Analysis of selected tests of aquifer characteristics, West Pakistan

Abstract

Selected pumping tests from the old province of the Punjab of West Pakistan were analyzed in detail, with a view to defining the hydraulic characteristics of the unconfined aquifer and evaluating the effects of various physical factors that govern the unconfined flow. An electric analog model study was made also to examine the effects of anisotropy, vertical components of flow, and the partial penetration of both the pumped well and the observation wells. From the analog study response curves were obtained for a representative pumping test in the Punjab0 These were used to determine the effects of anisotropy, partial penetration, and vertical flow components and to evaluate the results obtained from the application of equations of radial flow to an unconfined flow system. The significance of the delayed yield from storage was also studied using Boulton's method (1963). The response of the aquifer appeared to be artesian in the initial stages, which reflected the release of internal storage, but later changed to water table as additional yield accrued from lowering of the water table. The results of the study indicate that pumping tests in an Unconfined aquifer require much longer duration to meet the qualifying conditions for application of equations of radial flow that assume instantaneous release from storage and complete penetration of the aquifer. Further, it is shown that observation wells which are located at greater distances, but within certain practical limits, provide a better basis for interpretation of the test results than do wells in the region of flow convergence. The average transmissibility of the upper section of the aquifer, which represents a few hundred feet of aquifer thickness, was estimated as 0.5 ft^2/sec with an average lateral permeability of 0. 0015 ft/sec (1, 000 gpd/ft^2). The ultimate value of T for the entire thickness of the aquifer appears to be more than twice the value found from the tests of partial penetration on the basis of the electric analog study. The internal storage due to elastic properties of the aquifer is 0. 002, and the average specific yield is 0, 15. The time when the effect of delayed yield ceases to be significant ranges from 2 to 12 days, with an average of about 7 days. It is expected that results achieved by the present study will lead to improvements in existing methods of aquifer testing and analysis of results in the areas of investigation in West Pakistan.