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Harding Lawson Associates <br /> ' The potentiometric surface maps show a general southerly hydraulic <br /> gradient. Some deviations from this (Plates 9-I1) are likely due to extraction <br /> pumping in Wells 1, 15 and 17. A detailed study of the magnitude and direction of <br /> ' the lateral hydraulic gradient on site was conducted. The results are presented on <br /> Table 1. The well triplets between which hydraulic gradients were calculated are <br /> ' also shown. The results show that both the hydraulic gradient magnitude and <br /> direction changed very little between measurement periods. The average hydraulic <br /> ' gradient magnitude observed on the site is 0.0074, in a direction 155 degrees <br /> ' counterclockwise from north (south 25 degrees west). Vertical hydraulic gradients <br /> may play a minor role in the hydraulics of the aquifer zone on site; however, no <br /> data exist with which to make this estimation. <br /> The present lateral hydraulic gradient direction implies that the San <br /> ' Joaquin River acts as a source of recharge to the aquifer zone on site. Water level <br /> elevations on the site are likely affected by tidal fluctuations. It is possible that <br /> tidal fluctuations may have some effect on the lateral hydraulic gradient on and <br /> near the site, and may cause a local reversal of hydraulic gradient direction. Tidal <br /> fluctuations in both the San Joaquin River and the Burns Cutoff may possibly <br /> 1 cause a divide of ground-water flow on Rough and Ready Island, thereby <br /> impeding the flow of ground-water from the site south to the Burns Cutoff. Tidal <br /> ' fluctuation studies will be needed to quantify these effects on the site. <br /> 1 <br /> November 24, 1987 5 of 9 <br /> ' B2203-R <br />