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ckolog"l hchtucs Inc Page 2 ' 1"Quarter 2003 Groundwater Monitoring Report Project No 507 2 • May 15,2003 ' 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 --10 30 feet below msl on March 18, 2003, which corresponds to approximately 15 feet below grade surface (bgs) This ' elevation represents an increase of 0 75 feet since the December 20, 2002 monitoring event A groundwater gradient was calculated for the event and found to be flowing N70°E at 0 002 ft/ft This site consistently exhibits a flat groundwater gradient ' The gradient direction for the March 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 had been to the east-northeast in seven of eight quarterly events 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 • 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) • 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 piezometne 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