Characterizing Aquifers - Hydrogeology: Objectives, Methods, Applications - page 125

wells reveal themselves inadequate for measuring the piezometric level,
due to the clutter caused by their equipment (cables and piping), which
prevents the passage of a probe. It is also possible for other excavations
reaching the water table, whether natural (karst cavities) or artifi cial (mining
wells), can be used for this purpose. Finally, it is important in such an
analysis to account for the springs indicating preferential points of contact
with, or overfl ow of the water table. When such control points are too few
or even nonexistant, it becomes necessary to create specifi c access points,
called piezometers, which consist of small-diameter boreholes, equipped
with perforated casing in the saturated zone. The creation of piezometers
in deep aquifers, especially in the presence of confi ned aquifers, requires
particular precautions in order to adequately isolate possible overlying
aquifers and to prevent mixing between aquifers.
The measurement of the water level is taken from the surface, with
a piezometric probe. It can be automated with the emplacement of a
piezograph (following the surface of the water) or a pressure probe
(following the water column above a fi xed point). In the case of artesian
aquifers, the piezometer head must be perfectly waterproofed and equipped
with a pressure gauge to measure the hydraulic head in the piezometer.
4.2 Piezometric maps
Piezometric studies require very precise knowledge of the elevation of
the measurement points (wells, boreholes, piezometers, springs), which
guarantees precision when creating a piezometric map. This map is drawn
by interpolating between the measured elevations, based on isohypsal
contours (lines of equal altitude of the piezometric surface), the quality
and spacing of which will depend on the density of data points and the
scale of the study.
The piezometric map of an aquifer allows an instantaneous view of its
state at a particular moment. It must therefore be established over a very
short time period, in order to be representative, over the entire study area,
of identical conditions in terms of local infl uences and peripheral events
(stream discharge and rainfall, in particular).
The piezometric surface can be interpreted, in the same way as a
topographic surface, by its morphology, its slope, its detailed variations
and its anomalies.
The tracing of contour lines allows the defi nition, by orthogonal
lines following the greatest slope, of the piezometric surface's fl ow lines,
highlighting the aquifer's fl ow patterns.