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Technical
Resources >> Water Sources
>> Ground Water
Groundwater constitutes
the largest source of usable, unfrozen fresh water on the planet.
Approximately 21% of the Earth's freshwater is in the form of ground
water. Roughly fifty percent of the U.S. population derives some
portion of their fresh water from ground water. In California, the
majority of the usable water supplies are from ground water sources.
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Ground water is water which is located below the
soil surface and contained in the pore spaces of bedrock, sand, gravel,
and other such materials. Most ground water originates from precipitation
that soaks into the ground. The ground water system as a whole leads
the water in the general direction of the ocean. This can either occur
by movement through the ground or by flowing out and joining streams
or rivers. For the most part, there is a continuous process of exchange
between stream flow surface waters and ground water.
The movement of ground water occurs both vertically and horizontally
in response to gravity and hydraulic pressure. This movement is
through the aquifers which are the well-saturated zones. These zones
consist |
| of a layer of rock or sediment that is permeable to
ground water flow in usable amounts. |
| The main method of ground water movement is through
percolation. This movement is much slower than the of surface water
and is dependent upon the amount of slope in the water table. |
| The two most common means to obtain
supplies of ground water are springs and wells. A spring is the simplest
source of ground water as it occurs naturally. Springs occur in areas
where there is a significant change of permeability in the horizontal
direction.
The change in permeability must occur with the upper sedimentary
material more porous than the lower material. Thus, flow is greater
through the more porous matter and as the flow reaches the less
porous matter, a spring emerges. Although, once this water emerges,
it is considered surface water. |
| Excavating a well to obtain ground
water is a much more complicated process. In order for planners and
developers to determine the location of a well, they must find a reliable
source of ground water. In general, sedimentary rocks have the greatest
porosity. Such rock types include sandstone and limestone. These define
excellent aquifers.
This issue of under-ground water storage is very important when
considering a well. Water is stored in the voids of rock and soil.
In order to determine which areas are in a favorable condition for
drilling, one must consider many different factors. Precipitation
or the source of infiltrated water is probably the most important.
There must be a sufficient amount of water to fill the available
pore spaces. The zone where the water is to infiltrate must have
a high porosity in order to contain an adequate amount of water.
Plus, the area in which the water is to be stored needs sufficient
permeability to allow water to flow into the well and replace the
water that is pumped out of the well.
Ground water may pick up minerals and metals as it passes through
the aquifer, or may become contaminated by surface water run-off
from agricultural, industrial or residential development. |
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