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Geological rechudcs#(C. Page 3 Groundwater Monitoring Report Project No. 1030.2 November 18,2004 1.3.1 Lithology and Soil Characteristics t The shallow geology of the site is characterized by clay and silt primarily gray in color with varied degrees of plasticity. A dirty sand layer was encountered throughout the site at 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 October 14, 2004, the depth to groundwater was measured in all of the monitoring wells prior to purging. The average groundwater elevation for the October 2004 event was 64.65 feet AMSL. Site-specific groundwater bearing and slope were calculated and found to be N410E @ 0.0035 ft/ft. Groundwater elevation decreased 2.91 feet since the duly 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 October 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 following conclusions were farmed based on the equation: v =K t/n n=effective porosity