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below msl, which coincided with Lithologic Zone 4,and monitoring wells MW-2 and MW-3 were subsequently replaced by deeper wells MW-2A and MW-3A in 1995. Lithologic Zones 3 and 4 are essentially parts ofthe same unconfined aquifer in which the water table elevation fluctuates. Since 1991, ` groundwater elevations have rebounded to approximately 18.61 feet below msl (WESTON, 2006). ` To study the effect of tidal fluctuations on groundwater elevation at the site, WESTON measured groundwater levels during a 4.5 hour period in November 1991. Between high and low tide,a total groundwater elevation decrease of0.18 feet in monitoring well MW-4 was observed. During the same period,the tidal elevation at the Port of Stockton decreased by approximately 1.7 feet. Based on this data, it appears that groundwater beneath the site is tidally influenced. 2.2.3 Flow Direction and Gradient ` Regionally,groundwater flows east toward a large groundwater depression approximately 8 miles east of the site (WESTON, 1992; San Joaquin County Department of Public Works, 2004). Based on groundwater contour maps prepared for the quarterly monitoring reports,groundwater flow beneath the site has been thought to be in a trough-shaped pattern(WESTON,2006). However,this does not seem to correlate with regional groundwater flow and could be due to an incorrect top ofwell casing survey. With the groundwater elevation in the anomalous well MW-6 removed,the groundwater flow is towards the southeast along a gradient of 0.009 feet per foot(ft/ft) as shown in Figure 2-2. _ 2.2.4 Hydraulic Conductivity Hydraulic conductivities ofLithologic Zone 4 were estimated using slug tests performed in October 1991 (WESTON, 1992). The geometric mean hydraulic conductivity for the site is 8.6 x 10'centimeters per second(cm/s)or 2.45 feet per day(ft/day),which is typical for silty sand and sand(Freeze and Cherry, 1979). 2.2.5 Effective Porosity ` Due to the difficulties involved in accurately determining effective porosity,accepted literature values for the type of soil comprising Lithologic Zone 4 were used to estimate this parameter. Freeze and Cherry ` (1979)give a range of effective porosities for silt between 0.25 to 0.5. Tobe conservative,the effective porosity of Lithologic Zone 4 is assumed to be 0.4. L L 4 L