ARCHIVED REPORTS_XR0002060 - 1 - Laserfiche WebLink

Home Browse Search

Workplan for Vertical Groundwater Investigation And Bench Test for In-Situ Oxidation 7835 Thornton Road, Stockton, CA Page 2 of 8 Between August 24, 1987 and January 25, 1991) 15 soil borings and 8 groundwater monitoring wells were drilled/installed at the site by three consulting companies (GTI, WaterWork, RESNA Industries) to investigate vertical and lateral extent of soil and groundwater contamination. Analytical results of soil samples collected during initial investigations indicated gasoline hydrocarbon impacted soil was present in the vicinity of the former underground fuel storage tanks and fuel dispensers from approximately five feet below grade to the water table (approximately 50-55 feet below grade). Locations of borings and monitoring wells are shown on Figure 3. Tabulated summaries of analytical results are included in Attachment 1, In January 1990, approximately 3,000 cubic yards of contaminated soil was excavated from beneath the former USTs. Excavation proceeded to a depth of approximately 35 to 40 feet below ground surface (bgs). Confirmation soil samples collected from the excavation floor indicated residual petroleum hydrocarbons remained in place at depths below 35 feet bgs. Confirmation samples collected from the excavation sidewalls indicated that the lateral limits of soil contamination had been removed. Excavated soils were stockpiled onsite and. aerated under the authorization of the San Joaquin County Air Pollution Control District (APCD). Aerated soils were confirmed clean through laboratory analyses and subsequently backfilled into the excavation under the authorization of EHD. Figure 4 shows the limits of the excavation. Ground Zero has maintained a quarterly groundwater monitoring and sampling program at the site, installed a replacement well for the downgradient well (MW8) that was destroyed to allow for the widening of Thornton Road, and supervised the destruction of two additional monitoring wells (MW6, MW7). SITE CHARACTERIZATION Site Geology Subsurface soils beneath. the site consists predominately of clays from surface grade to approximately 12 to 20 feet bgs; clayey sand to well graded sand from approximately 12/20 feet to 30 feet bgs; clay and silt from approximately 30 feet to 50 feet bgs; clayey sand to well graded sand with some silt and clay from approximately 50 feet to 70 feet bgs; and clay and silt from approximately 70 feet to 99 feet bgs, the maximum depth explored. Poorly graded sand was encountered from approximately 95-99 feet bgs in boring DB-la, the only boring at the site advanced deeper than 70 feet bgs. Figure 5 shows lines of cross sections and Figures 6 and 7 present east-west and north-south cross sections,respectively, depicting lithology beneath the site. Site Hydrology Groundwater beneath the site was encountered at approximately 49 feet bgs during the initial subsurface investigation in September 1987. The water table subsequently fell to as low as 59 feet below grade in September 1992. The water table in November 2005 was approximately 37.5-38.5 G:IGROUNDZE%KNO W LES.HAMIREPORTSI W KPLAN_deepwel l_bench2.doc