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i N Site Background Information - Guardino and Crawford Page 5 of 6 encountered. Alternating layers of silty and sandy material were identified from approximately 41 feet bsg to the total depth of boring CPT- 1 . I HYDROGEOLOGIC MODEL Based on the CPT results and comparison with soil descriptions from borings previously advanced on the site, AGE has developed a preliminary subsurface model of the site soil stratigraphy comprised of six informal zones - a vadose (unsaturated) zone and five hydrologic units. Hydrologic units may be defined as significant vertical intervals of soil having significant lateral extent and having similar hydrologic characteristics (i.e. hydraulic conductivity, transmissivity, etc.) over their vertical and horizontal extents. A ` flow unit' may be defined as a hydrologic unit characterized by significantly greater hydraulic conductivity values than the overlying and underlying units. A flow unit is capable of serving as a preferred migration route, and could potentially allow migration of large contaminant masses over great distances. The model presented is interpretational and will be modified as more data is collected. The six vertical zones identified are as follows (depths are approximate and can vary by several feet across the site) : Vadose Zone - Extending from surface grade to approximately 17 feet bsg, the vadose zone is composed primarily of silt and sandy silt underlain by an approximately 5-foot layer of clayey silt, silty clay, and clay. This zone is characterized by low to moderate pore pressures. HUl - HUl extends from approximately 17 feet to 33 feet bsg and is composed primarily of fine sand with low to moderate to pore pressures in the upper 5 feet of the zone, with sandy silt grading to gravelly sand below. This zone is characterized by low pore pressures and may be considered a flow unit. HU2 - HU2 extends from approximately 33 feet to 43 feet bsg. This zone consists primarily of clay, clayey silt, and silty clay. This zone is characterized by generally low pore pressure, except for a high-pressure peak at the bottom ofthe unit associated with very fine sands or less permeable clayey material. HU3 - Extending from 43 feet to approximately 55 feet bsg, HU3 is composed primarily of layers of sandy material (sand and fine- to coarse-grained sand) This zone exhibits uniformly low pore pressures and may be considered a flow unit. HU4 - Extending from 55 feet to approximately 70 feet bsg, HU4 is composed primarily of thin, alternating layers of silty material (silt, and silt with clay). HU5 - Extending from 70 feet to approximately 80 feet bsg, HU5 is composed primarily of layers Advanced GeoEnvironmental, Inc.