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<br /> 30 November 2005,
<br /> { AGE-NC Project No 96-0235
<br /> ' Page8of13
<br /> A! l Y 4 I I
<br /> 'STRATIGRAPHY
<br /> Dominantly fine-grained'soil consisting of silt, clayey silt, silty clay and clay were encountered at
<br /> depths from the near surface to approximately 185 feet bsg ,Significant sand units with variable silt
<br />' to gravel were encountered between 20 feet and 40 feet bsg, 62 feet to 90 feet bsg and 165 feet to I`
<br /> V 1,91 feet bsg The shallow sand unit(20 feet to 40 feet bsg)appears to have limited'lateral continuity, ,
<br /> The'sand unit,encountered at 165 feet bsg appears to have lateral continuity and a documented v
<br />' F thickness of greater than 40 feet Lessor, thin, laterally discontinuous sand units were encountered `
<br /> gat approximately 50 and' 100 feet bsg at the southern UST No , 3 area" CPT-computer-generated s
<br /> ` boring logs are included in Appendix B Based on the findings from the previous CPT soil borings '
<br /> and existing monitoring well and soil borings,cross sections A-A' was constructed to depict general
<br /> lithology beneath,the=UST No" 3 area and directly-south of the site (Figure 7)
<br /> 3 4 HYDROGEOLOGICAL MODEL Y`
<br /> Based on the CPT results, AGE has developed a preliminary subsurface model of the site sediment
<br /> stratigrphy comprised' of informal 'zones including a vadose (unsaturated) zone and various.
<br /> hydrogeologic units Hydrogeologic or hydrostratigraphic units (HUs) can be defined as'a body of {
<br /> sediment that has hydrogeologxc characteristics(1'e hydraulic conductivity,pore pressure,etc )that t
<br /> ",make it distinct from,surrounding bodies ofsediment(Sanders, 1998), or a part of a body sediment
<br /> ''that forms a distinct hydrologic unit with respect to the flow of ground water(Maxey, 1964)"A`flow
<br /> " `,'i,',umt' may be defined as a hydrogeologic unit and is characterized by significantly-greater hydraulic
<br /> ' 'conductivity values than overlying and underlying units, a flow unit is capable of serving as a
<br /> ' preferred migration route;,and could potentially allow migration of large contaminant masses over'
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<br /> great,distances
<br /> Based on` the CP1 results, AGE had previously identified' seven informals zones, a vadose I '
<br /> (unsaturated) zone and six hydrogeologic units as follows
<br /> , 'se ' 1 t r ' f ~'L
<br /> VadoZone,- Extending from surface[ grade to approximately 18 feet bsg, the depth to'the water
<br /> table -The vadose zone is predoffiniantly fine-grained iiiateniil from surface grade to;20 feet bsg The '
<br /> zone is characterized by low pore pressures Hydrogeologic Unit 1 (HU 1), at the top of the current r
<br /> ,r saturated zone', extends from 18 feet bsg to a maximum'depth of 40 feet bsg and is comprised I
<br /> predominantly of clay and fine sand,interbedded with silt HU1, is characterized`by generally low
<br /> t ' to moderate pore pressures Hydrogeologic Unit 2 (HU2)extends from a minimum depth of 31 feet
<br /> bsg to a maximum depth of-72`feet bsg and is composed,dominantly of stiff, fine-grained soils, ,
<br /> including silt and ,clay„ interbedded with laterally discontinuous fine-sand intervals HU2 is
<br /> 1 characterized by,generally modeiate,to high pore pressures Hydrogeologic Unit 3'(14U3) extends
<br /> 1 ; from a'minimum depth of 62 feet bsg to a maximum depth of 90 feet bsg, HU3 is composed 3 '
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