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Third Quarter 2002 Monitoring,Sampling and Remediation Progress Report 4 7-Eleven, Inc,Store#19976, 1399 North Main Street, Manteca,CA October 31 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 2) Benzene and TPH-G concentrations in this well have ranged up to 6,990 pg/L and 140,000 pg/L, respectively The highest MTBE levels have fluctuated over 1 time between wells MW-1 and MW-2 D� kyr►ik V), 'j ON oIn3 During the third quarte 21 2, MTBE concentrations In MW-1 and MW-2 have decreased significantly from Lthe previous quarter ;G 0 NgIL and 11,000 pg/L, respectively) to 970 NgIL and 1,80 pg/L, respectively Low to non-detectable concentrations in downgradient wells MW-5, MW-5A, MW-6, and MW-7 Indicate ' adequate lateral and vertical plume delineation (Figures 2 and 3) Low MTBE levels relative to benzene/TPH-G concentrations In well MW-2 likely indicate a relatively old release his h'9� dave' 1fl- rns p e-hem,e'.A& ' Geology/Hydrogeology d.ncer^%/P%- 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 Iles 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 20 15 to 22 55 feet below top of casing (TOC) The groundwater flow direction is historically to the north or northwest ' Sensitive Receptors ' The site is located in a light commerciallretail 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 upgradient/crossgradient 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 MWAA, 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) 1 0 19976023 DOC