CHAPTER IV. WATER AND WATER-SUPPLY. Physical Properties of Water.-Pure water is a colorless, odorless, and tasteless liquid, of neutral reaction, and is taken as the type of all liquids, as air is the type of all gases. Chemical Composition.-Pure water consists of 2 parts by weight of hydrogen and 16 parts by weight of oxygen, having a molecular weight of 18. Two volumes of hydrogen combine with 1 volume of oxygen to form 2 volumes of water gas, having a density of 9. The chemical formula for water is H2O. The percentage composition of water is hydrogen, 11.11; oxygen, 88.89. water. Chemically pure water does not exist in nature, but is made in the laboratory by mixing the required amounts of hydrogen and oxygen gas and then passing an electric current through the mixture. The gases unite and form From the hygienic standpoint, water as found in nature is either "pure" or "impure." Hygienically pure water is one which does not contain any foreign matter which is injurious to health. Impure water is one that is unfit for domestic use. Water as it exists in nature contains a great variety of substances derived from the air through which it has fallen as rain or snow, and from the soil over and through which it has passed. The nature and quantity of the mineral salts dissolved out of the soil by water are dependent upon the chemical composition of the soil. The nature and amount of organic matter contained in the water are dependent largely upon the nature of the soil-covering over which the water has passed. Water as it occurs in nature may be divided into rainwater, spring-, river-, lake-, and sea-water. Each of these natural waters varies somewhat according to the locality from which it is derived, though in a general way all of these natural waters possess characteristics which are common to all the waters of that particular class. Rain-water.-If rain-water were collected in a chemically clean vessel at the moment when it was condensed, it would presumably be chemically pure, but in falling through the atmosphere it takes up some of the impurities in the air. Rain-water is one of the purest of the natural waters, but it varies in purity with the nature of the atmosphere through which it has fallen. It is always. purer at the end than at the beginning of a shower. It contains dissolved gases derived from the atmosphere— on an average 25 cubic centimeters per liter, of which about 64 per cent. is nitrogen, 34 per cent. oxygen, and 2 per cent. carbon dioxid. The relatively large amount of carbon dioxid, in comparison with the proportion contained in atmospheric air, is due to its large absorption coefficient. Ammonia is also commonly present. The average amount of solid matter in rain-water is 39.5 parts per 1,000,000. Sodium chlorid is the most abundant salt, while nitric acid and nitrates, sulphuric acid and sulphates, and a little organic matter are also commonly present. Spring-water. That portion of the rain-water which penetrates the ground exercises a powerful chemical action on the substances present in the soil and underlying rocks. This action consists of solution, hydration, oxidation, etc. Rain-water is an oxidizing agent on account of the considerable proportion of dissolved oxygen that it contains. Springs may be divided into two classes: Common springs, yielding fresh, potable water; and mineral springs, yielding mineral, thermal, or medicinal waters, in which the dissolved mineral matters render them unfit for ordinary domestic use, though of great value for therapeutic purposes. Ordinary spring-water usually contains the gases of the atmosphere in solution. It also contains various mineral salts in solution, such as calcium carbonate and sulphate, magnesium carbonate and chlorid, sodium chlorid, alkaline sulphates and nitrates, and silicates. The amount of organic matter is usually small, and the content in free and albuminoid ammonia is low. The temperature of spring-water is usually lower than that of the surrounding air. Well-water, if derived from a deep well, is similar in character to spring-water; but if derived from a shallow well it is contaminated with surface washings. Springwater is usually soft, while some well-waters are moderately hard because of the presence of calcium and magnesium salts in the rocks of the locality. Spring- and well-waters are usually not rich in bacteria unless specially polluted. River-water.-The course of a great river may be divided into three portions-the mountain track, the valley track, and the plain track; and the composition of the water varies considerably in these three portions of its course. In the first part it is comparatively pure and partakes of the nature of spring-water; in the second and third parts it is usually more or less polluted, depending upon the density of the population along its course. The composition of river-water is complex, as in the case of spring-water, as the water of rivers is largely derived from springs. The proportion of organic matter and of free and albuminoid ammonia is usually higher than in spring-water; and if polluted with sewage the proportion of chlorin is also considerably higher. The character of the water varies greatly with the amount of rainfall and with the nature of the soil-covering of the valley and plain tracks. River-water is usually rich in bacteria, the number and variety of species varying greatly with the season of the year and the amount of sewage pollution. Lake-water.-Lake-water is of variable composition, the water of salt lakes being loaded with mineral constituents, while that of fresh lakes is usually of great purity. Fresh-water lakes act as settling basins for the inflowing water. The sudden diminution in the velocity of the current causes the subsidence of suspended matters, while oxidation of organic matters takes place from exposure of so large a surface to the atmosphere, and from the action of microscopic plants and bacteria. Sea-water.-Sea-water is appreciably alkaline from the presence of carbonates. The proportion of solids in solution is about 3.5 per cent.; chlorin being the chief constituent, while sodium, calcium, and magnesium are next in amounts. It also contains considerable amounts of atmospheric gases, even at great depths, the average amount being from 2 to 3 per cent. by volume. Impurities in Water.-By the term impurities is meant such substances as are directly injurious to health, or that from their association are indicative of pollution though in themselves they may be harmless. The impurities in water may be either in suspension or solution, and they may be either gaseous or solid, organic or inorganic. Many of the inorganic constituents of water are injurious only when present in considerable amounts—as, for instance, the salts of calcium and magnesium. These, when present in large amounts, render the water hard and therefore unsuited for domestic use, aside from the fact that they are productive of disordered function of the gastro-intestinal apparatus. Under certain conditions the salts of magnesium are also believed to produce goiter. The amount of sodium chlorid commonly found in natural waters is not objectionable, because it is much smaller than the amounts constantly used in seasoning food. Sodium chlorid is, however, a most important indication of the pollution of surface-waters with sewage, since sewage is rich in chlorin derived from urine. In determining the significance of the amount of chlorin found in any water, it is necessary to know the normal chlorin content of the surface-waters of the locality, since the amount of chlorin normally present in surface-waters varies greatly. The amount is influenced by the proximity to the ocean or other bodies of saline water, by the proximity to natural deposits of salt, and by the geologic formation of the locality. The chlorin content of surface-waters of the natural gas and oil regions is especially high, and this fact must be borne in mind in determining the significance of chlorin present in water from such a locality. The amount of nitrates and nitrites commonly found in surface-waters is without influence upon health. These substances are, however, of great interest and importance as indications of the length of time that has elapsed since the water has been polluted and the extent of the pollution. The organic impurities in water are of two kinds, dead organic matter of vegetable and animal origin, and living organisms. The amount of dead organic matter commonly found in surface-waters is without effect upon health. It serves, however, as a most important indicator of the extent and character of pollution. The organic matter present in water is usually divided into the nitrogenous organic matter and the oxidizable organic matter. The nitrogenous organic matter usually represents animal organic matter, and is estimated in the form of free and albuminoid ammonia, though it is not always of animal origin, as certain vegetable compounds also yield ammonia on distillation, and, therefore, are nitrogenous in character. The oxidizable organic matter is usually of vegetable origin, and is determined by its bleaching effect upon a solution of potassium permanganate. Neither the nitrogenous organic matter nor the oxidizable organic matter is, as a rule, directly injurious to health, and these also are of importance principally as indicators of the nature and extent of pollution. In making the estimates of the amounts of these various organic and inorganic impurities in water it is |