Geology Reference
In-Depth Information
use fluoride treatments, such
that the population as a whole
has seen a reduction in cavities
and tooth decay. Despite the
benefi cial effect of fl uoridation
in reducing tooth decay, there
has been an increase in oppo-
sition to fl uoridating munici-
pal water supplies because of
possible health risks.
◗
Figure 13.18
Groundwater Contamination
Septic tank
Drain pipes
High water
table
Zone of aeration
Average water
table
ACTIVITY
Hydrothermal
is a term re-
ferring to hot water. Some
geologists restrict the meaning
to include only water heated
by magma, but here we use it
to refer to any hot subsurface
water and the surface activity
that results from its discharge.
One manifestation of hydro-
thermal activity in areas of
active or recently active volca-
nism is the discharge of gases,
such as steam, at vents known
as
fumeroles
(see Figure 5.2).
Of more immediate concern
here, however, is the ground-
water that rises to the surface
as
hot springs
or
geysers
. It may
be heated by its proximity to
magma or by Earth's geother-
mal gradient because it circu-
lates deeply.
Zone of saturation
a
A septic system slowly releases sewage into the zone of aeration. Oxidation, bacterial degradation,
and fi ltering usually remove impurities before they reach the water table. However, if the rocks are very
permeable or the water table is too close to the septic system, contamination of the groundwater can
result.
Unlined
landfill
1
Water table
3
2
4
b
Unless there is an impermeable barrier between a landfi ll and the water table, pollutants can be
carried into the zone of saturation and contaminate the groundwater supply: (1) Infi ltrating water leaches
contaminates from the landfi ll; (2) the polluted water enters the water table and moves away from the
landfi ll; (3) wells may tap the polluted water and thus contaminate drinking water supplies; and (4) the
polluted water may emerge into streams and other water bodies downslope from the landfi ll.
A
hot spring
(also called a
thermal spring
or
warm spring
)
is any spring in which the water temperature is higher than
37°C, the temperature of the human body (
replaced by sodium (Na
+
) ions through the use of an ion ex-
changer or a mineral sieve. Thus, the amount of calcium and
magnesium is reduced and the water is more desirable for
most domestic purposes. However, people on low-sodium
diets, such as those with hypertension (high blood pressure),
are cautioned not to drink softened water because it contains
more sodium.
Not all dissolved materials in groundwater are undesir-
able, at least in small quantities. Fluoride (F
−
), for instance,
if present in amounts of 1.0 to 1.5 parts per million (ppm),
combines with the calcium phosphate in teeth and makes
them more resistant to decay. However, too much fluo-
ride—more than 4.0 ppm—gives children's teeth a dark,
blotchy appearance.
Fluoride in natural waters is rare, so few communities
benefi t from its presence. However, many cities and towns add
fl uorine to their drinking water, and many dentists routinely
Figure 13.19a).
Some hot springs are much hotter, with temperatures up to
the boiling point in many instances (Figure 13.19b). Another
type of hot spring, called a
mud pot
, results when chemically
altered rocks yield clays that bubble as hot water and steam
rise through them (Figure 13.19c). Of the approximately
1100 known hot springs in the United States, more than
1000 are in the far West, with the others in the Black Hills
of South Dakota, Georgia, the Ouachita region of Arkansas,
and the Appalachian region.
Hot springs are also common in other parts of the world.
One of the most famous is in Bath, England, where shortly after
the Roman conquest of Britain in A.D. 43, numerous bathhouses
and a temple were built around the hot springs (
◗
◗
Figure 13.20).
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