ARCHIVED REPORTS XR0011757 - Laserfiche WebLink

Home Browse Search

Report: Groundwater-quality Monitoring—June 23-28,2005, 7500 Wes[Eleventh Street, Tracy, CA. Page 6 r milar strata at another location in Tracy, and Based on pump tests that SJC conducted in si from the observed rate f migration of methyl tertiary butyl ether (MTBE) through the [_=' is estimated that the sands beneath the site have a mean horizontal subsurface, a - hydraulic conductivity of approximately 1.5 to 10-z cm/sec (The San Joaquin ornpany Inc. 1994, 2002c). The verticalerm fromr ground surface to 50 ftof the clay strata aBGS was assessed by . e the several aquifers present at depths in the range g eability testcond a constant-head perm5 BGS The vertical permeability of that apple was in April 2004 from a depth of 7 =<r found to be 10.8 x 10-7 cm/sec. [-J 1.5 Distribution of Hydrocarbons in the Subsurface The several stages of tank removal, excavation and removal of contaminated soil and groundwater, site characterization and groundwater-quality monitoring that have been conducted at the 7500 West Eleventh Street site since December 1998 have permitted synthesis of historic, geologic, hydrostratigraphic and geo-chemical data. That synthesis has resulted in the following interpretation of the distribution of hydrocarbons in the subsurface. ' 1.5.1 Prim The site is affected by a primary plume of diesel and gasoline that has affected both soil 3 through 9, it emanates from the area where and groundwater. As is shown on Figures i r? the pump islands were formerly located on the 7500 West Eleventh Street property and 4 extends north-northeast down the groundwater gradient. When it was at its maximum c:# extent, the long axis of the plume measured some 750 ft down the groundwater gradient. The main body of the plume includes groundwater affected by both gasoline and diesel, but ahead of that mass is a fringe of groundwater affected solely by MTBE. ,... <) Historically, in the interior of the primary plume, LNAPL in the form of free floating product was present beneath the pump islands of the former fueling station at the Navarra i Site where it had leaked from piping, and by April 2002, it had migrated down-gradient ' as far north as groundwater-quality Monitoring Well MW-7 (see Table 2). The locations of the pump islands at the former fueling station and of Monitoring Well MW-7 are both shown on Figure 2. :1 pumping of floating product and heavily contaminated groundwater from the pits from ; which the underground tanks were exhumed in December 1998 successfully eliminated =;,; und Monitoring Well MW-3. Based on consideration o floating product from the area aro the hydraulic conductivity and hydrostratigraphy of the subsurface and the subsequent I.: reduction in analytes of concern in Monitoring Well MW-3 and at other locations along ;a roduct and {{ the south side of West Eleventh Street, it was estimated that floating p groundwater containing high concentrations of components of fuel hydrocarbons were removed from the groundwater over a radial distance of some 70 to 100 ft (The San Joaquin Company Inc. 2004c, 2002c). �. SJC