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1 CrtalogtcalTecknkslnr Page 2 Yd Quarter 2004 Groundwater Monitonng Report Project No 507 2 November 4,2004 ' have decreased by half A report on the pilot test is under development and it will contain recommendation for additional remedial actions 1.0 GROUNDWATER MONITORING 1.1 Hydrogeology of Site ' The geology of the site is predominately clays and sandy clays with lesser amounts of inter bedded silts and sands The average groundwater elevation was —12 44 feet below msl on August 25, 2004, which corresponds to approximately 17 75 feet below grade surface (bgs) This average elevation represents a decrease of 146 feet since the June 4, 2004 monitoring event Groundwater gradient for this monitoring event was 0 0042 ft/ft flowing to the N72°E direction This site consistently exhibits a flat groundwater gradient The gradient direction for the August 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 Table 5 provides a summary of monitoring well construction The groundwater gradient is relatively flat and the direction of groundwater flow has varied during the course of the investigation As evident in Figure 3, groundwater exhibited a strong ' bifurcated flow regime in 2000, trending predominantly to the southwest or north-northeast at approximately 180-degree direction reversal Since the installation of wells MW-7 and MW- 10 in May 2001, the direction has been predominantly to the northeast The site is quite close ' to the San Joaquin River and tidal fluctuations may contribute to some of 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 ' o Determine the 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-9) ' e 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 piezometric 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 1