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1 �{ CjeoCogicalTechcnics 2nc. Page 2 Groundwater Monitoring Report Project No. 770.2 January 10,2002 1.0 GROUNDWATER MONITORING 1.1 Hydrogeology of Sife The site exhibits a primarily clayey geology with minor sand layers/lenses, pebbles, black organics, and iron staining occasionally present. The groundwater elevation on December 7, 2001 averaged —36.14 feet below MSL (-61 feet bgs). This represents a decrease of 0.17 feet since the September 2001 monitoring event. r A groundwater gradient was calculated for the December 2001 monitoring event and found to be flowing N70°E at 0.0004 ft/ft. Groundwater elevation and gradient data is summarized in Table 1, Appendix A. Gradient direction is shown on Figure 2: Gradient Map. The historical gradients are illustrated in Figure 3: Groundwater Gradient Rose Diagram. The site displays highly variable shifts 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 prcedure is used to calculate vertical groundwatergradient: • 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-105) and the mid- point between the groundwater elevation and the bottom seal in the shallow well (MW- 5). • 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 f 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 5 gradient in this report, MW-5 and MW-105. Tables 1 and 2 show Summary of Water Levet and Gradients Slope and Bearing, and Table 3 shows the calculated vertical gradients. The information used in the calculations is shown below. Vertical gradient calculationformulas 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] 1