NUMERICAL-SIMULATION OF THE NOORDBERGUM EFFECT RESULTING FROM GROUNDWATER PUMPING IN A LAYERED AQUIFER SYSTEM

Numerical simulations were carried out to investigate the so-called No ordbergum effect (i.e. reverse water-level fluctuation) resulting from groundwater withdrawal. Two specific cases were analyzed: a three-lay er aquifer system composed of two permeable aquifers separated by a se mi-impermeable aquitard, and a corresponding single-layer aquifer syst em composed of an equivalent lumped material, In the numerical simulat ion of the layered aquifer system, the Noordbergum effect is observed during the early time period of pumping in the overlying aquitard and unpumped aquifer while the underlying aquifer is pumped. In contrast, the numerical simulation results of the lumped aquifer system do not s how such a reverse water-level fluctuation throughout the entire domai n. These results strongly suggest that the Noordbergum effect can be e xplained by the difference in poroelastic responses of the layered (he terogeneous) and lumped (homogeneous) aquifer systems to the hydraulic pumping stress. The Noordbergum effect is caused by two mechanisms: a faster mechanical propagation (deformation) of the pumping stress tha n its hydraulic propagation (drawdown) from the pumped aquifer into th e adjacent aquitard and unpumped aquifer due to relatively lower hydra ulic conductivity of the aquitard, and a mechanical amplification (exc essive compression) in the lower part of the relatively soft aquitard. However, the pumping stress is evenly distributed throughout the enti re domain of the lumped aquifer system without such mechanisms since i t has homogeneous hydraulic and mechanical properties. (C) 1997 Elsevi er Science B.V.