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QUARTERLY 13MUNOWA7M MONPi IMG T ' 6131 Pacific Avenue Stockton,CW forma 1.0 INTRODUCTION This report documents the results of groundwater monitoring conducted by Aegis Environmental, Inc (Aegis), during October 1991, at the Shell Oil Company (Shell) service station located at 6131 Pacific Avenue in Stockton, California (Figure 1) The monitoring included quarterly collection of water samples and monthly measurements of depth to liquid in the three groundwater monitoring wells, MW-1, MW-2, and MW-3, located on site Additionally, monitoring and bailing of floating liquid hydrocarbons in well MW-3 was performed either weekly or twice monthly This report is based, in part, on information obtained by Aegis from Shell and is subject to modification as newly acquired information may warrant I2.0 SITE DESCRIPTION The Pacific Avenue Shell site is an active service station that retails regular-leaded, regular-unleaded, and super-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 in a commercial/residential area of Stockton 3.0 BACKGROUND During March 1990, Aegis drilled and sampled fourteen soil borings on site (report to 1 Shell dated June 18, 1990) Analysis of the samples indicated concentrations of total 1 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 up to 19,000 ppm Three new UST, of double-wall fiberglass construction, were installed in a new tank basin (Figure 2) A horizontal slotted vapor extraction piping run was installed beneath the former product piping 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 (Figure 2) were installed in three of the borings (report to Shell dated January 4, 1991) Vadose zone vapor extraction I rreii5, %,—-1 c'u id V E-2, were insianed m me remaining two borings Analysis of soil samples collected from the five borings indicated concentrations of TPH up to 12,000 ppm and benzene up to 110 ppm Groundwater samples have been collected on a quarterly basis and depth-to-liquid measurements have been collected on a monthly bass since the installation of the monitoring wells Analytical results of groundwater Isamples collected from the wells are summarized in Table 2 90.020E RPT 1