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analytical technique loses some accuracy when the percentage of aquifer contribution <br /> during the pumping test is less than 70 percent. In the case of GW-N9, the percent <br /> aquifer contribution was approximately 68 percent so the error was assumed negligible. <br /> The percent aquifer contribution for Well GW-N15 was only approximately 50 percent, so <br /> some error is assumed in the calculated transmissivity value. The aquifer test data plots <br /> for Wells GW-N19 and GW-N20 match the theoretical response curves well except for the tail <br /> portion of the water level recovery. Possible causes for this include the incorrect <br /> measurement of the initial water level following removal of a volume of water, fracture <br /> flow and the interception of impermeable boundaries. <br /> Because the modified slug test analysis is a relatively new procedure, some additional <br /> discussion regarding the technique is warranted. Briefly, the modified slug test accounts <br /> for the opposing theories of an aquifer's response to slug and pumping stresses. The slug <br /> test assumes that there is no aquifer discharge until after the well is "slugged". The <br /> standard pumping test (Theis assumptions) solution assumes that aquifer discharge only <br /> occurs during the pumping phase of a test and that there is no casing storage. The <br /> modified slug type curves account for the two opposing scenarios, thereby allowing the <br /> pumping test recovery data to be analyzed in a similar manner to an instantaneous slug <br /> test. The solution is arrived at by superimposing the data plot on top of the modified <br /> type curves, which take into account non-instantaneous stresses to the aquifer. Appendix <br /> 7-2 contains the modified type curves in tabular and graphical form. <br /> McWhorter (1982) also provides an alternate method for analyzing recovery data. In those <br /> cases where the volume of water pumped from the aquifer is 70 percent or more of the total <br /> volume of water pumped, the standard recovery theory can be modified to account for the <br /> effects of continued aquifer discharge or afterflow. The afterflow can be determined <br /> using the following relationship: <br /> Q1 =Tr rc2(S0-S1 Mt 1-t <br /> where: <br /> 01 = Aquifer discharge at time increment t1 <br /> t = Time pumping ceases <br /> 0 <br /> t1 = Time increment since pumping ceased <br /> S = Drawdown at time t <br /> 0 0 <br /> S1 = Drawdown at time increment t1 <br /> 7-26 Revised 04/11/88 <br />