Basic Groundwater Characteristics
Rain water that falls on porous
terrains, such as sand or sandy loam, drains or percolates into the ground. In
these cases, the water encounters a wide variety of minerals arranged in
complex layers, or strata.
The minerals may include granite,
basalt, and shale. In some cases, there may be a layer of very permeable sand
beneath impermeable clay.
Ground water tend to be low in
turbidity, high in dissolved minerals, and low in biological content than
surface water.
In contrast to surface water, ground water
sources are relatively free from suspended contaminants, because they are
filtered as they move through the soil strata. This filtration also removes
most of the biological contaminants.
Some groundwater sources with a high iron
content contain sulfate reducing bacteria. These are a source of fouling and
corrosion in industrial water systems.
Groundwater may contain an undesirable
level of scale forming solids, but due to its fairly consistent chemistry it
may be treated effectively.
As groundwater encounters different
minerals, it dissolves them according to their solubility characteristics. In
some cases chemical reactions occur, enhancing mineral solubility.
A good example is the reaction of
groundwater with limestone. Water percolating from the surface contains
atmospheric gases. One of these gases is carbon dioxide, which forms carbonic
acid when dissolved in water. The decomposition of organic matter beneath the
surface is another source of carbon dioxide.
Limestone is a mixture of calcium and
magnesium carbonate. The mineral, which is basic, is only slightly soluble in
neutral water. The slightly acidic groundwater reacts with basic limestone in a
neutralization reaction that forms a salt and a water of neutralization. The
salt formed by the reaction is a mixture of calcium and magnesium bicarbonate.
Both bicarbonates are quite soluble. This reaction is the source of the most
common deposition and corrosion problems. The calcium and magnesium (hardness)
form scale on heat transfer surfaces if the groundwater is not treated before
use in industrial cooling and boiler systems.
Structurally, limestone is porous. That
is, it contains small holes and channels called "interstices". A
large formation of limestone can hold vast quantities of groundwater in its
structure. Limestone formations that contain these large quantities of water
are called aquifers, a term derived from Latin roots meaning water bearing.
If a well is drilled into a limestone
aquifer, the water can he withdrawn continuously for decades and used for
domestic and industrial applications.
Unfortunately, the water is very hard,
due to the neutralization / dissolution reactions described above. This
necessitates extensive water treatment for most uses.
source: gewater.com
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