The drainage from these is into the lower fields, so that a well driven into the high ground not far from the house is presumably safe.] The Danger from Outbuildings.

This danger is further increased by the fact that for the same reason--the vital need of plenty of water for all living creatures--the hen coop, the pig pen, the cow stable, and the horse barn are all likely to be built clustering around this same well.
If the fertilizer from these places is, as it should be in all intelligent farming, protected from the rain so as not to have all its strength washed out of it, and removed and spread on the soil at frequent intervals, the well may even yet escape contamination; but the chances are very strongly against it.

If you will figure out that a well drains the surface soil in every direction for a distance from ten to thirty times its own depth, and that the average well is about twenty-five feet deep, you can readily see what a risk of contaminating the well is caused by every barn, outhouse, or pen within from sixty to a hundred and fifty yards from its mouth.
Every well from which drinking water is taken should be at least fifty, and better, a hundred and fifty, yards away from any stable, outhouse, or barn; or set well up-hill from it, so that all drainage runs away from its basin.

This, of course, is possible only in the country, or in villages or small towns, where houses have plenty of ground about them.
Consequently, the health laws of most cities and states forbid the use of shallow wells for drinking purposes in cities of over 10,000 population.
Causes which Produce Pure Well Water.

Occasionally a well will be driven through a layer of rock or hard water-proof clay, before the water-bearing layer of soil, or sand, is struck, so that its water will be drawn, not from the rain that falls on the surface of the ground immediately about it, but from that which has fallen somewhere at a considerable distance and filtered down through the soil.