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The source of any water supply determines the kinds and amounts of its impurities. Groundwater obtained from deep wells usually contains high concentrations of dissolved minerals. This water is usually clear and colorless due to its filtration through rock and sand. It also may contain various types of pollution, including detergents and industrial wastes. It is now known that such forms of pollution may travel quite some distance in water. Shallow wells provide water with varying amounts of mineral impurities. There is also the danger that water from such sources may become contaminated with human and animal wastes.
Surface waters contain many impurities, silt, sand and clay, which give them a muddy or cloudy appearance. If run-off to them passes over agricultural land, it may also absorb chemical wastes and toxic refuse from animals.
Where water flows sluggishly through swampland, it may acquire objectionable taste, odor and plant color. During periods of flooding, these swamps may discharge their decayed vegetation, color and microorganisms into moving streams and rivers.
Deep wells and large lakes alone provide water that is more or less consistent from season to season. Smaller bodies of water, shallow wells and springs often reflect seasonal-even daily variations in their mineral content.
To understand why water from different sources varies in quality, it is necessary to know something about basic water chemistry.
When suspended in the atmosphere, water vapor approximates distilled water. It is free from impurities and remains thus as long as it stays aloft. When water vapor condenses sufficiently to fall to earth, it comes into contact with gases in the surrounding air-carbon dioxide, nitrogen and oxygen. Atmospheric dust may also contain minute particles of silica, oxides of iron and other materials together with dust, pollen and some microorganisms.
In falling, moisture absorbs amounts of the atmospheric gases because these are partially soluble in water. The colder the water, the more of the surrounding gaseous content it dissolves.
If we chemically diagram the action of water as it dissolves some of the carbon dioxide in the air, it would look this way:
H2O + C2 --> H2CO3
Water dissolves and collects Carbon Dioxide to produce Carbonic Acid
Normally when such water reaches the earth, it is slightly acid, corrosive and relatively soft (though not as soft as man can make it through his skill in the treatment of water). After water reaches the ground, it may pick up additional amounts of carbon dioxide from decaying vegetable matter. Equipped with this booster action it acquires even greater potential for dissolving minerals and other impurities on or below the surface. Water at the surface is slightly acid. If, however, it has the opportunity to seep into the soil and pass through a limestone stratum, the acid condition due to the carbon dioxide will be neutralized. At the same time, the water will get a large amount of mineral content. Chemically this can be diagrammed:
H2CO3 + CaCO3 --> Ca(HC03)2
Carbonic Acid reacts with Insoluble Calcium Carbonate to produce Soluble Calcium Bicarbonate
Limestone, a common rock formation, contains varying portions of both calcium and magnesium carbonates. These are the unseen hardness minerals which plague so many supplies. The basic reaction shown in the above diagram holds true for both minerals. Iron and manganese are found in water supplies less frequently. But again, their basic chemical reaction in water is quite similar. Because it is a solvent, water also picks up the soluble chlorides, sulfates and nitrates of calcium and magnesium. Similarly, it absorbs the carbonate, bicarbonate, chloride, sulfate and nitrate compounds of sodium as well as quantities of silica. Close scrutiny of a water supply after exposure to many common gases and minerals will give a good idea of the active solvent that water can be.