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1992 Quarterly Groundwater Monitoring Report Western Stone Products, Tracy, California =i Page 2 Depth to groundwater measured in wells MWl, MW2, MW3 and MW4 was subtracted from wellhead elevation to determine groundwater elevation (Table 1). Water levels in October had dropped approximately 0.8 feet since last measured in July 1992. Figure 2 depicts the groundwater gradient on October 7, 1992. Gradient magnitude was approximately 0.004 ft/ft or 21 ft/mile in a north-northwesterly direction. Monitoring data is presented in Attachment 1. On October 7, 1992, after purging and stabilization of pH, conductivity, and temperature groundwater samples were collected from downgradient wells MW3 and MW4. Disposable bailers were used to collect the samples. Purge logs are included in Attachment 1. The groundwater samples were delivered to the Sequoia Analytical Mobile laboratory on site and analyzed for TPHD and BTEX utilizing EPA methods 3510/8015 and 503018020, respectively. No petroleum analytes were detected in the groundwater samples collected from wells MW3 and MW4. Table 2 summarizes analytical results. Attachment 2 contains laboratory data sheets, and chain-of-custody documentation. EXCAVATION AND SAMPLING ACTIVITIES Source removal excavation activities were conducted on October 7, 8, and 9, 1992, under the direction of Mr. Gregory P. Stahl and a field geologist. Approximately 375-400 cubic yards of soil were removed. The former diesel tank pit was opened to an approximate depth of 14 feet below grade (5-6 feet below groundwater) in order to remove contaminated soil previously found to exist above and below the water table. Excavation continued until photoionization detector readings and visual and olfactory evidence indicated that no petroleum hydrocarbon contamination remained on the northern, eastern and southern sides of the pit. To verify complete removal of the petroleum hydrocarbon impacted soils, confirmation samples were collected from the three "clean" sides of the excavation. The western wall of the former tank pit was excavated to the maintenance structure flooring edge. Samples were collected from the western wall to determine the level and extent of contamination left in place. Depicted on Figure 3 is the extent of excavation and the locations of the soil samples collected from the sidewalls. The confirmation soil samples were analyzed for TPHD and BTEX by the Sequoia Analytical mobile laboratory. Sample analysis indicated that petroleum hydrocarbon impacted soils were totally removed from the northern, eastern and southern walls of the tank pit. Complete removal of petroleum hydrocarbon impacted soils from the west wall was not possible because impacted soils were beneath the maintenance shed and further excavation in this direction would have threatened the footings of the structure. Sol] sample analytical results are QR102592.FNL t i