Deep wells are those reaching to the deeper water-bearing strata of the earth, thus drawing water from the truly underground sources of ground-water storage. Surface water is usually excluded from such wells by means of water-tight casings or linings extending from above the surface downward to the underground water-bearing stratum that furnishes the supply. Deep wells are driven, bored, or drilled, depending on whether the casings have been driven through soft strata, by means of blows, or bored through clay and similar easily penetrable material by means of augers, or drilled where harder strata are encountered. In some cases all three methods may have been employed on the same well.

Artesian wells are those that flow, owing to underground hydrostatic pressure. This pressure is usually developed by a warped, porous, water-bearing stratum being inclosed between two impervious layers, and receiving its water at a higher elevation than that of the ground surface at the point tapped by the well. Deep, nonflowing wells are frequently but incorrectly termed artesian wells.

Owing to the force of gravity, underground waters, unless impeded, are in continuous motion. Because of various geological formations of impervious materials, the underground flow may be forced to the surface and form a spring. Such springs frequently exist in hilly country with underlying rock formations; they are oftentimes of considerable volume and may thus furnish suitable sources of water supply. Springs frequently form the sources of surface streams or emerge along stream channels, thus becoming a part of the surface supply.

Infiltration galleries are usually tunnels, collecting pipes, or chambers constructed across the underground flow channels in such a manner as to divert the normal course of the underground water to central collecting points from which it may be taken as desired. The most advantageous locations for such works are generally at low points along the base of inclined impervious strata upon which pervious water-bearing strata are superimposed. Infiltration cribs have frequently been placed along stream channels, below the water surface, thereby drawing part of the supply from underground sources and part from the surface supply of the stream. Owing to the uncertainty of the quality of water thus obtained, it is inadvisable to use such a source unless further treatment is contemplated.

Quantity of Water Available.

An important factor in selecting a source of water supply is that of sufficient quantity from the contemplated source. In reaching a proper determination on this question, capable engineering advice should be sought, for varying factors exert a most important rôle in the yield of different watersheds. Moreover, the determination of

the probable maximum demand of water requires careful study. It should be borne in mind, however, that the amount of rainfall is always the maximum limit of available supply. Hence the volume and distribution of rainfall, the size of the catchment area, and the percentage of the rainfall that is actually available are functions of the yield that may be expected from any source. Each of these factors can be further analyzed into its component parts so that an intelligent estimate can be formed of the weight or value of the whole. Thus, in the matter of a surface supply, the percentage of the rainfall actually running off is altered by such conditions as permeability of the top soil, evaporation, transpiration, temperature, and other factors.

In estimating the yield to be expected from underground sources, still other conditions must be given consideration. The nature, extent, and position of the various geological formations largely determine the volume of underground water available in any region as well as the quality of the supply. A considerable knowledge of the geology of a region is therefore required if one is to judge competently the extent of an underground source of water.

In many cases the total amount of surface water available from a given source is ample, but owing to various conditions the rate of run-off may be exceedingly variable and hence at certain seasons be entirely inadequate. To tide over such dry periods, impounding or storage reservoirs may be necessary in order to retain part of the excess flow during the wet season and release it when needed. Many instances of such equalization systems are in operation.

The problem of securing a suitable quantity of water is therefore very frequently an engineering problem that requires considerable study and experience for its proper solution, and no such problem should be undertaken without the assistance of competent engineering advice.

Quality of Water.

In the selection of a source of water supply, quality is a prime requisite, and this fact must be kept clearly in view. If a supply is deficient in quality, it can never reach the degree of usefulness that consumers have a right to demand regardless of the many other points it may have in its favor. There are numerous characteristics that determine the quality of a supply, but in general they may be grouped under three heads: (1) sanitary quality, (2) chemical quality, and (3) physical characteristics.

1. SANITARY QUALITY.

The sanitary quality of a water has to do with its relative freedom from pathogenic organisms or those which cause disease or otherwise affect the health of persons drinking it. The most common disease

organisms transmitted by water are those of typhoid fever, dysentery, and cholera, which diseases, because of this common mode of transmission, are frequently termed water-borne diseases. In some waters there are other forms of organic life present, organisms whose action is not definitely known but which are certainly not beneficial and may even be detrimental to health. To be of satisfactory quality, a drinking water must be practically free of pathogenic organisms, and the standards are being constantly increased to the point where practical freedom from all organic life may soon be required.

For ascertaining the sanitary quality of a water, carefully standardized methods have been developed which permit the making of detailed comparisons; and complete information should be had, in the light of these procedures, in regard to any contemplated source. These methods relate, in brief, to the bacteriological analysis and to the sanitary survey of the watershed. One bacteriological analysis is of comparatively little value, but a series of such analyses, covering a considerable period of time and taken under a variety of conditions, gives a valuable index of the sanitary quality of the water. Such analyses, combined with a carefully made sanitary survey of the watershed with a view to locating and weighing the importance of all existing sources of pollution, will produce valuable evidence from which safe decisions may be made.

It is not the intention to discuss here the details of bacteriological technique of water analysis-water analysis has become a highly developed and well standardized science-but a few remarks on the broad features of its foundation may be of value in explaining its applicability to the question under discussion.

