ARCHIVED REPORTS XR0011755 - Laserfiche WebLink

Home Browse Search

Evaluation ofNatural Attenualton: 7500 West Eleventh Street,Tracy,C4. Page 10 Accordingly, when drawing the contours shown on Figure 2, the elevation of the groundwater in well MW-7 was based on an assumed depth to groundwater of 9.18 ft. rather than the depth of 9.20 ft. actually measured. This yields a corrected groundwater table elevation of 39.04 ft. MSL compared to the uncorrected value of 39.02 ft MSL given in Table 1. In addition, the groundwater elevations in Wells MW-3A, MW-313 and MW-12A were {{ not considered when the groundwater contours were drawn because, although small in h' magnitude, differences between the elevations of the groundwater in Wells MW-3, MW- 3A and MW-313 and in MW-12 and MW-12A are sufficiently great to indicate that the is=j groundwater monitored by the shallow well at the locations of each of those well clusters has a different piezometric pressure from that of the deeper wells in the same cluster. It must therefore be assumed, at least locally, that the shallow wells monitor aquifers different from those monitored by the deeper wells. During the February 24, 2005 sampling round, the groundwater level in Monitoring Well MW-3B, whicb is screened in an aquifer the top of which is some 37 ft. BGS,was 2.04 ft. lower than that in Monitoring E Well MW-3, which is screened in the shallow, near-surface aquifer, while the level in Monitoring Well MW-3A was at the same level as that in Monitoring Well MW-3. 1 In the period from October 27, 2004, the date on which the depths to groundwater in the monitoring wells were last measured, to February 24, 2005, the groundwater table generally rose/fell between 0.2 ft. and 0.5 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.) As has been observed previously, the groundwater table rose more in the southern half of the well array than in the extreme northern end of the well array. 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-3A and 12A), the piezometric head rose, on average, by 0.44 ft. In the deepest aquifer that is monitored at the site (by Monitoring Well MW-313) the pieziometric head fell by 3 ft. The observed changes between October 2004 and February 2005 in depths to groundwater in the groundwater-quality monitoring wells that are described above are consistent with the annual changes in groundwater elevations that SJC has noted in previous reports. They are not related to seasonal weather cycles, but agricultural irrigation practices that dominate variations in groundwater elevations in the area around Tracy. When crops are being irrigated, very large volumes of water are imported to the fields around Tracy via the Delta-Mendota canal, and water pumped from deep ii 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 March. Inspection of Table 1 reflects this phenomenon and demonstrates that the rise and fall in the groundwater table elevation beneath the 7500 West Eleventh Street site usually occurs I with little or no relationship to seasonal precipitation in the San Joaquin Valley where little rain falls between May and September of each year. sic - 1