68
Samples Nos. 1, 2, 3 and 4 show that practically the whole volume of water in the
lake is full of contamination washed. off the land, and that up to this point, the only sign
that any purification has commenced is the rapid disappearance of oxytocus perni-
ciosus and organism No. 69. During these three weeks from October 28th to Novem-
- b e r 2 3 r d a v e r y l a r g e q u a n t i t y o f r a i n f e l l , a b o u t 3 0 i n c h e s , a n d f o r a c o n s i d e r a b l e
portion of this period the sky was very overcast. Further, up till nearly the end. of
November the supply channels were bringing in a very large volume of highly polluted
water washed off "paddy fields" ; analysis 4 (a) shows that even the surface sample
contained a considerable amount of silt. It should be observed that coli communis was
only once isolated in this period, in spite of the fact that a constant. stream of this
bacterium was coming in. Note the great number of varieties present, and the truly
fœcal nature of the organisms shown in each analysis.
Sample No. 5 taken on November 29th shows that the action of sunlight has
already commenced ; for the surface layers of the lake are considerably purer than
the bottom and. middle. This improvement goes on steadily till sample No. 8 ( a),
taken in December, and shows that the surface water is extremely pure. Note that
with the improvement lactis ærogenes becomes common.
On December 5th a heavy downpour brought in to the lake a further rush of
very highly polluted water, though it does not appear to have raised the total number
of lactose fermenting organisms very much. As this new pollution was chiefly shown
by the presence of coli communis, improvement in quality of the water occurred very
rapidly. After the rain on December 5th, note the disappearance of lactis ærogenes
and its preponderance a few weeks later.
The sample taken on January 6th is very unusual. In this case the surface
water is distinctly inferior in quality to that in the lower depths. The reason for this
is, that when the sample was taken heavy rain was falling, probably a layer of warm
highly polluted surface water was lying on the surface of the lake, and thorough
mixing between the purer water and this contaminated layer had not yet taken place.
The sample taken a week later shows that mixing of the whole volume of water had
occurred, for the lake has risen nearly half a foot, and surface, middle, and lower
layers are equally contaminated.
Samples 11 a, b, c, taken on the 28th January, are quite unexplainable and
probably there is an error somewhere.
As has already been pointed out the samples taken week by week do not show a
progressive purification. There is a considerable variation particularly between samples
15 a, b, c, and 16 and 17 a, b, c. The explanation of this irregularity is due to an
in-rush of irrigation water from paddy fields down one or other of the inlet channels.
In spite of this the surface water remains always very pure.
In the table showing these analyses there is one point, which will certainly
come as a surprise to anybody perusing this report, namely, that in a fair proportion
of the samples taken, the number of water organisms and fœcal bacilli is greater
in the middle layers than it is either on the surface or at the bottom. The number of
instances in which this occurs is too great to be explained by the supposition that some
error occurred either in taking or analysing the sample. These were taken, in every
instance but one, by the same man, and from as nearly as possible the same spot in the
lake, a considerable distance (600-700 yards) from the edge, all usual precautions
being observed. Further examination of this point is necessary before it can be
accepted as constant, or before an explanation is sought for.
One point is amply demonstrated in these results, is, the fact that there is an
enormous preponderance of bacillus cloacœ in all the samples taken from the bottom of
the lake. A glance at the table shows this is beyond doubt. Hence it would appear,
that this organism is more resistant than others to whatever influences are at work in
this situation.
The rarity of bacillus coli communis on the whole is also remarkable. Only
on two occasions were they found and in only one were they present in any consider-
able quantity (vide samples 6 a, 6, c, taken on December 7th). They were probably
brought into the lake by a downpour of rain, which occurred in the very early
part of December. Samples taken from some of the inflow channels, about this time,