ARCHIVED REPORTS XR0009295 - Laserfiche WebLink

Home Browse Search

Geatogtcatreckntcsl1tc Page 18 ' Site Conceptual Model Report(2006 Revision) Valentine(Castle Motors) Project No 723 2 June 16,2006 ' pathway for off site contaminant migration The CPT data obtained in February 2006 did not p Y g identify significant layers of greater hydraulic conductivity layer that were missed during past ' field investigations because of the five foot sampling interval Sandy horizons of limited thickness (1 - 3 feet) were identified on the CPT logs but not laterally continuous like the 60 - 75 feet bgs sands Extraction well EW-2 was installed at the site on September- 28, 2005 for tthe groundwater pump & treat system and it is screened from 67 — 77 feet bgs to specifically address this preferential pathway situation ' A work plan for interim site remediation has been submitted to the SJC/EHD for consideration The work plan was approved and the installation of a dual phase extraction system to address the TPH-G contamination is in the process of being implemented Well ' MW-208 should be an adequate indicator of the system effectiveness when the proposed remedial system is placed in operation and together with the Shell wells will alleviate the need for additional down gradient monitoring wells ' 4.2 Conclusions Based on our interpretation of the data collected over the course of this subsurface investigation, GTI has reached several conclusions These conclusions are based on the premise that the data considered, although incomplete, are representative of actual site conditions We acknowledge that there may be undiscovered conditions, which would upon their consideration, change our interpretation and thus our conclusions ' Geological Technics Inc makes the following conclusions • The geology of the site consists primarily of alternating layers, lenses and ' interstratified beds of clays, silts and sands Units containing gravels are infrequent • Groundwater flow trends northeast to east • High levels of BTEX and TPH-G are present in the groundwater of all six on-site shallow and intermediate monitoring wells (MW-1 through MW-6) The soil contamination above the water table appears to be limited an lateral extent to the areas directly within 15 feet of the former dispenser island ' • The residual soil TPH-G contamination in the vadose zone and sorbed hydrocarbons below the water table continues to provide a source for dissolved hydrocarbon groundwater plume ' • The groundwater plume contamination is stratified with the main mass of contaminants having migrated to a depth of 45 to 70 ft bgs • Sandy units below the depth of 60 — 75 feet below grade may be acting as a preferential pathway for submerged plume contaminant migration to the northeast of the site as depicted in Figure 16 • The low levels of contamination in well MW-101 and lack of contaminants in MW- 201 suggests that these wells define the vertical limit of the plume beneath the site