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

摘要

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