C£SS POOL 
Deep bored or driven wells are less liable to pollution than shallow 
dug or bored wells, since in the first case the wells are usually im- 
perviously cased and the surface water must filter through a depth 
of soil equal to the depth of the well before gaining access to it, while 
in the second case the wells are usually loosely lined with brick or 
stone, and the surface water, having only a short distance to filter, 
seeps in through the entire depth. 
That both deep and shallow wells are subject to contamination is 
shown from many examinations which have been made. Of 177 deep 
and 411 shallow farm wells examined in Indiana, 1 116 of the deep well 
waters were of good quality, 45 were bad, and 16 were doubtful; 159 
of the shallow well waters were good, 209 were very bad, and 43 were 
doubtful. 
The safety of water supplies when near sources of possible surface 
pollution often depends largely on the character and quality of the 
material in which the 
well is sunk. Surface 
waters " in sinking 
through sandy soils or 
surfaces are filtered, 
and in the finer sands 
much of the polluting 
matter which they 
carry is frequently re- 
moved. In coarser 
sands or gravel the 
degree of filtration is 
less, but water taken 
from sands and gravels at a considerable depth may be considered 
relatively safe. Waters from wells in clay are not often polluted, 
since surface pollution filters through clay very slowly. 
Waters from wells in limestone are frequently polluted, owing to the 
fact that limestone soils usually contain passages and channels at 
different depths which sometimes form a continuous passage for 
underground water for a considerable distance and which are very 
often directly connected with sinks and basins occurring here and 
there on the surface. It is a common practice to dump manure, 
trash, and garbage into such sinks or basins, and rain water falling 
into these plunges directly into the underground channels, carrying 
with it the impurities from the basin to those points where wells are 
sunk. In this manner garbage or refuse dumped anywhere in the 
neighborhood of or even at a considerable distance from a well in 
limestone may pollute the water. Figure 6 shows how the pollution 
of wells and springs may occur in limestone. 
Fig. 5. — Effect of pumping on ground water. 
i Ann. Rpts. Ind. Bd. Health, 27 (1908), p. 345; 29 (1910), p. 349. 
