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Results of Groundwater Pumping Tests - 152 East 11 th Street, Tracy Page 12 Early-jime Data As noted previously, although the pumping tests performed on January 18th 1994, were adversely affected by intermittent freezing of the pump under the adverse weather conditions, useful early-time data was obtained from those tests through use of the electronic data logger and pressure transducer array Specifically, when Well NAV 6 was first pumped, good early-time data in the first minutes after the pump was started were obtained with drawdowns recorded at closely spaced intervals These data are plotted on Figure 10 That Figure shows that there is a distinct change in slope of the drawdown versus logarithm of time plot at 0 18 min after the pump was started Prior to that time, the plot is relatively steep and equates to a computed transmissivity of 864 gpolft Thereafter, the slope flattens and yields a computed transrrussivity of 3,895 gpolft which is in good agreement with the transmissivity of 4,324 gpolft , which was derived for the aquifer from the longer duration test that was performed on December 28th 1993 (See Figure 8 ) The bi-modal nature of the time-drawdown plot on Figure 10 is characteristic of early- time plots for wells where the time-drawdown relationship is initially dominated by evacuation of water from within the casing and only later by the hydraulic properties of the aquifer This phenomenon is known as the casing effect However, such bi-modal plots can also be produced in aquifers that are being recharged, or there is some boundary effect due to hydrostratigraphic variations within the zone of influence of the pumped well To check that the slope change of the seen on Figure 10 can be attributed to the casing effect, Schafer's (1978) procedure was used to estimate the duration of the casing effect using the equation Time (in Min ) after which the Casing Effect is Negligible, t, = 0 6 (dC2 - dp2) s / Q where Q = Steady State Pump Discharge Flow Rate (in GPM) dc = Inside Diameter of Well Casing (in in ), and dp = Outside Diameter of Pump Riser Line (in in ) For well NAV 6 the internal diameter of the casing is 1 78 in and the outside diameter of the piping installed in the well to evacuate the water was approximately 1 in Applying these and the other applicable parameter values to solve the above equation iterivly produced an estimate for to of 0 48 min which is close to the time at which the slope of the data plotted on Figure 10 changes This confirms the assumption that the very early drawdown data shown on the figure is influenced by the casing effect but that drawdown