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Site Background Information Sutter Office Center
<br /> Page 4 of 7
<br /> silty sand, sand, or gravelly sand at a thickness of approximately 15 feet or less, and are located at
<br /> depths of approximately 50 feet bsg, between 60 and 75 feet bsg, between 95 and 110 feet bsg,
<br /> between 120 and 130 feet bsg, and between 140 and 160 feet bsg The sands appeared to be
<br /> continuous throughout the site, with the exception of the uppermost sand layer, which may be
<br /> limited laterally to the northernmost portion of the site Measured hydraulic conductivities in the
<br /> sand-bearing intervals, based on CPT dissipation tests, were typical for sand mixtures
<br /> HYDROGEOLOGIC MODEL
<br /> Based on the CPT results,AGE has developed a preliminary subsurface model of the site sediment
<br /> stratigraphy comprised of ten informal zones a vadose (unsaturated) zone and approximately nine
<br /> hydrogeologic units A hydrogeologic, or hydrostrati graphic, unit (HU) is defined as a body of
<br /> sediment that has hydrogeologic characteristics that make it distinct from surrounding bodies of
<br /> sediment, or a part of a body of rock that forms a distinct hydrologic unit with respect to the flow
<br /> of ground water A flow unit may be defined as a hydrogeologic unit characterized by significantly
<br /> greater hydraulic conductivity values than overlying and underlying units, a flow unit is capable of
<br /> serving as a preferred migration route and could potentially allow migration of large contaminant
<br /> masses over great distances The model presented is interpretational and will be modified as more
<br /> data is collected The ten vertical zones identified are as follows (depths are approximate and can
<br /> vary by several feet across the site)
<br /> Vadose Zone - Extending from surface grade to approximately 35 feet bsg, the depth to the water
<br /> table, the vadose zone consists primarily of fine-grained material from surface grade to 25 feet bsg,
<br /> with a sand unit occurring at 25 feet bsg at CPT-1 on-site Pore pressures in this unsaturated zone
<br /> are characteristically low or at zero
<br /> I Hydrologic Units IA, 2, 4, 6, and 8 (HULA, HU2, HU4, HU6 and HU8) - These are sand-bearing
<br /> units characterized by low CPT pore pressures HUTA, HU2, and HUS are predominantly sand units,
<br /> HU4 and HU6 are composed of inter-bedded layers of sand and sandy silt Hydraulic conductivities,
<br />' based on CPT soil behavior types(SBTs)should range between I X 10-6 to 1 meters per second(m/s)
<br /> for sandy silt to gravelly sand, measured hydraulic conductivities, based on CPT pore pressure
<br /> dissipation tests, were between 1 X 10' to 1 X 10 8 m/s
<br /> I
<br /> Hydrologic Units 1, 3, 5, 7, and 9 (HU1, HU3, HU5, HU7, and HU9) - These units consist of fine-
<br /> grained intervals composed of clay, silty clay, clayey silt, silt, and stiff, fine-grained soil These
<br /> Iintervals are characterized by low to moderately low pore pressure in the clay and silt, generally
<br /> moderate pressures for the clayey silt to silty clay,and pore pressures varying fiom moderately low
<br /> to high in the stiff,fine-grained soil Hydraulic conductivities,based on SBTs,should range between
<br /> 1 X 10-10 to 1 X 10-7 m/s for clay to silt, and between 1 X 10' to 1 X 10' m/s for stiff, fine-grained soil
<br /> Advanced GeoEnvironmental,Inc
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