Causes of non-linear step pumping test responses

Abstract

Step pumping tests are frequently used to examine the performance of wells. It is usually assumed that the drawdown in a pumped well due to a discharge Q can be divided into two parts, the formation loss BQ and a non-linear loss CQ n which depends on the conditions in the vicinity of the well. This paper demonstrates that the actual response due to step pumping tests is often more complex. As an aid to understanding the complexity of step tests, the effect of various conditions in the vicinity of the pumped well and in the aquifer are represented by a radial and vertical flow numerical model. Conditions which are represented include changes in hydraulic conductivity with discharge rate, a seepage face forming in the well, zones of lower hydraulic conductivity or higher hydraulic conductivity within the aquifer, the aquifer changing from the confined to unconfined state in the vicinity of the well and a reduction in the effective depth of penetration due to the collection of sediment in the well. Results from numerical model simulations indicate that there are distinctive discharge-drawdown relationships for each situation. Field examples are presented which illustrate how certain of these responses can be observed in practical situations.