ARCHIVED REPORTS XR0011754 - Laserfiche WebLink

Home Browse Search

Extended PlumeDefnition:-7500 W Ilth St.,Tracy,GI. Page 30 9.0 CONCLUSIONS The scope of work performed to implement the extension of the site characterization program at the 7500 West Eleventh Street site in Tracy, California has permitted additional details of the hydrogeologic regime and the distribution of analytes of concern within the primary plume of affected groundwater emanating from the 7500 West Eleventh Street property to be documented. However, these improvements in detailed understanding did not yield findings that required SJC's previous evaluations and understanding of the distribution of analytes of concern in the subsurface or the factors affecting contaminant transport at the site to be modified in any way that might be of engineering significance, either with respect to the environmental characteristics of the site or to the development of practical means of corrective action that might be required to "close"the Navarra Site. The previously-defined lateral extent of the affected areas of soil and groundwater was confirmed. This included confirmation of the limits of the primary plume along its western boundary by installation of additional groundwater-quality monitoring wells. Figure 11 is a site plan showing the plumes of affected groundwater. Stratigraphic, geochemical and groundwater-quality data recovered from new groundwater- quality and floating product monitoring wells installed along the axis of the primary plume confirm SJC's earlier assessment that contaminant transport within the plume is significantly influenced by the presence of silts, sands and gravels that are present in the subsurface beneath and adjacent to South Chrisman Road, which material are significantly more permeable than the materials to the east and west, where clay strata are more predominant. The concentrations of analytes of concern in groundwater samples recovered in April 2004 {' are presented in Table 4. Aquifers deeper than the near-surface aquifer at the site are protected by overlying clay aquitards and continue to be entirely free of components of hydrocarbons. When monitored in April 2004, no LNAPL was detected floating on the groundwater in any of the wells present on the site at that time. However, the concentrations of analytes of concern in groundwater-quality monitoring well MW-7, where apparent thicknesses of LNAPL up to 0.55 R were previously observed in October 2003,prior to implementation of a program of periodic purging of that well, continue to be elevated and have increased significantly since last sampled in January 2004. No components of fuel hydrocarbons were detected in samples recovered in early April 2004 from six potable water supply wells in the neighborhood. sic