The Exploitation of Aquifers - Hydrogeology: Objectives, Methods, Applications - page 186

￿ In the case of wells and boreholes, the same processes apply, and can
be aggravated by pumping conditions, particularly the transport of fi ne
particles into the zone of pronounced drawdown, and the formation
of precipitates generated by this depression.
Wells, due to the open drainage channels in their draining section, are
subject to the risk of silting up in their lower portion or in their enveloping
gravel fi lter.
The sensitivity of boreholes is also due to the corrosion risk for their
metallic portions (support tubing, backfl ow tubing, pumping mechanism),
when the water passing through is chemically or bacterially aggressive, or
due to electrolytic reactions if there are multiple components.
The only means of preventing or attenuating such effects consists of
taking them into account during the construction and exploitation of the
catchment system: choice of equipment (construction material, isolating
joints, cathodic protection or surface coating of tubing), choice of perforation
type and gravel sheath, prolonged development phase prior to exploitation,
adaptation of the pumping rates.
Reclamation procedures are numerous, but of varying effectiveness:
mechanical scraping of surfaces, drainage channels, and perforated tubing,
alternating pumping or injection of compressed air or pressurized water,
injection of selected chemical substances (hydrochloric or muriatic acid,
sulfamic acid, polyphosphates, chlorine).
4 EXPLOITATION OF COASTAL AQUIFERS
4.1 General cases
4.1.1 Study of the saltwater intrusion
Study of the aquifer should permit the identifi cation of sectors in which
contamination is possible; a structural and paleogeographic analysis is
therefore essential. Electric surveying, which, on the one hand, can help
clarify the geometry of the aquifer (depth to bedrock, permeability), on
the other hand highlight the interface, characterized by an abrupt drop
in resistivity, and such surveys are a well-adapted tool. For example, in a
sandy aquifer, the resistivity for freshwater is on the order of 100 to 200 Ω·m,
whereas it drops to 2 to 3 Ω·m once salt water is reached (Figure 99).
Since contamination occurs from downslope to upslope and from
bottom to top, aquifers must be equipped with sensors so as to enable the
3D dynamic monitoring of the intrusion, with a network of piezometers with