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1 GeologicalTeclucicslKc. Page 2 !T Groundwater Monitoring Report Project No.750.2 I November 18,2003 A vertical groundwater gradient calculation was made for the September 2003 groundwater- monitoring event. The vertical gradient was found to be in a downward direction with a slope that is less than the water table horizontal gradient. 1.0 GROUNDWATER MONITORING i 1.1 Hydrogeology of Site The dominant it - so type m 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 measured during the September 6, 2003, event was 55.47 feet above mean sea level, which is consistent with previously measured average elevations at the site. The groundwater elevation represents a decrease of.approximately L 1.49 feet since the monitoring event of June 5, 2003. r- , Groundwater gradients were calculated for the September 2003 event using MW-1, MW-2 and MW-3. Groundwater was flowing N89°W at 0.0020 ft/ft. The elevation, gradient and bearing data are summarized in Table 1, Appendix A. The groundwater gradient is ' illustrated on Figure 3: Groundwater Gradient Map. Historical groundwater directions and gradients are presented in Figure 4: Rose Diagram, which shows a historically westerly flow direction., L 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 iwith horizontal flow conditions, the diameter of the well or piezometer is irmnaterial. 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- 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. U