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Credldgi alTeckxusInc Page 2 ' Yd Quarter 2001 Groundwater Momtonng Report Project No 507 2 • November 6,2001 ' 1.0 GROUNDWATER MONITORING 1.1 Hydrogeology of Site ' The geology of the site is predominately clays and sandy clays with lesser amounts of silts and sands The average groundwater elevation was —9 62 feet below MSL on September 17, 2001 This represents a decrease of 2 01 feet since the June 2001 monitoring event 1 A groundwater gradient was calculated for the event and found to be flowing N72'E at 0 002 ft/ft The gradient direction for the September 2001 monitoring event is shown in ' Figure 2, and the historical groundwater directions and gradients are shown in Figure 3, Groundwater Gradient Rose Diagram The groundwater elevation data are summarized in Table 1, Appendix A ' The groundwater gradient is relativelyflat and the direction of groundwater has varied during the course of the investigation As evident in Figure 3, groundwater had a strong bifurcated flow regime, trending predominantly to the southwest or northeast at approximately 180-degree direction reversal With the installation of wells MW-7 and MW-10 in May 2001, the direction has been to the east-northeast for two consecutive events This change may be due to the new spatial orientation of the wells used in the computer-contouring program The site is quite close to the San Joaquin River and tidal ' fluctuations might also be responsible for the observed changes in groundwater direction Vertical Groundwater Gradients Under vertical groundwater flow conditions, the water level in a monitoring well is a function of the length of the well screen and its depth or vertical position in the aquifer As with horizontal flow conditions, the diameter of the well or piezometer is immaterial ' The following procedure is used to calculate vertical groundwater gradient • Determine vertical distance between the two measuring devices by the distance from the ' mid-point between the top and bottom seal of the deep well (MW-109) and the mid- point between the groundwater elevation and the bottom seal in the shallow well (MW- • Measure the head in both wells used in the calculations • if the lateral distance between the well pair is greater than a few feet, then calculations ' must be made to correct the down-gradient piezometnc head to account for the sloping water table between the wells The calculation considers the slope of the water table and the distance in a down-gradient direction between the two wells used in the calculations ' Figure 2 shows the location of the well cluster used for calculating the vertical groundwater g g gradient 1n this report, MW-9 and MW-109 Table 1 shows Summary of Water Level and