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243
of flood, to raise large quantities of water that has passed through
the natural beds of gravel and sand, which are characteristic of
the neighbourhood. "The gravel beds in the Thames Valley"—
we are told by the Water Examiner—" form in some localities a
valuable source of supply, the importance of which is being
recognized; and which, moreover, is probably capable of being
far more largely utilized while the integrity of the ground
in respect of pollution is assured"; for, of course, with the
growth of population, and its nearer approach to such sources of
supply, the risk of sewage-pollution increases; such risk, arising
in a greater or less degree from the manuring of surface lands.
"This being the case, there exists always the possibility of the
introduction of specific matter of a noxious character."
Filtration, as already stated, was effective in freeing the water
from all traces of turbidity. Turbid water is very difficult to
filter. The solid impurities in suspension, consisting chiefly of
clay, marl, and chalk, in a very finely divided state, can be got
rid of only by long subsidence previous to filtration. Turbid
water must of necessity be sometimes admitted, and the filters
are then liable to be over-taxed.
The fact that the impurity of the water in the river is so
largely increased during periods of flood, renders it of great
importance that there should be sufficient provision of subsidence
and storage reservoirs, to enable the necessary daily supplies to
be delivered during such periods, while keeping the intakes closed.
The position of the several Companies varies in this respect very
considerably. The following statement shows the number of
days' supply, represented by the capacity of the reservoirs for
unfiltered water, belonging to each of the local Companies:—
Chelsea, 14.1; West Middlesex, 7.4; Grand Junction 3.5. But
the Grand Junction Company's resource, during floods, of pumping
from the gravel beds adjoining the Thames, is practically equivalent
to an addition to their provision of storage reservoirs. For sake
of comparision it may be added that the East London Company