of 2.2 µg/L. No diesel or gasoline was detected in the sample recovered from Monitoring Well MW-6 on October 25, 2002, although 220 µg/L of gasoline had been detected in the sample recovered from that well on July 29, 2002. The concentration of MTBE in the groundwater at the location of Monitoring Well MW-6 rose from 180 µg/L to 290 µg/L over the period from July 29 to October 25, 2002. r`'✓'= The sample of groundwater recovered from Monitoring Well MW-2, located on the western periphery of the principal plume of affected groundwater, on October 29, 2002 remained free of any analytes of concern expect for a trace of MTBE at a concentration of 2.5 gg/L. Groundwater recovered on that date from Monitoring Wells MW-8 and MW- 9, which are beyond the eastern lateral extent of the primary plume of affected groundwater, remained entirely free of any detectable concentrations of any analytes of concern. On the northeastern fringe of the primary plume, gasoline-range hydrocarbons, were detected for the first time in the sample of groundwater recovered from Monitoring Well MW-10 on July 29, 2002. That well was free of that analyte on October 25, 2002, g p µglL, which is not although MTBE continued to be resent at a concentration 55 f significantly different from the concentration of 58 gg/L seen at that location on July 29, 2002. Groundwater recovered from Monitoring Well MW-t 1 on October 25, 2002, 4 remained free of any detectable concentration of analytes of concern, except for a trace of r MTBE, at a concentration of 9.8 p.g/L, which is only slightly higher than the concentrations of 5.1 µg/L and 8.3 µg/L, respectively, detected in samples of groundwater recovered from that well on April 11 and July 29, 2002. No detectable concentration of any analytes of concern that are components of fuel hydrocarbons were detected in samples of groundwater recovered on October 25, 2002 from any of the monitoring wells screened in the aquifers that lie beneath the first clay i sic