Analysis of hydrologic performance tests in unconfined aquifers

Abstract

Evaluation of aquifer characteristics was one of the facets of hydrological investigations started in l95 in the Indus plains of West Pakistan. Rechna and Chaj Doabs (area between Ravi and Chenab rivers; Fig. 1) were the first to be investigated. Analysis of the pumping tests in these areas was made after methods of Theis and Jacob. The values of storage coefficients obtained indicated artesian conditions but this was in conflict with the available geological evidence. Transmissibilities were also questionably high, and there was a disparity between results calculated from time-drawdown versus distance-drawdown analyses. The purpose of the present study was to understand the flow regime around a pumping well in the area of study; to identify the causes of inapplicability of Theis' analysis method, and to evaluate alternative approaches for analysis of pumping tests in this area. Analysis of four specially designed long duration pumping tests indicates that vertical components of flow due to partial penetration and a variable storage coefficient due to slow drainage are the two major factors which control the aquifer response and make Theis' method inapplicable. In the initial pumping period the effect of partial penetration on drawdown is very significant whereas the effect of delayed yield is inappreciable. In general, the effect of partial penetration is more pronounced throughout the pumping period as compared to delayed yield. However, with increasing distance from the pumping well, the partial penetration effects decrease rapidly and thereby become less important. It was found that analysis of the first 20 - 40 minutes of pumping data, with allowance for partial penetration, gave an accurate value of "P" and artesian storage. In most cases the effective depth of aquifer could also be estimated. Observation wells at about l.5b or farther from the pumped well, with adjustment for delayed yield, gave good values of specific yield and transmissibility. Close agreement was observed between the values of "T" from both methods. The use of data of nearer wells for the partial penetration analysis, and the farther well data at later times for the delayed yield analysis provided the best approach for aquifer characteristic determination. In the present analysis the effects of anisotropy and vertical component of flow due to lowering water table are so small relative to the partial penetration influence that they do not affect the early data analysis. However, a method has been suggested to account for these factors if in any test their magnitude appears significant. The average value found for permeability was 7.5 x 10^-4 ft/sec, the artesian storage coefficient was in the range of 0.0001 - 0.005, and specific yield was 0.2 - 0.25. The effective depth of aquifer taking part ranged from 720 feet to over 1500 feet.