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Warner M&A% fn SKeAsmeowwaBa,Few 14, sree BACKGROUND A tidal study was conducted on well M WA on November 9, 1991 where a 0.101'wt decrease in elevation was observed hetween high tide and low tide over a C5 ],am period. 'Ihe tidal change recorded at the port of Stockton was a decrease of 1.7 feet. A buried stream channel leading from McLeod Lake, located 0.33 mile northwest of MW-4, is likely the tidal hydraulic conduit. A historic map of Stocktw fmm 1849 was reviewed and stows a historic slough Connecting McLeod Lake to Mormon Slough running through the Site. The hydraulic conductivity of the shallow groundwater was estimated by conducting slug test in wells MW-4, MW-6 and MW-8 in October 1991. "Die rising head data was analyzed using Agtesolv'n^ subsume or determix the hydraulic coruluctivity. WCIIsMW-4and MW-61hadsimilar hydraulic conductivity of 0.0015 fooUminute for die sift Name material. Well M W-8 had a slightly graterhydiaulicc nductivity O170.032 fwUminute forthe sill and sand aquif material. The top of seeing of the shallow wells at the Site were surveyed by a Cxlifomia licensed Land Surveyor in Mamh 2007 using North America Vertical Datum of 1988 to the nearest 0.01 foot. Well M W-10 was surveyed by WESTON relative to well M W-2 he January 2011. The historic groundwater elevation measurements for wells MW-3A, MW-6 and M W-9 rwm Max], 199810 present were recalculated using the updated survey data. "These wells were selected based on well construction having similar screened intervals. The groundwater flow direction at We Site is consistently eastward at a gradient ranging fmm 0.002 W O.0up4 fset/foot. 2.3 PREVIOUS INVESTIGATIONS AND RESULTS In April and May 1988, WESTON collected a total of 13 soil samples fro benath the nine USTs during their removal. Petroleum hydrocarbons were detected in soil samples collected fmm beneath thessrne of the tanks. In December 1988, WFS'fON conducted a soil and groundwater investigation at the site which included drilling soil borings (BH-1 through 1111-12) and the installation of groundwater marketing wells M W-1 through MW-3. The borings were drilled to depths ranging from 15 re 30 feet bgs. Samples Collected fmm two ofthe borings (BH-3 and B14-4) collsoed in the area of the two 10,000 gallon UST9 contained petroleum hydrocarbons at maximum concentrations of