No practicable method has yet been devised to isolate the typhoid organism from water. Moreover, it has been done so rarely that the procedure is of no significance. It is entirely practicable, however, to isolate and enumerate a group of organisms similar in many respects to the typhoid bacillus, and one whose natural habitat is the same as that of the typhoid organism, namely, the intestinal tract of human beings, and hence always present where typhoid organisms are found. This group, called Bacillus coli, is therefore in reality a more sensitive index than the typhoid organism itself; for while the latter would be found only where typhoid bacteria were actually present, the presence of B. coli is indicative of excremental pollution, either animal or human, which may or may not be of a dangerous nature, since the B. coli group itself is nonpathogenic.

The permissible extent of occurrence of B. coli in a water supply has been a much discussed question, and the fixing of any standard of this nature is difficult. The Treasury Department of the Federal Government, responsible for the sanitary quality of drinking water on interstate carriers, has set a bacteriological standard for such

supplies that is admittedly high. Briefly, this standard requires that the total bacteria developing on agar plates in 24 hours at 37 C. be less than 100 per c.c. of the sample and that not more than one of five 10-c.c. portions of a specimen shall contain B. coli when examined by the standard procedure.

Bacteriological examination should not be the sole means of gauging the sanitary quality of a water supply, however, but should be made in connection with a thorough inspection of every part of the area with which the water may come in contact. All possible sources of continuous, intermittent, or accidental pollution should be examined and noted in order to estimate the real or potential influence of each such source on the sanitary quality of the water. The nature and extent of such possible sources of pollution will largely influence the methods to be inaugurated for the protection of the supply. Such surveys should be repeated at frequent intervals in order that detailed information may be complete and up to-date. The sanitary quality of any supply is the most important factor to be considered, because even a temporary or periodic lapse of precaution may exact a toll of human lives from water-borne diseases. Conse quently too much care can not be taken to prevent such occurrences

CHEMICAL QUALITY.

While the chemical impurities found in water are not generall so detrimental to health as bacterial pollution, yet, from an economic standpoint, such impurities are often of such a nature as to demand careful consideration. Water in passing over or through certain mineral deposits, dissolves small amounts of these substances and carries them off in solution. Ground waters, because of the generally longer and more intimate contact with the earth, are, as a rule, more heavily impregnated than surface waters. The mineral content of ground waters is naturally closely related to the geology of the region from which they are obtained. The chemical features most ordinarily encountered are hardness, miscellaneous mineral content, and gases in solution.

Hardness in water is usually caused by the presence of salts of calcium and magnesium in the form of either sulphates or carbonates. These salts render the water objectionable for household purposes and some industrial uses because the calcium and magnesium unite with soap to form insoluble compounds, causing unsightly scums and precipitates, and thus render the soap ineffective. The added cost for soap in a community where hard water is used is greater than generally supposed. Whipple states (1, p. 26) that it requires ! pound of average soap to soften 24 gallons of water having a hardness

Interstate Quarantine Regulations o. the United States Misc. Publication No 7., United States Public Health Service, Washington, D. C., 1916. Page 40

of 200 parts per million; or in other words, before a lather can be produced, a pound of soap is consumed in 24 gallons of such water. Hardness is objectionable for many industrial purposes also, because of the deleterious chemical actions produced. Thus in dye and dyeing industries, sugar refining, textile trades, tanning, paper making, etc., soft water is necessary. Used in boilers, hard water forms a scale or incrustation on the inner surfaces and greatly reduces the efficiency for steaming purposes, because of the low heat conductivity of the scale. Excessive scale even causes the metal tubes to burn out, resulting in the expense of replacement. Wherever possible, therefore, hard water should be avoided as a source of supply. Water frequently contains other mineral substances that are objectionable; iron, manganese, and chlorides being among the number. Iron and manganese form red precipitates and stains and may affect the taste of the water as well. They can generally be removed by treatment processes such as aeration and precipitation. Many well supplies, especially along the salt-water coasts have a high chloride content which, on excessive pumping, tends to become even higher. The abandonment of such supplies is generally the only feasible remedy in such cases.

Waters frequently contain considerable amounts of dissolved gases, mostly carbon dioxide, and sometimes in the case of underground waters, hydrogen sulphide. Carbon dioxide imparts a rather pleasant taste but in considerable amounts, owing to its solvent qualities, may be accompanied by increased quantities of other minerals such as iron or manganese. If present in the free state, carbon dioxide will attack iron pipes and containers and cause serious trouble in distribution systems. Hydrogen sulphide imparts a disagreeable odor and taste. Oxygen and nitrogen are normally present in all surface waters and to a lesser extent in ground waters, and have a beneficial effect, especially the former.

PHYSICAL CHARACTERISTICS.

In addition to the above factors affecting quality, there is another group which, while not directly harmful to health, must yet be considered. Their presence in water may be said to offend the aesthetic senses and in that way divert consumers to the use of even unsafe sources of supply in which these factors are absent. Such factors are color, odor, taste, turbidity, and temperature.

Color in water is usually of organic origin and of a complex chemical composition little understood. Compounds of iron are sometimes responsible for mineral coloring in water. Vegetable color usually originates from water that has been in contact with swampy areas, dead leaves and vegetation, peat, etc., and varies in intensity at dif

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