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1 <br /> Fourth Quarter 2002 Monitoring,Sampling,and Remediation Progress Report 4 <br /> 7-Eleven, Inc,Store#19976 1399 Borth Main Street,Manteca,CA December 24, 2002 <br /> ' Groundwater analytical data trends over time indicate that the highest dissolved BTEX and TPH-G levels <br /> are consistently present in well MW-2 (Table 1A) Benzene and TPH-G concentrations in this well have <br /> ranged up to 6,990 fag/L and 140,000 pg/L, respectively The highest MTBE levels have fluctuated over <br /> ' time between wells MW-1 and MW-2 <br /> Comparison of groundwater data over time Indicates that MTBE concentrations in wells MW-1 and MW-2 <br /> have decreased significantly, from early 2002 levels of between 11,000 pg/L and 9,200 dig/L, to current <br /> levels of between 100 pg/L and 1,600 tag/L Low to non-detectable concentrations in downgradient wells <br /> MW-5, MW-5A, MW-6, and MW-7 Indicate adequate lateral and vertical plume delineation <br /> (Figures 2 and 3) <br /> ' Geology/Hydrogeology <br /> The site is located in the middle of the northern San Joaquin Valley, in a region mapped as Quaternary <br /> ' dune sand overlying arkosic alluvium of the Quaternary Modesto Formation (California Division of Mines <br /> and Geology, 1991) Soil types at the site consist of sand and silty sand to about 10 feet bg underlain by <br /> clayey silt and silt to about 20 feet bg, below which lies fine-to coarse-grained sand and silty sand to 35 or <br /> 40 feet bg <br /> <(P- <br /> First groundwater is encountered between 14-18 feet bg During the current sampling event, equilibrated <br /> depth to groundwater ranged from 195 to 21 75 feet below top of casing (TOC) The groundwater flow <br /> direction Is historically to the north or northwest <br /> ' Sensitive Receptors <br /> The site is located in a light commercial/retail land use area There are no active public water supply <br /> wells within 2,000 feet downgradient of the site (IT Corporation, 1999) There are two reported wells of <br /> unknown status within 2,000 feet upgradient/crossgradient of the site <br /> Groundwater Monitoring Data <br /> 1 Before monitoring, each well cap was opened to allow the water level In each well to stabilize to <br /> atmospheric pressure The DTW was measured to the nearest 0 01 foot from the top of the well casing <br /> ' using an interface probe <br /> Monitoring results are for two depth intervals, shallow(about 30 feet bg wells MW-1, MW-2, MW-3, <br /> ' MW-4, MW-5, MW-6, and MW-7) and deep (about 50 feet bg wells MW-'IA, MW-4A, and MW-5A) <br /> Groundwater elevations in the deeper wells are usually slightly deeper than in the respective shallower <br /> ' wells, indicating a slight downward vertical gradient Seasonally, water levels fluctuate between about 14 <br /> to 20 feet bg (about 7 to 13 feet above mean sea level) <br /> 19976024 DOC <br />