Environmental Engineering Reference
In-Depth Information
hold enough water to support billions of people for
centuries. The quality of water in these aquifers may
also be much higher than the quality of the water in
most rivers and lakes.
Two major concerns arise regarding tapping these
mostly one-time deposits of water.
First,
little is known
about the geological and ecological impacts of pump-
ing water from deep aquifers.
Second,
no international
water treaties exist governing rights to, and ownership
of, water found under several different countries.
Without such treaties, legal and physical conflicts
could ensue over who has the right to tap into and use
these valuable resources.
Major
irrigation
well
Well contaminated
with saltwater
Water
table
S
a
l
t
w
a
t
e
r
S
ea le
vel
Fresh
groundwater
aquifer
S
e
a
f
l
o
o
r
Interface
Interface
Normal
interface
Saltwater
intrusion
Figure 11-15
Natural capital degradation:
saltwater intrusion
along a coastal region. When the water table is lowered, the
normal interface (dashed line) between fresh and saline
groundwater moves inland, making groundwater drinking sup-
plies unusable.
Find out where in the United States aquifers are being
depleted, polluted, and contaminated by saltwater at
Environmental ScienceNow.
Science and Economics: Desalination
Removing salt from seawater will probably not be
done widely because of high costs and questions
about what to do with the resulting salt.
Desalination
involves removing dissolved salts from
ocean water or from brackish (slightly salty) water in
aquifers or lakes. It represents another way to increase
supplies of fresh water.
One method for desalinating water is
distillation
—
heating salt water until it evaporates, leaves behind
salts in solid form, and condenses as fresh water. An-
other method is
reverse osmosis
—pumping salt water at
high pressure through a thin membrane with pores
that allow water molecules, but not most dissolved
salts, to pass through. In effect, high pressure is used
to push fresh water out of salt water.
Today about 13,500 desalination plants operate in
120 countries, especially in the arid, desert nations of
the Middle East, North Africa, the Caribbean, and the
Mediterranean. These plants meet less than 0.3% of the
world's water needs.
There are two major problems with the widespread
use of desalination. One is the high cost, because it
takes a lot of energy to desalinate water. Currently,
desalinating water costs two to three times as much as
the conventional purification of fresh water, although
recent advances in reverse osmosis have reduced the
energy costs somewhat.
The second problem is that desalination produces
large quantities of briny wastewater that contain lots
of salt and other minerals. Dumping concentrated
brine into a nearby ocean increases the salinity of the
ocean water, threatening food resources and aquatic
life in the vicinity. Dumping it on land could contami-
nate groundwater and surface water.
Solutions
Groundwater Depletion
Prevention
Control
Raise price of water
to discourage waste
Waste less water
Subsidize water
conservation
Ban new wells in
aquifers near
surface waters
Tax water pumped
from wells near
surface waters
Buy and retire
groundwater
withdrawal rights
in critical areas
Do not grow
water-intensive
crops in dry areas
Set and enforce
minimum stream
flow levels
Reduce birth
rates
Figure 11-16
Solutions:
ways to prevent or slow groundwater
depletion.
Critical thinking: which two of these solutions do you
believe are the most important?
With global water shortages looming, scientists
are evaluating deep aquifers—found at depths of 0.8
kilometer (0.5 mile) or more—as future water sources.
Preliminary results suggest that some of these aquifers
