The Noordbergum Effect and Characterization of Aquitards at the Rio Maior Mining Project

Abstract

ABSTRACTThe planned exploitation of the Rio Maior lignite deposits will require a major dewatering program involving a thick leaky sand aquifer, and a heterogeneous and anisotropic aquitard complex. Diatomite, lignite, and clay are the main lithologies found in this aquitard complex. Characterization of the sand aquifer was accomplished by performing pumping tests with interpretations for steadystate and nonsteady‐state conditions. Aquitards were characterized by using both laboratory and field tests to determine hydraulic conductivity. Field methods were either those commonly adapted to leaky aquifers using water‐level data taken from observation wells tapping the underlying sand aquifer or those using water‐level data taken from piezometers located at the same distance from the pumping well and screened in the aquitard and in the aquifer. Field values of hydraulic conductivity are greater than laboratory values. In several piezometers the Noordbergum effect was observed during pumping tests. Redistribution of the loads originated by the pumping in the underlying sand aquifer is considered to be responsible for the reverse water‐level response in the overlying lignite‐diatomite complex. The Noordbergum effect has some influence in the application of the ratio method. On a practical basis, it seems reasonable to use the Noordbergum effect to compute drawdowns in the aquitards, and this leads to somewhat higher values for vertical hydraulic conductivities.