Well radius of influence and radius of investigation: What exactly are they and how to estimate them?

Abstract

The radius of influence and radius of investigation of a pumping well are fundamental concepts in hydrogeology with various uses in well hydraulics and aquifer testing. However, numerous definitions have been given of these radii, which have often been equated while they are fundamentally different. Here radius of influence is qualitatively defined as the maximum distance from a pumping well up to which the influence of pumping is significant. On the other hand, radius of investigation is qualitatively defined as the maximum distance from a pumping well up to which aquifer properties have a significant influence on drawdown at the well. The exact meaning of the term significant is open to interpretation and, consequently, various operational definitions (i.e., definitions that enable quantification) have been given of radius of influence and radius of investigation. We comprehensively review these definitions and corresponding estimation methods, and discuss the relevance of each definition in view of different potential uses. We also propose new definitions that are more adapted to several applications. Next, we quantitatively compare the various definitions of radius of influence and radius of investigation. The results show that while remaining of the same order of magnitude, the radii are significantly affected by the choice of operational definition and associated parameters. As a rule of thumb, the radius of investigation is half of the radius of influence. We eventually propose a practical guide for choosing a suitable operational definition of radius of influence or radius of investigation depending on the objective.