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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 CW-N 15 was only approximately 50 percent, so <br />some error is assumed in the calculated transmissivi ty value. The aquifer test data plots <br />for Wells CW-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 />NcWhorter (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 of terflow. The of to rflow can be determined <br />using the following relationship: <br />pl = ?f rcz(So-sl)/(tl -to) <br />where: <br />Ol = Aquifer discharge at time increment tl <br />t =Time pumping ceases <br />0 <br />tl = Time increment since pumping ceased <br />S =Drawdown at time t <br />0 0 <br />S1 = Drawdown at time increment tl <br />7-26 Revised 04/11/88 <br />