Geology Reference
In-Depth Information
3
Overview of the Hydrogeology
of Hard Rock Aquifers:
Applications for their Survey,
Management, Modelling and
Protection
P. Lachassagne
Resource Assessment, Discontinuous Aquifers Unit, Water Division, BRGM,
French Geological Survey, 1039, rue de Pinville, 34000 Montpellier, France
INTRODUCTION
Hard rocks (granites, metamorphic rocks) occupy large areas throughout the
world (Africa, South and North America, India, Korea, several areas in
Europe, etc.). Their groundwater resources are modest in terms of available
discharge per well (from less than 2-3 m
3
/h up to 20 m
3
/h), compared to
those in other types of aquifers (sedimentary, karstic or volcanic aquifers).
They are, however, geographically widespread and therefore well suited to
scattered settlement, and small to medium size cities. These resources
contribute largely to the economic development of such regions, especially
in arid and semi-arid areas where the surface water resource is limited.
However, these aquifers are considered as highly heterogeneous. For
example, two neighbouring wells may exhibit very contrasted behaviours:
one of them yielding several cubic metres per hour, the other being of very
low discharge. As a consequence, for most authors, their hydrodynamic
properties feel quite unpredictable at the local scale and they are thus
considered as "discontinuous aquifers". Moreover, their properties also seem
to be unpredictable at the catchment scale.
Significant advances have recently been made in our knowledge of the
structure and functioning of hard rock aquifers. Their hydrodynamic properties
appear to be mainly related to the existence of ancient weathering profiles.
The spatial distribution of such weathering profiles, or their remains after
