Depletion of Freshwater Resources
In
many parts of the world people are extracting water from aquifers more quickly
than the aquifers are replenished by recharge. In addition
to draining aquifers, excessive groundwater pumping changes groundwater flow
patterns around wells and can drain nearby rivers and streams. This happens
because pumping changes the natural equilibrium that exists in an undeveloped
aquifer with discharge balancing recharge.
When
pumping starts, groundwater stores are depleted in the vicinity of the well,
creating a cone of depression in the hydraulic head. If a
new water source such as a river or stream is available close by, the well may
capture (draw water from) that source and increase its recharge rate. Until this
inflow matches the pumping rate. If no such source is available and pumping
draws the water table down far enough, it will dry up
the aquifer or deplete it so far that is it not physically possible or affordable
to pump out the last stores of water.
Pumping
quickly lowers the pressure within confined aquifers so that water no longer
rises to the surface
naturally. Fifty years ago artesian aquifers were common, but today they have
become rare because of widespread groundwater withdrawals. In unconfined
aquifers, air fills pores above the water table, so the water table falls much
more slowly than in confined aquifers.
As
aquifers are depleted, water has to be lifted from much greater depths. In some
parts of the world, the energy costs of lifting groundwater
from deep beneath the surface have become prohibitive. Overuse of groundwater
can also reduce the quality of the remaining water if wells draw from contaminated
surface sources or if water tables near the coast drop below sea level, causing
salt water to flow into aquifers.
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