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there is an extremely strong subsoil stream, the water being fairly pure ; and in cases
where there is no sub-soil stream the river itself acts as the supply, the water-
percolating through the sand. into . the pipes placed. below the sand of the bed acting
as a natural filter. When the river bed. is dry it should be carefully protected from
pollution for at least a mile above the gallery.
The advantages of this arrangement are (1) it is cheap, (2) the fact that the
river contains no water for several months in the year or for several years consecutively
does not affect the supply when there is a subsoil stream. (3) There is no necessity
to filter the water as it is usually free from silt.
The difference between a true sub-soil water and a filtered river water can nearly
always be demonstrated by the chemical tests (vide the infiltration gallery in Madura),
the chloride and nitrate figures in the river water ands the gallery water showing con-
siderable difference. In cases like Triehinoply and Tanjore, where a river water is
obtained, the results of the chemical analyses of both river and gallery water are the
same in these particulars.
The disadvantage of this arrangement is that the amount of purification which
goes on in the sand varies considerably with many factors such as the rate of pumping.
It can never be absolutely guaranteed that the coarse sand in the river bed will
filter the water adequately. Experience has shown, that the best water is obtained,
when there is slight trickle in the river ; this washes away the added pollution rapidly,
but keeps the sub-soil supply well filled up. Therefore though this arrangement
possesses many advantages, the quality of the water obtained may be very variable
and at times dangerous.
(2) Supplies derived from lakes, etc.—Supplies derived from these sources are
superior to others, because full advantage is taken of the action of the sun in puri-
fying the water. During or immediately after heavy rain it may be necessary to
filter lake water, to remove the gross contamination, but with a floating outlet it is
possible to obtain from a lake a water that contains no fœcal organisms in 50 cc.
without any difficulty. Houston's recent work of storage and our own work on the
action of sunlight all go to demonstrate that water exposed to the sun in this country
requires little or no other treatment. Consequently, whenever possible, a settling tank
should be attached to river and irrigation canal supplies ; and a lake supply should
always be chosen, in preference to a river, or surface spring, provided the surface
of the catchment area is not very heavily contaminated.
(3) Springs and well waters.—The subject of well water has been discussed
in Chapter VI and very little more need be said. There can be no doubt that the
surface well yields water of most uncertain quality, especially in some porous soils.
We have comparatively few rock springs in India and therefore their importance is
limited.
(4) Artesian wells and deep borings are supplies which have not been properly
investigated in this part of India. In Pondicherry there are some excellent deep
borings mostly about thousand feet. Analyses of this water shows that it is exceed-
ingly pure. In Kumbakónam a boring about three hundred feet deep yielded a
satisfactory but not very large supply. There can be no doubt, that we in India are
distinctly backward in this matter, and that for small places deep borings would yield
a satisfactory, and probably adequate, supply of water.
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