Absolutely pure water is a rarity and a chemical curiosity. It can be made by burning pure hydrogen in an atmosphere of pure oxygen. Water, being the greatest of all solvents, immediately dissolves a little of everything it contacts. Water is a tough, resilient substance which never wears out, but it does become contaminated.
In theory, water can be re-used endlessly. However, there is considerable public concern about, and a growing interest in water supplies – in the amount, future sources, and methods of treatment. Public health authorities, industrial firms, commercial firms, hospitals and institutions, farmers and homemakers all have special requirements in terms of water quality and availability. Major problems result when these fundamental needs are not met.
The difficulty in obtaining sufficient water does not stem from the lack of water in general. Over 97% of the world’s water is in its oceans, but sea water is not usable for most purposes without extensive treatment. The difficulty, then, is due to the fact that the remaining 3%, which is fresh water, is not evenly distributed and often unaccessible.
Two-thirds of the world’s fresh water is tied up in the polar ice caps and glaciers. The remaining one-third is in ground waters, surface water, the atmosphere, and soil moisture. The only two sources of water supply available to man are ground waters – wells and springs, and surface water – lakes, rivers, streams, and reservoirs.
In providing the right water for any demand, one must consider both a precise analysis of the raw water supply, and the end use of the water. Analysis of a water can show the existence and amount of dissolved minerals and gases, sediment, color and organic matter, taste and odor, and/or microorganisms. Whether any of these impurities could be harmful depends on (a) the nature and amount of the impurities, (b) the tolerance permissible for each of these impurities, and (c) to what use the water will be put.
A given quality of water that is unacceptable for one function may prove satisfactory for another. For example, exceptionally hard water that may be objectionable for laundering and bathing may be satisfactory for sprinkling the lawn. Both the quality of a raw water and its end use must always be determined before it can be treated economically.
It is almost impossible to find a source of water that will meet basic requirements for a public water supply without requiring some form of treatment. Generally, these requirements are that it be free of disease-producing organisms, that it be colorless, clear, odorless and good tasting, that it be non-corrosive and free of objectionable gases and staining minerals, and that it be plentiful and low in cost.
The source of any water supply determines the kinds and amounts of its impurities. There are tremendous variations in the quality of water from area to area. In some cases there are variations in quality even on a day to day basis.
The Earth’s water cycle, or hydrological cycle, is the continuous circulation of moisture and water on our planet. Radiation from the sun evaporates water from the ocean into the atmosphere. The water vapor rises, then collects to form clouds. Under certain conditions, the cloud moisture condenses and falls back to earth. Precipitation that falls upon land areas is the source of essentially all our fresh water supply. Approximately 70% of precipitation is returned to the atmosphere by evaporation and transpiration.
The 30%, of precipitation which is not quickly evaporated either seeps deep into the soil or finds its way into lakes and rivers and eventually flows into the oceans. Surface topography and vegetation, porosity of the soil, the degree of its saturation at the time of a rainfall, and atmospheric conditions are all factors that help to determine the distribution of water after precipitation.
When suspended in the atmosphere, water vapor approximates distilled water. It’s free from impurities and remains so as long as it stays aloft. When water vapor condenses sufficiently to fall to earth, it comes into contact with atmospheric dust, which may contain minute particles of silica, oxides of iron and other materials, pollen, and some microorganisms. It also absorbs amounts of atmospheric gases in the surrounding air – nitrogen, oxygen, and carbon dioxide. Water dissolves and collects the carbon dioxide to produce carbonic acid. Water’s solvent action which permits it to have a cleansing action on the atmosphere continues after it reaches the earth.
Normally when this water reaches the earth, it is slightly acid, corrosive and relatively soft. Once on 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 any other impurities on or below the surface.
Surface water 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 swamp land, 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. Surface waters are relatively low in mineral content, but possess a high degree of contamination and are unsafe to use for human consumption unless properly treated.
Water must travel through various strata before becoming groundwater. Below the surface it moves first through the subsoil, the intermediate layer, the capillary fringe, and finally into the groundwater bed. As water seeps into the soil and passes through a limestone stratum, the carbonic acid content, which increases the solvent power of water, becomes neutralized. During this process, however, the water dissolves a certain amount of the mineral matter of the soil or rock with which it comes in contact. Groundwater supplies are usually higher in mineral content than surface waters in the same area. The nature and kinds of impurities vary widely indifferent sections of the country, as the rocks and sand in the soil consist of many kinds of minerals and chemical substances.
Usually, shallow wells will not contain as high an amount of hardness and other dissolved materials as deep wells. Shallow wells may, at times, be turbid and there is the – danger that water may become contaminated with human and animal wastes. Groundwater from deep wells typically contains high concentrations of dissolved minerals. It’s usually clear and colorless due to filtration through rock and sand, but may contain various types of pollution, including detergents and industrial wastes, which can travel a considerable distance in water.
Springs provide another source of groundwater. It’s a popular belief that spring waters are clear, sparkling, and absolutely pure. In fact, spring waters contain rather large amounts of mineral matter and often show a marked degree of turbidity. As for potability, no spring water should be considered safe to drink unless it is given periodic bacterial examinations.
A water supply is the product of its environment. The amount of vegetation and presence of limestone and other minerals directly affect a water supply’s hardness and corrosiveness. The sources of water in any given area can serve as a guide to the nature and amounts of impurities which may occur. To further understand why water from different sources varies in quality, it is helpful to know something about basic water chemistry.