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GeologicaI ncluucs//u. Page 6 Groundwater Monitoring Report Project No.750.2 April 1,2005 Figure 2 shows the location of the well cluster used for calculating vertical groundwater gradient in this report, MW-1 and MW-101. Table 2 of Appendix A summarizes the calculated vertical gradients. 2.3.4 Average Groundwater Velocity The lithology at the COE site will be divided into four general layers for the purposes of calculating the average horizontal groundwater velocity. o Layer 1 consists of silts and sands from 5- to 30-feet bgs o Layer 2 is primarily sand with some larger clasts from 30-to 45-feet bgs o Layer 3 is primarily silt from 45-to 50-feet bgs o Layer 4 is primarily sand from 50-to 70-feet bgs. r The average depth to groundwater at the site suggests only Layer 4 will contain groundwater ,in a saturated state. The following calculations are based on the equation v = K i/n, where v is the average horizontal groundwater velocity, K is the average hydraulic conductivity, i is the average hydraulic gradient, and n is the effective porosity. K and n values are estimated from tables in Groundwater and Wells 2nd Edition (Driscoll,F.G., 1986). r • The average horizontal groundwater velocity (v) of Layer 1 is estimated to be approximately 0.065 to 6.5 ft/yr. —lo- • The average horizontal groundwater velocity (v) of Layer 2 is estimated to be approximately 5.7 to 2,850 ft/yr. • The average horizontal groundwater velocity (v) of Layer 3 is estimated to be approximately 5.6E 104 to 0.057 ft/yr. • The average horizontal groundwater velocity (v) of Layer 4 is estimated to be approximately 0.758 to 75.8 ft/yr. r r 3.0 GROUNDWATER MONITORING 3.1 Monitoring Well—Groundwater Sampling Procedure On February 16, 2005, Del-Tech mobilized to the site to conduct a groundwater-monitoring event. Del-Tech personnel opened the wells and measured the depth to water with an electrically actuated sounding tape. The water level reading was recorded to an accuracy of 0.01 foot. No floating product was detected during the monitoring event. Stagnant water in the well casing was purged using a Waterra© pump and dedicated tubing. The rate of well purging was monitored. The well was purged of at least three casing volumes until the groundwater parameters (temperature, conductivity and pH) had stabilized (Appendix C) indicating that water representative of actual aquifer conditions was entering