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Geo(ogica(?ecFtnics Zne, Page 2 Groundwater Monitoring Report Project No. 750.2 June 15,2401 +r 1.0 GROUNDWATER MONITORING 1.1 Hydrogeology of Site The dominant soil type in the upper 75 feet of subsurface geologic soils investigated is sand, with fine to coarse-grained texture. Varying amounts of silt, silty sand and laterally discontinuous clay lenses were encountered from borehole to borehole. A thick clay layer was encountered starting at 95-feet in S13-101/MW-i 01 and continues to at least 115-feet. The average groundwater- elevation on May 15, 2001 was about 56.63 feet above MSL, which represents a decrease of approximately 0.14 feet since the February 22, 2001 monitoring event. This elevation corresponds to about 61 feet below ground surface. A groundwater gradient was calculated for the event and groundwater was flowing N78°W L., at 0.0024 fi/ft. Data is summarized in Table 1, Appendix A. The gradient direction is shown on Figure 2: Gradient Map. Historical groundwater directions and gradient are presented in Figure 3: Groundwater Gradient Rose Diagram. The groundwater gradient and �. direction for the May 15, 2001 event are consistent with past measurements utilizing monitoring wells MW-1, MW-2 and MW-3 for the calculations. +- 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-101) and the mid- ,_, point between the groundwater elevation and the bottorn seal in the shallow well (MW- 1). • 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. Figure 2 shows the location of the well cluster used for calculating vertical groundwater gradient in this report, MW-I and MW-101. Table 1 shows a Summary of Water Level and Gradients Slope and Bearing and Table 3 shows the calculated vertical gradients. The information used in the calculations is shown below. L..