ARCHIVED REPORTS XR0007840 - Laserfiche WebLink

Home Browse Search

I SUBSURFACE INVESTIGATION REPORT INSTALLATION OF WELLS MWS, MW6 AND MW7 I640 N El Dorado Street, Stockton, CA I1.0 INTRODUCTION I Ground Zero Analysis, Inc (Ground Zero) submits this report to summarize additional subsurface investigation conducted at the subject site (Figure 1) at the request of the San Joaquin County Environmental Health Division (EHD) to further evaluate the nature and extent of soil and groundwater I contamination associated with a former underground storage tank (UST) The work was conducted pursuant to a workplan dated September 6, 2002, which was approved, with conditions, by EHD in a letter dated January 24, 2003 I1.1 Site Description and Previous Investigations ' The site currently houses a specialty automotive sales and service center Previous site occupants operated one 250-gallon UST for storage of waste oil The UST was removed under permit from EHD on March 26, 1997 A soil sample collected from beneath the tank contained elevated levels of petroleum hydrocarbons, lead, and volatile organic compounds(VOCs) ' A total of eight soil borings were advanced in the vicinity of the former UST in subsurface investigations conducted in January and October 1999 to investigate the vertical and lateral extent of soil contamination and to determine if groundwater beneath the facility had been impacted by past site operations Laboratory analyses of soil and groundwater samples collected during the investigations indicated that petroleum hydrocarbons,VOCs, and lead had impacted soil and groundwater in the immediate vicinity of ' the former UST The vertical extent of soil contamination was defined at the source (boring SB3) No contaminants were detected deeper than 31 feet below ground surface (bgs) The lateral extent of soil contamination was also largely defined in previous investigations Soil contamination in the vadose zone ' appears to be limited to a small area in the immediate vicinity of the former UST, and soil contamination in the capillary fringe zone appears to extend no more than 25 feet north, 20 feet west, 10 feet south, and 5 feet east of the former UST ' Based upon analytical results of groundwater samples collected from soil borings and groundwater monitoring wells MW1 through MW4 (installed in December 2000) the extent petroleum hydrocarbon ' contamination in the groundwater does not extend more than approximately 40 feet downgradient from the source Various chlorinated solvents were also detected in all site wells, however, and free product was noted in well MW3 Subsequent quarterly groundwater sampling at the site confirmed that the lateral extent of petroleum hydrocarbon contamination is within close proximity to the former UST ' Groundwater analytical results and the periodic presence of free product in well MW3 indicate that petroleum hydrocarbon contamination is present in groundwater in the immediate vicinity of the former UST but does not extend to wells MW1, MW2, or MW4 Various chlorinated solvents are present in ' groundwater at low levels, however, only PCE routinely exceeds its Maximum Contaminant Level (MCL) The MCL for PCE in groundwater is 5 parts per billion (ppb) Figure 2 shows the location of the former UST as well as soil borings and monitoring wells advanced through December 2000 Tables 1 ' through 4 summarize soil and groundwater analytical data from previous investigations and Table 5 summarizes free product measurements in well MW3 Ground Zero continues to remove free product from well MW3 using a passive oil-wick skimmer I -1- I