Deep lakes furnish a better supply and clearer water than shallow ones. The solid matter brought into the lake by the brooks or rivers which feed it does not remain long in suspension, but soon settles at the bottom, and in this way some lakes acquire the wonderfully clear water and the beautiful bluish-green color for which they are far famed.
Strong Winds Dangerous on Lakes
Strong winds or currents at times stir up the mud from the bottom; hence, in locating the intake, the direction of the prevailing winds should be considered, if practicable. The suction pipe should always be placed in deep water, at a depth of at least fifteen to twenty feet, for here the water is purer and always cooler.
Settlements on the shores of a lake imply danger of sewage contamination, but the larger the lake, the less is the danger of a marked or serious pollution, if the houses are scattered and few.
Pools and stagnant ponds are not to be recommended as a source of supply. In artificially made lakes there is sometimes danger of vegetable pollution, and trouble with growth of algæ. The bottom of such lakes should always be cleared from all dead vegetation.
Surface water may be obtained from brooks flowing through uninhabited upland or from mountain streams. Such water is very pure and limpid, particularly where the stream in its downward course tumbles over rocks or forms waterfalls. But, even then, the watershed of the stream should be guarded to prevent subsequent contamination. Larger creeks or rivers are not desirable as a source of supply, for settlements of human habitations, hamlets, villages, and even towns are apt to be located on the banks of the river, which is quite generally used—wrong as it is—as an outlet for the liquid wastes of the community, thus becoming in time grossly polluted. Down-stream neighbors are sure to suffer from a pollution of the stream, which the law should prevent.
The Water of Springs
The water of springs is subterranean, or ground water, which for geological reasons has found a natural outlet on the surface. We distinguish two kinds of springs, namely, land or surface springs, and deep springs. The former furnish water which originally fell as rain upon a permeable stratum of sand or gravel, underlaid by an impervious one of either clay or rock. Such water soaks away underground until it meets some obstacle causing it to crop out on the surface. Such spring water is not under pressure and therefore cannot again rise. Water from deep springs is rain water fallen on the surface of a porous stratum on a high level, and which passes under an impermeable stratum, and thus, being under pressure, rises again where an opening is encountered in the impervious stratum; these latter springs are really artesian in character.
Deep-spring water is less apt to be polluted than water from surface or land springs, for it has a chance in its flow through the veins of the earth to become filtered. Land springs always require careful watching, particularly in inhabited regions, to prevent surface contamination.