Management of Aquifers - Hydrogeology: Objectives, Methods, Applications - page 213

3 ACTIONS ON AQUIFERS
3.1 Quantitative action
These are meant to restore equilibrium in an aquifer or to counter effects
induced by exploitation. An aquifer is, indeed, in a state of equilibrium
defi ned by the intrinsic characteristics of the aquifer system and by the
infl ows and outfl ows of water. This equilibrium oscillates between extremes
linked to climatic variations. Exploitation provokes an additional constraint
which defi nes a new equilibrium. The effects can be as follow:
￿ lowering of the piezometric level;
￿ drying of superfi cial terrain;
￿ reduction in the discharge of peripheral springs;
￿ subsidence settling;
￿ brutal collapses, particularly in gypsic terrain;
￿ saltwater intrusions in coastal zones;
￿ withdrawal of poor quality surface water.
In order to restore a satisfactory level to the aquifer, possible
interventions include assisting infi ltration, artifi cally recharging the aquifer,
or blocking natural outlets in the case of karst systems (see chap. C4).
Due to the effects of withdrawals from a confi ned aquifer below Shanghai,
the city underwent signifi cant settling. The subsidence recorded since 1921
reached 1 cm per year in the years 1956-1959. In 1964, river water injection
projects began in the aquifer. In 1966, a hundred or so injection wells allowed
recharge directly into the confi ned aquifer. This stopped the subsidence, and in
1976 the city had risen back up by 34 mm (Detay, 1997). Nevertheless, due to
its runaway urbanisation, the city of Shanghai is still subsiding today. Mexico
City, built on drained marshland, is experiencing the same phenomenon,
accentuated by earthquakes, which liquefy the soil.
In the United States, 18 major sites are affected by subsidence, and for 6
of these, the decision has been taken to stop pumping of groundwater and
to use surface water instead. Re-injection is, indeed, not always possible.
In addition, the terrain has been disorganized by settling, and a return to
initial conditions is rarely possible.
3.1.1 Aquifer recharge methods
a) Assisted autochthonous infi ltration
One frequently employed method is the construction of small dams along
a river in order to regulate surface water fl ow and to encourage infi ltration