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GEOMATRIX (Figure 3). The sand intervals vary between 2 and 10 feet thick, and comprise primarily very fine sand to silty sand material. The intervening clay intervals consist of moderately plastic silty clay to clay materials. The sediments are generally blue-green in color, suggesting a reducing depositional environment. The sand interval from approximately 70 to 90 feet bgs is relatively continuous beneath ' the Facility and vicinity and has been designated the intermediate zone. The intermediate zone is currently monitored by five wells, including MW-9, MW-15, and MW-17 through MW-19 (Figure 2). Between about 100 and 150 feet bgs (the maximum depth of investigation beneath the Facility) is another sequence of interbedded clay and sand layers. Relatively continuous sand intervals were identified at approximately 120 and 150 feet bgs (Figure 3). The sand interval at about 120 feet bgs varies between 10 and 15 feet thick, and comprises primarily very fine sand to silty sand material. This interval appears to become finer grained towards the west. The sediments are generally blue-green in color, suggesting a reducing depositional environment. The sand interval at about 150 feet bgs varies in ' thickness and appears to pinch out to the west. Groundwater in the shallow zone is first encountered between 9 and 12 feet bgs. The most recent ' groundwater elevation data were collected on 23 March 1998 (Geomatrix, 1998b). On 23 March 1998, the depth to groundwater was measured in 14 shallow zone monitoring wells and the groundwater surface was found to vary between 0.14 and 1.05 feet below mean sea level (MSL). The depth to groundwater was also measured in five intermediate zone monitoring wells on 23 March 1998 and the groundwater surface in the intermediate zone was found to vary between 3.00 and 3.40 feet below ' MSL. The 23 March 1998 data indicate that there is a downward vertical hydraulic gradient between the shallow zone and intermediate zone; however, vertical migration of groundwater may be inhibited by the presence of fine-grained materials (silt and clay) between these zones. Water level elevations in intermediate zone monitoring wells ranged between 2.46 and 3.01 feet less than in the adjacent shallow zone monitoring wells. The average difference in groundwater elevation between the shallow zone and intermediate zone during March 1998 was 2.94 feet. The presence of the downward hydraulic gradient 1 beneath the Facility has been consistent over time(Geomatrix, 1997a,b, 1998b). ' Potentiometric surface maps prepared for the shallow and intermediate zones based on the 23 March 1998 groundwater elevation data show that the direction of horizontal groundwater flow beneath the facility in the shallow zone is predominantly to the east-northeast with a hydraulic gradient of about 0.00073, or approximately 3.8 feet per mile (Geomatrix, 1998b). The direction of horizontal ground- water flow beneath the facility in the intermediate zone is predominantly to the east-southeast with a ' hydraulic gradient of about 0.00186, or approximately 9.8 feet per mile. These data indicate that the direction and magnitude of horizontal groundwater flow in the shallow zone and intermediate zone are different beneath the Facility and vicinity. The presence of a consistent downward vertical groundwater ' gradient and the presence of fine-grained materials between the shallow and intermediate zones I\PROIECTCIWRD(YOI.ffNNX54l'l.CPc 5