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Report: Groundwater-quality Maiitoa-ing—danuaiy20, 2003: 7500 West Eleventh Street, Tracy, CA. Page 6 2.0 JANUARY 20, 2003 GROUNDWATER SAMPLING AND ANALYSIS On January 20, 2003,,the ninth round of groundwater-quality monitoring was conducted. . ; Monitoring Wells MW-1 through MW-3, MW-3A, MW-3B, MW-4 through MW-12, and MW-12A, which are at the locations shown on Figure 2, were sampled. 2.1 Groundwater Elevations and Flow Direction To initiate the January 20, 2003 sampling program, the depth to groundwater in each of the fifteen monitoring wells was measured using a conductivity probe. The water table elevations were computed relative to mean sea level (MSL). Those measurements and the computed groundwater table elevations are recorded in Table 1. In the period from October 25, 2002, the date on which the depths to groundwater in the monitoring wells were last measured, to January 20, 2003, the groundwater table rose between 0.1 and 0.3 ft beneath most areas of the site. However, the rise in the water table was not uniform over the whole of the area monitored. Groundwater rose 0.26 ft. in Monitoring Well MW-12, which is located close to the southern boundary of the West Eleventh Street property, but it rose only 0.10 ft. in Monitoring Well MW-11, located to the north of West Eleventh Street at the northern down-gradient limit the of the primary plume of affected groundwater. Between October 2002 and January 2003 considerable rain fell in the area and this probably accounted for the rise of the groundwater table in the shallow unconfined aquifer beneath the site. In most years, the groundwater table falls in the month of January because the primary influence on groundwater levels in the area is agricultural irrigation practices. When crops are being irrigated, very large volumes of water are imported to the area via l; the Delta-Mendota canal, and water from deep agricultural wells is being discharged to shallow aquifers. This activity begins in the spring of each year and typically continues through early- to mid-September, when it stops and groundwater levels in shallow aquifers begin to fall. Except in unusually wet winters, levels continue to fall until some time in February or March. Inspection of Table 1 shows that the rises and falls in the groundwater table elevation beneath the 7500 West Eleventh Street site usually occur with little or no relationship with precipitation failing in the San Joaquin Valley. Consistent with a groundwater system dominated by agricultural irrigation, the ground water level in Monitoring Well 3B (screened between 45 ft to 50ft. BGS in a confined or semi-confined aquifer that does not communicate with the unconfined aquifer beneath the site) fell 0.18 ft. in the period from October 25, 2002, to January 20, 2003. This occurred { because of the cessation of recharge of water into aquifers in the region to the south of the site when the importation of irrigation water from the Delta-Mendota Canal and the California Aqueduct ceased as the 2002 agricultural production season drew to a close. sic L.