SHOWING VARYING DANGER OF CONTAMINATION OF A SHALLOW WELL, ACCORDING TO LEVEL OF UNDERGROUND WATER AND RELATIVE POSITION OF CESSPOOL AND WELL (_after Galton_).]
=Deep Wells= are made by digging a surface well, as above, except that the ground water is prevented from entering the well by means of impervious steining; and then boring from the bottom down through the subjacent impervious stratum until a water-bearing stratum is reached. The difference between a surface well and a deep well is shown in Fig. 8 by A and B. Where the water in this stratum is retained under pressure, deep wells are known as =Artesian Wells=. Such Artesian wells have been sunk in London. Rain, falling on the chalk hills which lie to the south and north of London, percolates through the chalk downwards, and then laterally, until it lies in the concave London basin. Here the clay stratum above it prevents its escape upwards; and being confined under considerable pressure, it rises to the surface, or into a well in the superficial gravel, when the clay is tapped. In Fig. 8, B is an Artesian well if the pressure is such as to make the water rise through the London clay, when this is cut through and the underlying chalk is reached. C is a well in the chalk, which does not pass through an impervious stratum, and therefore comes within the above definition of a surface well; but as regards depth required to be dug before water is reached it is more like a deep well.
SHOWING DIFFERENCE BETWEEN DEEP AND SUPERFICIAL WELL. _A._—SURFACE WELL IN GRAVEL. _B._—DEEP WELL, GOING THROUGH GRAVEL AND CLAY TO CHALK. _C._—WELL IN CHALK DISTRICT.]
Among the deepest Artesian wells are Grenelle (1,800 feet), and Kissingen (1,878 feet.) The sinking of a deep well and severe pumping of its water may exhaust all the neighbouring wells for two or three miles. There is also danger of contamination from neighbouring cesspools when the upper part of the deep well is not properly constructed. The area exhausted by a deep well undergoing pumping is represented by an inverted cone, having a very wide base, and with a convex inner surface pointing towards the well.
For country places deep-well water is much preferable to water from streams, as streams are very liable to be contaminated by the sewage of houses higher up in their course, or even by that of houses close by. A good well should be at least thirty feet deep—preferably fifty feet and should always be lined with impervious material, except near its bottom. The absolutely water-tight and impervious condition as well as the distance of all drains or cesspools in the vicinity should be ascertained before deciding whether the drinking water from a given well is above suspicion. The direction of flow of the underground water should also be determined. This may be done by measuring the level of all the wells in the neighbourhood. Possible sources of pollution at points from which ground water is flowing towards the well are much more dangerous than those nearer than the well to the river towards which the underground water is flowing (see Fig. 7). Steam pumping greatly increases the area from which contamination may be derived.