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'- •: 4whgicatTechxiesInc Page 2 . Groundwater Monitoring Report Project No. 750.2 December 20,2001 \..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 SB-101/MW-101 and continues to at least 115-feet. The average groundwater elevation on November 16, 2001 was about 54.96 feet above mean sea level, which is the lowest measured at the site to date. The groundwater elevation represents a decrease of approximately 0.36 feet since the August 15, 2001 monitoring A" event. This elevation corresponds to about 63 feet below ground surface. A groundwater gradient was calculated for this event and groundwater was flowing N87°W 'r at 0.0021 ft/ft. The gradient was determined from computer software generated contour lines rather than a traditional three-point problem. The data for the November 16, 2001 event are consistent with past measurements utilizing monitoring wells MW-1, MW-2 and MW-3 for the gradient calculations. The elevation, gradient and bearing data is summarized in Table 1, Appendix A. The gradient direction is illustrated on Figure 2: Gradient Map. Historical groundwater directions and gradients are presented in Figure 3: Groundwater Gradient Rose Diagram. 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 bottom 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 piezometrie 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. +✓ Lo