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Geologrca1Tec(:nus Inc Page 2 Groundwater Monitoring Report Project No 724 2 March 13,2001 1.0 GROUNDWATER MONITORING 1.1 Hydrogeology of Site Geological Technics Inc installed five additional monitoring wells on November 20 - 22, 2000, to investigate the lateral and vertical extent of gasoline-range petroleum hydrocarbons in the groundwater at this site These wells included shallow/interinediate well pairs MW- 6/MW-106, MW-7/MW-107, and a discretely screened deep well, MW-104 (Figure 2, Site Map) Wells MW-6 and MW-106 were placed on the adjacent parcel to the east of the site, approximately 60 feet south of MW-5 Wells MW-7 and MW-107 were placed approximately 15 feet east of the former UST field Deep well MW-104 was placed adjacent to the former dispenser island on the western portion of the site ' A site-specific groundwater gradient and bearing was calculated from the depth to water measurements in wells MW-1, MW-2 and MW-3, as with past events The gradient was flowing S82°E at 0 0014 ft/ft, which is consistent with previous monitoring events The average elevation of groundwater under the site was —17 31 feet (below) mean sea level (msl) The water level has risen significantly at the site since October of 1992 when groundwater was —39 feet ms] Figure 2 is a groundwater gradient map for the January 4, 2001 monitoring event 1 Table 1 in Appendix A contains the shallow groundwater elevations and gradients for all events since the start of this project in October 1992 Table 2, Appendix A contains the intermediate/deep groundwater elevations for the January 2001 event Groundwater has predominately flowed to the east at an average of 0 0033 ft/ft 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-104, MW-106, MW- t07) and the mid-point between the groundwater elevation and the bottom seal in the shallow well (MW-4, MW-6, MW-7) • 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