Harmful impurities in rain water may be avoided by preventing the water entering the cistern before the air and the roofs of buildings have been thoroughly washed, and then keeping the cistern covered, to prevent the entrance of leaves, insects, etc.
Spring water necessarily contains some mineral matter - more in calcareous than in silicious regions - which it dissolved as it passed through the soil, and it may have dissolved some animal matter, if any dead carcass or animal excreta lay on the soil over which it passed. It usually contains some vegetable matter, obtained from dead leaves, though organic matter is usually found in small amount in spring water. Lakes which lie high up among the mountains are usually replenished by water which flows over uncultivated lands. Such lakes frequently have rocky bottoms, and are consequently quite free from contamination if they are remote from human habitations, and have an outlet, that they may be purified by subsidence, and kept fresh by constant change. Lakes which have no outlet contain much mineral matter on account of constant evaporation. Lakes in thickly-settled regions receive much mineral matter and other impurities brought by the streams which flow over cultivated regions. They may also receive garbage, sewage, etc., from cities and towns on their shores.
River water always contains mineral matter, the amount varying under different conditions. A river which has its source on high, uncultivated ground usually has purer water near its source than farther down. Much depends, also, on the formation of the soil through which it flows. It is also different when swollen by heavy and frequent rains than after protracted dryness. River water is very apt to be polluted by decaying animal and vegetable matters, as vegetation on its shores, the bodies of dead animals, as fish, etc. The refuse from factories and the sewage from cities frequently find their way into the near-by river. It sometimes becomes necessary to supply cities with water from a river or lake. In this case, some means of purifying is resorted to, as filtering through beds of sand, etc. Household filters can be had, but unless they are kept scrupulously clean, the water is worse with than without filtering.
The water of wells contains mineral matter dissolved from the soil and rocks through which it passed. Some well waters have much more mineral matter than others; this depends largely on whether the well is situated in a silicious or a calcareous region. Well water is not free from organic impurities, as water carries them long distances through porous soil. Of the three mentioned, artesian wells are least apt to be so contaminated. Shallow wells are the most commonly so polluted. There is more danger from such wells in the village than in the country, for, however cleanly a person may be, he has several neighbors near enough so that a little carelessness on their part may in a few days cause disease germs to enter the water of a well that has been used for years, and never found unwholesome.
Though there is less danger of pollution in the well water at the farm house, it is far from being free from danger. As the well is merely an opening into this underground lake or river, it will be as surely polluted by solid garbage thrown on the ground to be washed by the rains, and carried through the earth into it, as though it were thrown on its visible surface. The piggery, the barn, the henhouse, etc., are fruitful sources of contamination unless they be well removed and on lower ground. No one would think of throwing the kitchen slops into the well, but they sometimes find their way into it when thrown on the surface of the ground.
References: Johnson's Encyclopedia; Drinking Water and Ice Supplies - Prudden.