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QUARTERLY GROUNDWATER MDfirlOPING RPOFRr 5131 Pacific Avenue Stockton Caldornta 1.0 INTRODUCTION This report documents the results of quarterly groundwater monitoring conducted by Emcon Associates (Emcon) of Sacramento, California, on February 26, 1992, at the Shell Oil Company (Shell) service station located at 6131 Pacific Avenue in Stockton, California (Figure 1) The monitoring included collection of measurements of depths to liquids in the three groundwater monitoring wells (MW-1, MW-2, and MW-3) and groundwater samples from two of the wells (MW-1 and MW-2) located on site (Figure 2) This report is based, in part, on information obtained by Aegis from Shell and Emcon, and is subject to modification as newly acquired information may warrant 2.0 SITE DESCRIPTION IThe Pacific Avenue Shell site is an active service station that retails unleaded gasolines Details of the site's facilities were reported to Shell by Aegis in a "Site Assessment Report," dated January 4, 1991 The site is located to a commercial/residential area of Stockton 30 BACKGROUND ' During March 1990, Aegis drilled and sampled fourteen soil borings on site (report to IShell dated June 18, 1990) Analysis of soil samples indicated concentrations of total petroleum hydrocarbons (TPH), as gasoline, up to 17,000 parts-per-million (ppm) In June 1990, three underground fuel storage tanks (UST), of single-wall steel construction, and associated piping were excavated and removed from the site Analysis of soil samples collected from beneath the former tank locations indicated concentrations of TPH, as gasoline, up to 19,000 ppm Three new UST, of double-wall fiberglass construction, were installed in a new tank basin (Figure 2) A horizontal run of slotted pipe was installed beneath the former product piping locations for possible future use for soil vapor extraction I In July 1990, Aegis conducted drilling and sampling of five additional soil borings on site ` Groundwater monitoring wells MW-1, MW-2, and MW-3 were installed in three of the f borings (report to SI'ell dated January 4, 1991) Vadose zone vapor extraction wells, VEW-1 and VEW-2, were installed in the remaining two borings Analysis of soil samples collected from the five borings indicated concentrations of TPH, as gasoline up to 12,000 ppm and benzene up to 110 ppm I Depth-to-liquid measurements were collected monthly from the monitoring wells following installation through October 8, 1991, and are currently being collected on a quarterly basis Liquid level data collected during the past 12 months are summarized in Table 1 �} Groundwater samples have been collected on a quarterly basis since the wells were installed Analytical results of groundwater samples collected to date are summarized in Table 2 90-020A QMR i 1