ARCHIVED REPORTS XR0011753 - Laserfiche WebLink

Home Browse Search

Report.Groundwater-quality Monitoring--October 27,2004, 7500 West Eleventh Street, Tracy, CA. Page 8 _ 2.0 OCTOBER 2004 GROUNDWATER-QUALITY MONITORING ROUND On October 27 and 28, 2004, SJC conducted the sixteenth round of groundwater-quality monitoring at the 7500 West Eleventh Street site. The depth to groundwater was y measured in all of the 27 extant wells, including floating product monitoring wells MWFP-1 through MWFP-5,but sampling for groundwater-quality analysis was restricted to the 22 monitoring wells MW-1 through MW-19, which are at the locations shown on Figure 2. 2.1 Groundwater Elevations and Flow Direction To initiate the October 2004 sampling program, the depth to groundwater in each of the 27 monitoring wells was measured using a conductivity probe on October 27, 2004. The jj 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 July 26, 2004, the date on which the depths to groundwater in the monitoring wells were last measured, to October 27, 2004, the groundwater table fell between 0.33 ft. and 0.8 ft in the monitoring wells that are screened in the near-surface aquifer beneath the site. (See Figures 4 through 9 for aquifers and well screen intervals.) k As has been observed previously, the greatest rise in the groundwater table was observed in the wells in the approximate southern half of the well array, while the rise diminished j with distance north from there. In the monitoring wells that are screened in the first aquifer that is located deeper than the near-surface aquifer (i.e., Monitoring Wells MW- j 3A and 12A), the piezometric head fell, on average,by 0.67 ft. In the deepest aquifer that + is monitored at the site (by Monitoring Well MW-3B) the piezometric head rose by 2.05 The observed changes in depths to groundwater in the groundwater-quality monitoring wells that were observed between July and October 2004 reflect the fact that, as SJC has noted in previous reports, agricultural irrigation dominates variations in groundwater elevations in the area around Tracy. When crops are being irrigated, very large volumes 3 of water are imported to the fields around Tracy via the Delta-Mendota canal, and water pumped 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 usually continue to fall until some time in February or r March. Inspection of Table 1 shows that the rise and fall in the groundwater table elevation beneath the 7500 West Eleventh Street site usually occurs with little or no relationship to seasonal precipitation in the San Joaquin Valley where little rain falls between May and September of each year. The groundwater elevations presented in Table 1 were used to generate the groundwater contours shown on Figure 2. i` SJC