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Geological TeckTics Inc. Page 3 Groundwater Monitoring Report Project No. 2030.2 October 22,2004 approximately 15-feet bgs. A clay layer dominates the site between 19 and 22 feet bgs. Each of the sites five water table wells terminated in "plastic" clay. Groundwater was encountered ' between 9.5-11.4 feet bgs. The capillary fringe extended approximately 4 feet above the water table. ' The deep well (MW-101) terminated in highly plastic "fat" clay. Dirty sand and clay define the 25-35 feet bgs lithology near the former USTs. The deep geology of the site is primarily poorly sorted sand with varying amounts of silt and clay. The screen interval of the deep well ' (MW-101) is located in dirty sand. MW-101 is the only deep well and defining the deep geology is limited to this data. ' 1.3.2 Groundwater Gradient and Bearing On July 14, 2004, the depth to groundwater was measured in all of the monitoring wells prior to purging. Depths to groundwater, at the site, averaged 8.91-feet below the top of casing (TOC). The average groundwater elevation for the April 2004 event was 67.56-feet above mean sea level (AMSL). Site-specific groundwater bearing and slope were calculated and ' found to be N40°E @ 0.0042 ft/ft. Groundwater elevation increased 2.45 feet since the April 2004 monitoring event. These results are consistent with average historical groundwater bearing and slope for this site. Vertical groundwater gradients were calculated for the site using the MW-1/MW-101 well pair and found to be positive. The site-specific vertical groundwater gradient is 0.0006 ft/ft. ' MW-1 is screened from 5 to 20.feet bgs and MW-101 is screened from 29.5 to 34.5 feet bgs. Vertical groundwater gradient calculations have been performed at regular intervals since the installation of MW-101 on April 12, 2002. The historical vertical gradients have typically ' been positive and approximately one order of magnitude less than the horizontal gradients. Groundwater elevation, bearing and slope were calculated using data gathered from MW-2, ' MW-3 and MW-4 during the July 14, 2004, monitoring event and the results are summarized in Table 1 of Appendix A. Table 2 of Appendix A summarizes the vertical groundwater gradient calculations. ' 1.3.3 Average Groundwater Velocity The lithology at the ST site will be divided into three general layers for the purposes of calculating the average horizontal groundwater velocity. Layer 1 consists of silts and clays from 5- to 15-feet bgs; Layer 2 is primarily sand with some fines from 15- to 20-feet bgs; Layer 3 is primarily clay from 20- to 25-feet bgs. The average depth to groundwater at the ' - site suggests groundwater flows horizontally through each of the four layers. ---- La.'s er 1 ' The average hydraulic conductivity (K), estimated from typical K values for silt and clay mixtures (Driscoll, 1986), 0.12 feet per year (ft/yr), and the average hydraulic gradient (i) across the site is approximately 0.0038. Based on the equation v = K i/n, where n is effective