A text-book on hygiene and pediatrics from a chiropractic standpoint

by J. H. Craven

The greatest problem that a city getting its water supply from a lake has to solve is how to keep its sewage from polluting its own water supply. To prevent this, it is necessary to place the intake for the water supply far out in the lake. The danger from pollution has become so great in some places, as in Chicago, that special canals have been constructed to carry the sewage into other channels rather than empty it into the lake.

The impurities from the sewage may travel a great distance into the lakes. Serious sewage pollution was found ten miles out in the lake from the mouth of the Detroit River. Pollution has been found as far as eighteen miles from the shore in some places.

So it is readily observed that, notwithstanding the fact that the lakes and ponds furnish a more desirable public water supply, yet this source is not, by any means, free from objections.

The impounding reservoir, which is an artificial reservoir for the purpose of storing up water, is another very reliable source of public water supply. These reservoirs are often built in the mountains by placing a dam across a ravine or canyon. The largest dam in the world is the New Croton across the Esopus Creek in the Catskill mountains in New York. It is 248 feet high, 185 feet thick at the bottom and eleven feet thick at the top. This impounding reservoir furnishes an addition to the water supply of New York City.

There are many advantages to the impounding reservoirs, but there are also many disadvantages. One advantage is that the area drained is comparatively small and therefore the pollution from that source is lessened; also the storage factor which is experienced in this reservoir is an advantage. In this way many of the so-called pathogenic microörganisms die before they are carried to the consumer. A disadvantage is that they are open to the atmosphere and light and this is conducive to stagnation because the water is still. This results in an increase in the growth of algæ and other microscopic organisms. The stagnation of the water results in an increase of the products of decomposition. This, together with the microörganisms and algæ, produce the foul smell and bad taste of the water.

The stagnation of impounding reservoirs and small lakes may become a vital factor from a hygienic standpoint. If the water is less than twenty feet deep it will be kept in motion by the wind and in this way will not become stagnant, but if it is more than twenty feet deep the lower portion will remain still. This prevents the water from mixing and therefore the under portion will become stagnant while the surface will not. If the water supply is taken near the surface there will not be so much danger from the stagnant water, but even then there are times of the year when there is a complete stirring up of the water. This mixing of the surface water and the bottom water takes place twice a year, in the spring and in the fall. This mixing is the result of the changing of the temperature of the surface water. During the summer the surface water becomes warm and the temperature may reach 80° F. In this way the warm water remains on top but is kept stirred up by the wind. The wind will not usually create a disturbance for more than twenty feet except in very large lakes where it may be as great as forty feet.

As winter approaches the water cools until finally the temperature of the surface water becomes the same or more nearly the same as the bottom water. Then the wind exerts a deeper influence and the surface and deep water will mix. During this process vertical currents may be produced. This mixing continues until all the water has been thoroughly mixed and until the temperature of the surface water goes below the point of maximum density which occurs at 4° C. The cold water accumulates on the surface where ice is often formed. In the spring the process is just reversed. The fall mixing of the water is much more thorough and intense than the spring mixing.

The changes brought by this mixing are obvious. The surface water contains a large amount of oxygen. The bottom water contains much less oxygen but a great quantity of decomposed products. The oxygen is carried to the bottom and there oxidizes and neutralizes some of the products of decomposition.

The sources of impurities in surface water are various. One of the most menacing sources is from the sewage which is carried from the centers of population. In the rivers it is the sewage of the towns and cities situated above. In the lakes it is usually the sewage of the city itself that is likely to pollute its own water supply. Under the present conditions it seems that it is easier to purify the water supply than to purify the sewage or provide some other method for its disposal.

Naturally the water of rivers is purest near the source. The reason for this is that the rivers form a natural drainage for the land through which they flow. The waste products from every process carried on in that territory find their way into the streams as a natural result of the law of gravity. The water will be found to contain large amounts of mineral and organic matter after passing through populated and cultivated areas. Purification of rivers is considered under Water Purification.