Characterizing Aquifers - Hydrogeology: Objectives, Methods, Applications - page 127

Some of these parameters are defi ned by geologic and structural studies
(geometry of the overlying unit, thickness and cross-section of the aquifer,
facies variations of the reservoir), others are determined by specifi c tests
(permeability and transmissivity of water-bearing formations).
It is the combination of all this data that allows the defi nition, area by
area, of the nature of the factors directly governing the observed variations
in hydraulic gradient of the aquifer. These factors can, for example, include
the cross-section of the aquifer, which, in an alluvial plain, can result
in tightening valleys and increasing hydraulic gradients, or instead in
widening valleys and decreasing gradients. Similarly, in an area with a
constant or well-known cross-section, the variations in piezometric slope can
evolve as an inverse function of the aquifer's coeffi cient of permeability.
Major structural accidents (folds, faults) can also generate important
anomalies in the piezometric surface, both in map view and in cross-section,
due to the modifi cations they cause in groundwater fl ow patterns.
Finally, piezometric maps can provide useful information on the
exchanges occurring along the aquifer limit or along distinctive elements
in its environment (stream network, irrigation canals, lakes, sea). This
information depends on the confi guration of piezometric curves in the
relevant sector: lack of exchanges, infl ow to an alluvial aquifer by the
surrounding slopes or by the stream network (Figure 66), drainage of an
aquifer by a valley....
These developments can be illustrated by two concrete examples of
piezometric maps, established in two different hydrogeologic contexts and
at two distinct scales:
1) The piezometric map of a karst aquifer developed in a Triassic
gypsum unit in the town of Sospel (Alpes-Maritimes). This document
was established in October 1980, based on measurements from 22
piezometers and 4 basal springs (Figure 68). The aquifer is drained to
the south by the river bordering the hillside, with an average slope
of 0.06 to 0.07. The variations in hydraulic gradient are likely a result
of modifi cations in the drainage cross-sections and in the infl ows. For
example, the gradient increase in the NW area (0.15 to 0.16) corresponds
to a zone with high infl ows. Drainage appears generally concentrated
into three axes, the two westernmost of which are deeply entrenched
and lined with superfi cial indicators such as slumps and collapses
(see chap. D 2.3). These axes illustrate the trace of karst collectors,
as otherwise attested to by piezometric monitoring over time, which
has shown their high discharge and recharge capacity (piezometric
amplitudes of a few meters) within a more homogeneous environment
(piezometric amplitudes of a few tens of meters).