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Second Quarter 2002 Monitoring,Sampling,and Interim Remediation Report 4 7-Eleven Inc Store#19976, 1399 North Main Street,Manteca,CA July 30,2002 Groundwater analytical data trends over time indicate that the highest dissolved BTEX and TPH-G levels ' are consistently present in well MW-2 (Table ,Benzene and TPH-G concentrations in this well have ranged up to 6,990 lag/L and 140,000 pg/L, respectively The highest MTBE levels have fluctuated over ' time between wells MWA and MW-2, during the current quarter, MTBE concentrations in MW-2 were at 11,000 fag/L, while during the previous quarter, MTBE levels were higher in MW-1 at 9,200 pg/L Low to �V, non-detectable concentrations in downgradient wells MW-5, MW-5A, MW-6, and MW-7 indicate adequate� \ Rs L lateral and vertical plume delineation (Figures 2 and 3) Low MTBE levels relative to benzene/TPH-G %At concentrations in well MW-2 likely Indicate a relatively old release �2 ' Geology/Hydrogeology ' Regionally, the site Is located in the middle of the northern San Joaquin Valley, in a region mapped as Quaternary dune sand overlying arkosic alluvium of the Quaternary Modesto Formation (California Division of Mines and Geology, 1991) Soil types at the site consist of sand and silty sand to about 10 feet ' bg underlain by clayey silt and silt to about 20 feet bg, below which lies fine-to coarse-grained sand and silty sand to 35 or 40 feet bg First groundwater is encountered between 14-18 feet bg During the current sampling event, equilibrated depth to groundwater ranged from 18 96 to 21 42 feet below top of casing (TOC) The groundwater flow direction is normally to the north or northwest Sensitive Receptors The site is located in a light commercial/retail land use area There are no active public water supply wells within 2,000 feet downgradient of the site(IT Corporation, 1999) There are two reported wells of ' unknown status within 2,000 feet upgradienVcrossgradient of the site Groundwater Monitoring Data Before monitoring, each well cap was opened to allow the water level in each well to stabilize to ' atmospheric pressure The DTW was measured to the nearest 0 01 foot from the top of the well casing using an interface probe ' Monitoring results are for two depth intervals, shallow(about 30 feet bg wells MW-1, MW-2, MW-3, MW-4, MW-5, MW-6, and MW-7) and deep (about 50 feet bg wells MW-1A, MW-4A, and MW-5A) Groundwater elevations in the deeper wells are usually slightly deeper than In the respective shallower ' wells, indicating a slight downward vertical gradient Seasonally, water levels fluctuate between about 14 to 20 feet bg (about 7 to 13 feet above mean sea level) ' 19976022 BOC