Effets des gravières sur le comportement hydrodynamique des nappes d'eau souterraines

How gravel pits affect hydrodynamic groundwater behaviour

Ingénieur au B.R.G.M.

Abstract

Alluvial formations are very valuable sources of sand and gravel. They are frequently worked down to groundwater level. The aquifer, which mostly shows promising characteristics, frequently supplies a whole region with drinking water. Quarries in the aquifer system are liable to depress or raise the groundwater level, and especially to affect water quality by chemical or biological factors or by facilitating pollutant inflow. The severity of such effects is directly dependent on the amount of water exchange taking place between the full gravel pit and the aquifer. To evaluate such effects, therefore, it is necessary to establish hydrodynamic relationships existing between the aquifer and surface water. Author reviews the factors to consider in evaluating such hydrodynamic effects, which include warping of the river banks and bed, depth of excavation and position in relation to the aquifer system, relationships with rivers, shape and dimensions of excavations and position with respect to the general direction of groundwater flow. The effects of these factors vary and can be more clearly visualised with the aid of a series of diagrams. River bank and bed warping turns out to be the vital governing factor as regards flow exchange taking place between a gravel pit and aquifers. If warping is taking place the gravel pit is an obstacle to groundwater flow ; if not, it acts as a "privileged" flow path. Present means whereby these parameters can be evaluated are discussed in the second part of the article. Without going into conventional hydrogeological approaches in detail, special reference is made to local measurement of permeability, vertical permeability distribution through the various ground horizons and, especially, evaluation of river bank and bed warping. Closer methodology, however, seems to be required for some of the points considered. Interpretation and prediction methods vary from simple calculation to use of a three-dimensional model. The usefulness of electrical analogies of the conductive paper model type, which are particularly suitable for this type of problem, is also emphasized. Reference is made to two very different types of investigation. One relates to a gravel pit extension scheme in the Crau region of France, more especially its effects on the local water table level. The other is a more basic study of optimal gravel pit and water supply operation in the Toulouse region, in which the warping problem in quarries and its variation over a period of time are considered. Warping measurements have been carried out on site, from the results of which it is expected that the effects of the gravel pits on present and future water supply yields can be predicted.