ARCHIVED REPORTS XR0011094 - Laserfiche WebLink

Home Browse Search

GeologicalTeck�ucsl�c. Page 2 Groundwater Monitoring Report Project No. 750.2 October 5,2001 �.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, La 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 August 15, 2001 was about 55.32 feet above MSL, which represents a decrease of approximately 1.31 feet since the May 2001 monitoring event. This elevation corresponds to about 63 feet below ground surface. A groundwater gradient was calculated for this event and groundwater was flowing N770W at 0.001 ft/ft. Unlike previous events, the gradient on August 15 was determined from computer software generated contour lines rather than a traditional three-point problem. The data for the August 15, 2001 event are consistent with past measurements utilizing �.. monitoring wells MWA, 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- • 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-1 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.