ENVIRONMENTAL FACTORS OF WATER
THE HYDROLOGIC CYCLE
The sun draws about 70 percent of this daily precipitation
back up into the atmosphere through the process of evaporation almost
immediately-certainly before it seeps into the soil or goes far in the process
of run-off.
As shown in the diagram, the
upturned arrows indicate that the sun causes evaporation of water even while it
is falling.
The sun also draws water from
the soil, from surface run-off, vegetation, streams, lakes and oceans and
through the process of transpiration.
ENVIRONMENTAL
FACTORS
A water
supply is the product of its environment.
The gases carbon dioxide and
oxygen enter the water from the atmosphere. The carbon dioxide can unite with
water to form carbonic acid.
In vegetated areas, oxygen in
water is consumed and carbon dioxide increased through decay of vegetation.
In limestone areas, the water
containing carbonic acid reacts with limestone and becomes hard. Calcium and
magnesium bicarbonates are formed.
In granite or sandy areas, the
water retains as carbonic acid, but does not become hard, due to the absence of
limestone.
In arid regions, oxygen from
the atmosphere is not consumed to any degree, nor is carbon dioxide increased
by decay.
Where sand and granite
predominate, the water will be low in hardness and slightly acid.
In areas where calcium or
magnesium chloride or sulfate are found, the water will become very hard. The
hardness will be chiefly non-carbonate even though limestone is present.
This environmental background has important implications in regard
to the corrosiveness of well or groundwater, as follows:
1. In vegetated areas where limestone and
other hardness minerals are present, the hard water will not be corrosive due
to neutralization of carbonic acid and the virtual absence of dissolved oxygen.
When such water is softened by ion
exchange, the corrosion rate will remain low.
The corrosiveness of both the
hard and softened water will increase if they are aerated.
2. In vegetated areas where granite and sand
predominate, the water will be low in hardness and usually low in total
dissolved solids (conductivity.
It can be corrosive, however, due to the
presence of carbonic acid which can dissolve iron directly.
Such water supplies usually
produce-objectionable "red" (rusty) water but corrosion is usually
uniform rather than of the "pitting" type. Copper or other corrosion
resistant materials will be much more satisfactory than galvanized steel in such
supplies. Neutralization will control corrosion.
3. In arid regions where limestone and
non-carbonate hardness minerals are found, both the hard and softened water
supplies will tend to be corrosive due to their dissolved oxygen content and
conductivity.
4. In arid regions where granite and sand
predominate, water supplies will be low in hardness and conductivity.
They may still be corrosive, however, due
to their dissolved oxygen content, but usually are less corrosive than water
supplies from arid regions which have higher dissolved solids content (as in 3
above).
It is apparent that the
environment can be a guide to the corrosive nature of a water supply.
Note:
Contrary to the prevailing notion that oxygen-depleting reactions in the soil
zone and in the aquifer rapidly reduce the dissolved oxygen content of recharge
water to detection limits, two to eight milligrams per liter of dissolved
oxygen have been found in water from a variety of deep aquifers in Nevada,
Arizona and the hot springs of the Appalachians and Arkansas, Science Magazine
reports.
The prevailing opinion is that
the majority of dissolved oxygen in recharge water is consumed in the soil and
unsaturated zones by microbial respiration and the decomposition of organic
matter, or rapidly thereafter in the aquifer by various mineral-water and
organic oxidated reactions.
USGS researchers document the
widespread presence of dissolved oxygen in significant (two to eight mg/1)
concentrations in water several thousand to more than 10,000 years old from
deep aquifers in both arid and humid climates, and at distances as great as 80
km from recharge areas.
More puzzling is the presence
of dissolved oxygen in those Arkansan and Appalachian hot springs in which
water has passed principally through fractured siliceous rocks.
Perhaps all pertinent reactions
(organic or inorganic) involving dissolved oxygen have gone to completion
within the aquifer prior to entry of the extant groundwater, the USGS
scientists postulate.
RELATED POSTS:
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The Hydrological Cycle or Water Cycle
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