Groundwater is best thought of as a system of many different
parts. Some of these act as conduits and reservoirs, and others
as off-ramps and on-ramps for the groundwater system.
Recharge areas are where water enters the groundwater
system, and discharge areas are where water leaves the
groundwater system. In humid climates, recharge areas
encompass nearly the land’s entire surface (except for streams
and floodplains), whereas in desert climates, recharge areas
consist mostly of the mountains and alluvial fans. Discharge
areas consist mainly of streams and lakes.
The level of the water table changes with different
amounts of precipitation. In humid regions it reflects the
topographic variation, whereas in dry times or places it tends
to flatten out to the level of the streams and lakes. Water
flows faster when the slope is greatest, so groundwater flows
faster during wet times. The fastest rate of groundwater flow
observed in the United States is 800 feet per year (250 m/yr).
Aquifers include any body of permeable rock or regolith
saturated with water through which groundwater moves.
Gravel and sandstone make good aquifers, as do fractured
rock bodies. Clay is so impermeable that it makes bad
aquifers, or even aquicludes that stop the movement of water.
Springs are places where groundwater flows out at the
ground surface. They can form where the ground surface
intersects the water table, or at a vertical or horizontal
change in permeability, such as where water in gravels on a
hillslope overlie a clay unit and the water flows out on the
hill along the gravel/clay boundary.
Most wells fill with water simply because they intersect
the water table. However, the rocks below the surface are not
always homogeneous, which can result in a complex type of
water table know as a perched water table. Perched water
tables result from discontinuous bodies in the subsurface that
create bodies of water at elevations higher than the main
water table.
In many regions, a permeable layer, typically a sandstone,
is confined between two impermeable beds, creating a
confined aquifer. In these systems, water only enters the system
in a small recharge area—if this is in the mountains, then
the aquifer may be under considerable pressure. This is
known as an artesian system. Water that escapes the system
from the fracture or well reflects the pressure difference
between the elevation of the source area and the discharge
area (hydraulic gradient) and rises above the aquifer as an
artesian spring, or artesian well. Some of these wells have
made fountains that have spewed water 200 feet (60 m) high.
One example of an artesian system is in Florida, where
water enters in the recharge area and is released near Miami
about 19,000 years later.
Groundwater Dissolution
Groundwater also reacts chemically with the surrounding
rocks; it may deposit minerals and cement together grains,
causing a reduction in porosity and permeability, or form features
like stalagtites and stalagmites in caves. In other cases,
particularly when acidic water moves through limestone, it
may dissolve the rock, forming caves and underground tunnels.
Where these dissolution cavities intersect the surface of
the Earth, they form sinkholes.
Groundwater Contamination
Natural groundwater is typically rich in dissolved elements
and compounds derived from the soil, regolith, and bedrock
that the water has migrated through. Some of these dissolved
elements and compounds are poisonous, whereas others are
tolerable in small concentrations but harmful in high concentrations.
Groundwater is also increasingly becoming contaminated
by human and industrial waste, and the overuse of
groundwater resources has caused groundwater levels to drop
and has led to other problems, especially along coastlines.
Seawater may move in to replace depleted freshwater, and the
ground surface may subside when the water is removed from
the pore spaces in aquifers.
The U.S. Public Health Service has established limits on
the concentrations of dissolved substances (called total dissolved
solids, or t.d.s.) in natural waters that are used for
domestic and other uses. The table below lists these standards
for the United States. It should be emphasized that many
other countries, particularly those with chronic water shortages,
have much more lenient standards. Sweet water is preferred
for domestic use and has less than 500 milligrams (mg)
of total dissolved solids per liter (l) of water. Fresh and slightly
saline water, with t.d.s. of 1,000–3,000 mg/l, is suitable for
use by livestock and for irrigation. Water with higher concentrations
of t.d.s. is unfit for humans or livestock. Irrigation of
fields using waters with high concentrations of t.d.s. is also
not recommended, as the water will evaporate but leave the
dissolved salts and minerals behind, degrading and eventually
destroying the productivity of the land.
The quality of groundwater can be reduced or considered
contaminated by either a high amount of total dissolved solids
or by the introduction of a specific toxic element. Most of the
total dissolved solids in groundwater are salts that have been
derived from dissolution of the local bedrock or soils derived
from the bedrock. Salts may also seep into groundwater supplies
from the sea along coastlines, particularly if the water is
being pumped out for use. In these cases, seawater often
moves in to replace the depleted freshwater. This process is
known as seawater intrusion, or seawater incursion.
Dissolved salts in groundwater commonly include the
bicarbonate (HCO3) and sulfate (SO4) ions, often attached to
other ions. Dissolved calcium (Ca) and magnesium (Mg) ions
can cause the water to become “hard.” Hard water is defined
as containing more than 120 parts per million dissolved calcium
and magnesium. Hard water makes it difficult to lather
with soap and forms a crusty mineralization that builds up
on faucets and pipes. Adding sodium (Na) in a water softener
can soften hard water, but people with heart problems or
those who are on a low-salt diet should not do this. Hard
water is common in areas where the groundwater has moved
through limestone or dolostone rocks, which contain high
concentrations of Ca- and Mg-rich rocks that are easily dissolved
by groundwater.
Groundwater may have many other contaminants, some
natural and others that are the result of human activity, including
animal and human waste, pesticides, industrial solvents,
petroleum products, and other chemicals. Groundwater contamination,
whether natural or human induced, is a serious
problem because of the importance of the limited water supply.
Pollutants in the groundwater system do not simply wash
away with the next rain, as many dissolved toxins in the surface
water system do. Groundwater pollutants typically have a
residence time (average length of time that they remain in the
system) of hundreds or thousands of years. Many groundwater
systems are capable of cleaning themselves of natural biological
contaminants in a shorter amount of time using bacteria,
but other chemical contaminants have longer residence times.
See also HYDROLOGIC CYCLE.
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