1. Ground water represents percolation of about 15% of the precipitation
falling to the earth, accounts for .61% of the world's water, and is second
only to glaciers as a source of freshwater.
2. Porosity is the percentage of a rock's volume comprised of openings and
it measures the rock's ability to hold water. Most rocks hold some water in
either pores or joints, but porosity is highly variable. Permeability measures
the capacity of a rock to transmit fluids. Many porous rocks are permeable,
but shale has high porosity and low permeability.
3. Water percolates into the earth as far as porosity exists and saturates
the lower portions of the porous intervals (saturated zone). The upper surface
of the saturated zone is the water table. Above the water table, porosity
is filled by air and water, forming an unsaturated zone (= vadose zone). Water
is drawn by capillary action from the saturated zone into the vadose zone.
Perched water tables result from local variations in permeability, such as
shale lenses in sandstones.
4. Ground water movement reflects Darcy's Law (velocity = permeability x
hydraulic gradient; explained in Box 17.1). Water table slope influences ground
water velocity: steeper slopes have faster movement.
5. Aquifers are porous and permeable rock bodies through which ground water
moves easily. Aquitards have low permeability and/or porosity that prevent
ground water movement. Unconfined aquifers have water tables and exhibit rapid
ground water movement. Confined aquifers lack water tables and exhibit slow
ground water movement.
6. Wells must penetrate a saturated zone to produce ground water. Water tables
fluctuate with season. If the water table falls below the bottom of a well,
it is dry. Drawdown from pumping wells produces local lowering of the water
table. Recharge raises the water table. A cone of depression may form around
a well because of drawdown. In artesian wells, water rises above its confined
aquifer because the ground water is under pressure because of elevation differences.
7. Springs form where the water table intersects the surface, or structures
bring water to the surface. The surface of gaining streams is the water table,
while losing streams lie above the water table.
8. Rain can leach surface contaminants and move them into ground water. Human
activity produces potential pollution from pesticides, herbicides, fertilizers,
heavy metals and toxic compounds, bacteria, viruses and parasites from animal,
plant and human waste, acid mine drainage, and radioactive waste (both low
level and high level). Gasoline may float on the water table. Some pollutants
are naturally occurring. Some filtration and purification can be expected
through ground water flow, if it is slow. Heavily pumped wells near coasts
can be contaminated by saltwater intrusion.
9. Dropping water tables create problems with supply and subsidence, through
compaction. Artificial recharge may offset these problems.
10. Natural ground water is slightly acidic because of dissolved carbon dioxide
from the atmosphere or soil gases. Its contact with calcite in limestone causes
solution forming caves, sinkholes and karst topography. Calcium and bicarbonate
in solution can be precipitated as calcite in the form of stalactites, stalagmites,
columns and flowstone.
11. Ground water may also form petrified wood, concretions, geodes (bodies
in Figure 17.26 are amygdules), cement sedimentary rocks, and develop alkali
soils.
12. Hot springs have ground water warmer than the human body. Heating of
the water is by either proximity to a magma chamber, or through the geothermal
gradient. Geysers erupt periodically because of constrictions in conduits
to the surface allow the temperature of the ground water to rise to vapor,
which then condenses as the eruption proceeds. Hot ground water produces deposits
of sinter (silica) or travertine (calcite). Geothermal energy is derived from
hot ground water through the production of electricity from natural steam.
It is also utilized for heating, paper manufacturing, ore processing, food
preparation and other non-electric uses
