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C,ealogicat'r"h ics1/w Page 3 In Situ Remediation Pilot Test Work Plan Project No 507 2 June 10,2003 2.2 Hydrogeology 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 Figures b and 7 illustrate the historical range of groundwater elevation on the cross sections Table 5 provides a summary of monitoring well construction rThe groundwater gradient is relatively flat 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 north-northeast at approximately 180-degree direction reversal With the installation of wells MW-7 and MW-10 in May 2001, the direction had been to the east-northeast in eight of nine 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 t • 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 Figure 2 shows the location of the well cluster used for calculating the vertical groundwater ' gradient in this report, MW-9 and MW-109 Table I shows Summary of Water Level and Gradients Slope and Bearing, and Table 2 shows the calculated vertical gradients The ' information used in the calculations is shown below Vertical gradient calculation formulas are as follows ' • Vertical correction for gradient [(gw gradient slope) x (distance) = vertical correction] • Vertical head [(head of deep well) - (head of shallow + correction) = vertical head] • Vertical gradient [(vertical head) / (vertical distance) = vertical gradient]