ARCHIVED REPORTS XR0011749 - Laserfiche WebLink

Home Browse Search

Report: Groundwater-quality Moniloring—July 29, 2002: 7500 West Eleventh Street, Tracy, CA. Page 10 t 29, 2002 compared to the sample recovered from that well on April 11, 2002. This increase in the concentrations of analytes of concern in groundwater can be attributed to the rising water table submerging a zone of soil affected by significant concentrations of those analytes that was previously above the water table and, being so located, could not act as a source of components of fuel hydrocarbons that could be desorbed into the groundwater. i Inspection of Table 2 also shows increases in the concentrations of analytes of concern in the sample of groundwater that was recovered from Monitoring Well MW-12 on July 29, .; � 2002 compared to those in the sample recovered from that well on April 11, 2002. Monitoring Well MW-12 is located, as shown on Figure 2, in the area that is interpreted to be the source of the secondary plume of affected groundwater found beneath the 7500 -' West Eleventh Street property. The April to July increases in concentration of analytes of concern in the groundwater can again be attributed to the rising water table submerging soil affected by relatively high concentrations of components of fuel hydrocarbons. When groundwater in Well MW-7 was first analyzed in May 2000, it was affected by relatively moderate concentrations diesel- and gasoline-range petroleum hydrocarbons (at concentrations of 1,300 µg/L and 1,400 µg/L, respectively) and MTBE, at 22 µg& However, over the next two years, the concentrations of both diesel and gasoline in samples recovered from Well MW-7 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 chromatographs rather by its actual : . absence. By April 11, 2002, it was evident that there was floating product in Well MW-7 r= % and it was measured to have an apparent thickness of 0.3 ft. On July 29, 2002 the apparent thickness of floating product had reduced to 0.18 ft. f� >1 Reduction in the apparent thickness of floating product in a well that is perforated above and below the maximum elevation reached by groundwater is commonly seen in association with a rising water table, as was the case in the period from April to July 2002. 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 produces an apparent thickness of floating product in the well casing that may be thicker that the actual thickness of floating product in the groundwater in the area around the well. 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. Around the periphery of the principal plume of affected groundwater, only minor changes are seen in the concentration of analytes of concem in samples recovered from ?: '! monitoring wells in those areas when the results of the July 29, 2002 groundwater-quality monitoringround are compared with the Aril 11 2000 round. The p p 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 1.8 µg/L. No diesel-range hydrocarbons were detected in ` sic