Three-Dimensional Aquifer Heterogeneity in Pumping Test Analysis. A Numerical Investigation Using Radial Flow Models.

Abstract

Pumping test results are often analysed on the assumption of radial symmetry. This may be because analytical solutions are used, which are limited to two-dimensional problems, or because of the widespread acceptance that this assumption is satisfactory when estimating average values of the aquifer hydraulic characteristics. A three-dimensional R-θ-Z model, built using an object-oriented approach, is used to investigate the effects of spatial heterogeneity on time-drawdown curves. Particular attention is paid to the variation of aquifer properties in the circumferential direction, a factor ignored in most pumping test analyses. The mechanisms in the numerical model include a logarithmically increasing mesh spacing in the radial direction, features operating at the pumped borehole and a moving water table. The model is applied to two and three-dimensional idealised aquifers to establish the impact of aquifer heterogeneity. Threedimensional aquifer heterogeneity is shown to produce time-drawdown curves that can be matched using two-dimensional numerical models, giving a misleading interpretation of the aquifer flow processes. For example, a delayed response to pumping at an observation borehole due to a combination of low radial permeability and a fracture in the circumferential direction can be mistaken for large aquifer storage when radial symmetry is assumed. A field example is presented where this mechanism is believed to operate