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n s Results of Groundwater Pumping Tests - 152 East 11 th Street, Tracy Page 13 . data gathered later than 0 5 min after pumping had started reflects the hydraulic characteristics of the geologic formation A second concern related to the use of early-time data from pumping tests is that the Cooper and Jacobs' (1946) method of analysis is not accurate if used to compute transmissivity from drawdown data obtain from the early stages of a test when rt = 187Y4S /Tt X005 where r= Distance (in ft ) from the Center of the Pumped Well to the Point where the Drawdown is Measured (For a pumped well constructed with a filter pack, this is equal to the radius of the boring ) S = The Coefficient of Storage T = The Transmisszvity of the Formation (►n gpd/ft ) ,and I= Time Since Pumping Started (in days) Using the above formula and the parameter values obtained from the tests conducted on December 28th 1993, the time, t ,after which the Cooper and Jacobs' method is accurate is computed to be 0 028 mm However, in practice, drawdown data obtained within the first 10 min after the pump start is not normally used to compute transmissivity This practice was followed when we used the data plotted on Figure 8 to compute the transmissivity of the strata beneath the 152 East Street site For the observation well, NAV 3, at a radius of 86 ft from the pumped well, a similar calculation shows that values of a do not fall below 0 05 until more than 203 min have elapsed after the start of pumping Accordingly, because, for the reasons previously explained, such late-time data was not available from that well, drawdowns measured there were not used to compute transmissivity Hydraulic Conductivity and Groundwater Flow Velocity Hydraulic Conducttvrtv I The horizontal hydraulic conductivity of the strata influenced by a pumping tests can be computed from the relationship Hydraulic Conductivity, K = T / b where