The Exploitation of Aquifers - Hydrogeology: Objectives, Methods, Applications - page 184

2.4 Water drilling

Boreholes (also commonly termed wells) are characterised by their small
diameter (under 1 m and usually between 0.2 and 0.5 m), in comparison
with their depth, which can reach several hundreds of meters, but rarely
extends further than 1,000 meters (with the exception of geothermal and
oil drilling projects).
Boreholes therefore can provide access to all underlying aquifers, both
confi ned and unconfi ned. They can tap very specifi c horizons by using
perforated tubing, and isolating the undesirable horizons with fi lled tubing
and cement. Similarly, a ring of cement is put in place around their upper
portions in order to prevent contamination by surface water (Figure 98).

3 MAINTENANCE OF CATCHMENT SYSTEMS

Catchment systems often present a decreased productivity over time. This
degradation is caused by deposits cloggin waterways and/or by corrosion
of the draining elements of a borehole. It depends on the construction
conditions of the catchment system, the chemical composition of the water,
and the mode of exploitation of the catchment.

￿ In the case of spring diversions and drainage galleries, decreased
discharge is usually caused by clogging, which can have various
origins:

- mechanical, when fi ne soil particles carried by the water settle out

and reduce the cross-sectional area of the drainage channels or the
empty volume of the drainage material;

- physico-chemical, when the obstruction is caused by deposits

precipitated from the water, such as calcium or magnesium
carbonates or sulfates, or even iron hydroxides;

- biochemical, when the clogging is caused by gelatinous ooze

produced by the biologic cycling of certain microorganisms,
particularly when the water contains iron salts (iron bacteria).

Noticeable improvements are generally possible through regular
maintenance, when the catchment systems are accessible. If they are in large
part made up of drained masses without access points, intervention options
are limited. It is therefore essential that maintenance be considered during
the design and construction of a catchment system. Similarly, it is advisable
to avoid planting on a catchment area, and to regularly remove shrubs or
trees that may appear, as their root systems are capable of extending all the
way down to the drainage channels and to the aquifer, where they can also
facilitate the precipitation of chemical deposits.