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ti <br /> Results of Groundwater Pumping Tests - 152 East 11th Street, Tracy Page 14 <br /> T = the Transmissivity of the Subject Stratum, and <br /> b = the Thickness of the Subject Stratum <br /> Using the value of transmissivity of 4,324 gpd/ft computed on Figure 8 and an aquifer <br /> thickness of 11 ft for the confined sand stratum shown on Figure 7, its horizontal hydraulic <br /> conductivity is computed to be <br /> Horizontal Hydraulic Conductivity (Sand Stratum) = 393 gpd/ftz (1 8 x 10--2 cm/sec ), <br /> which is within the range expected for sands of the subject type (Terzaghi and Peck, 1967) <br /> However, the above computation assumes that all of the groundwater flowing to the <br /> pumped well was contributed by the sand layer, the top of which occurs approximately 14 <br /> ft below the ground surface (i e 6 ft below the water table) In actuality, as has been <br /> shown previously, the sand stratum is not a tightly confined aquifer ( i e the sand layer is a <br /> "leaky" aquifer) and some of the flow to the well was contributed by the 6 ft thickness of <br /> silty clay in the interval between the water table at a depth of 8ft and the top of the sand <br /> at a depth of 14 ft Thus, the value of 393 gpd/ftz must be treated as an upper-bound for <br /> the hydraulic conductivity of the sand <br /> If it is alternately assumed, for the purposes of making an estimate of the mean horizontal <br /> hydraulic conductivity of the stratified formation as a whole, that the aquifer influenced by <br /> the pumping should be taken to be 17 ft thick the calculation yields an upper-bound <br /> estimate of <br /> Mean Horizontal Hydraulic Conductivity = 254 gpd/ftz (1 i x 10­2 cm/sec) <br /> With respect to the hydraulic conductivity of the silty clay, that occurs at and below the <br /> water table, an estimate can be made by considering the results of the pumping test that <br /> show that the sand stratum behaves as a confined aquifer, but with a Coefficient of <br /> Storage close to the upper limit of the range for such systems For the overlying silty clay <br /> to form an aquitard that would fully confine the groundwater in the sand, it would have to <br /> have a relative vertical hydraulic conductivity several orders of magnitude lower than that <br /> of the sand - say around 0 2 gpd/ftz Such a value is within the range for materials with the <br /> lithology of the silty clay as it was observed in samples obtained when the wells were <br /> installed [See Terzaghi and Peck, 1967, Tablell 1 for typical values of hydraulic <br /> conductivity (permeability) of clastic materials] However, considering that the Coefficient <br /> Storativity of the system was computed to be at the high end of the range normally found <br /> for confined aquifers, and to ensure a conservative estimate with respect to groundwater <br /> flow-rate parameters of importance in the context of the present evaluation, the following <br /> upper-bound value will be used <br /> is Horizontal Hydraulic Conductivity (Silty Clays) = 39 gpd/ft2 (1 7 x 1 0--3cm/sec ), <br />