Derivative-assisted evaluation of well yields in a heterogeneous aquifer

Abstract

The evaluation of well yields conventional time–drawdown methods is based on the assumption of infinite-acting radial flow (IARF) of groundwater to a well. However, long-term well yields are controlled by heterogeneities and, as suggested here, by the presence of linear features and aquitard leakage, and the subsequent departures from IARF. Accurate prediction of long-term well yields therefore requires an evaluation of aquifer heterogeneities. Derivative techniques combined with aquifer geology and conventional methods aid in the evaluation of long-term well yields in the heterogenous aquifers. Our case study involves the estimation of long-term well yields from relatively short-term aquifer tests in an aquifer near Calgary, Alberta. The wells are completed in a gravel-floored cahnnel incised into bedrock. On the regional scale, the floor gravels appear to form a continuous and homogeneous aquifer. Aquifer-test responses indicate internal heterogeneity at a scale below the resolution attainable with the available well control. Reliable estimates of aquifer parameters are obtained by applying a derivative technique to the analysis of time–drawdown data. Derivative analysis allows us to isolate test segments for which the assumption of IARF is valid. Characteristic time–drawdown and derivative curves are then integrated with geology to identify the nature of heterogeneities and assess their impact on long-term aquifer response to pumping. Key words: aquifer-test, derivative, heterogeneity, well yield, buried valley aquifer.