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.
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.
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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:
- 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.
- 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.
- 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.
- 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 a 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.