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contains is derived mainly from swamps and is very slightly disposed to decomposition, and also to the fact that all soil and vegetable matter was carefully removed from the bed of the reservoir before it was filled with water. On the other hand, Jamaica Pond is in a populated region and the bottom contains much readily decomposable organic matter.
In connection with this investigation of the composition of water at different depths it was of interest to determine the amount of dissolved oxygen in the water in the different layers. This determination is best made on the spot, since there is danger of the samples absorbing oxygen from the atmosphere during the transportation to the laboratory.
During the summer of 1891, a considerable number of determinations of the dissolved oxygen were made at different depths, in the waters of many ponds and reservoirs.* The water was pumped up, under proper precautions to prevent access of air, into bottles of known capacity and the chemical reagents added immediately.t
Following are the results of some of the determinations thus obtained. The amount of oxygen present is in all cases expressed as percentages of the amount required to saturate the water at the temperature when collected.
DISSOLVED OXYGEN AT DIFFERENT DEPTHS IN JAMAICA POND, Boston.
The collection and analysis of these samples were made by Dr. A. H. Gill of the Massachusetts Institute of Technology.
+ L. W. Winkler's process (described in the volume on Purification of Sewage and Water, 1890) was used and it is to be recommended for its convenience, simplicity and accuracy. Winkler's original article appeared in the Berichte, vol. 21, p. 2843.
In all cases in Jamaica Pond the layers of water which contain no dissolved oxygen have an offensive odor from carburetted and sulphuretted hydrogen. The above series also well illustrates the increase in the thickness of the oxygen-free, stagnant layer as the summer advances.
Glen Lewis Pond, Lynn, is an artificial reservoir, from which the surface soil was not removed. It was first flooded in the latter part of 1889. The water has always contained a large number of animal and vegetable organisms which give it, at most seasons of the year, a very disagreeable appearance and odor. The fact that the oxygen was proved to be absent ten feet from the surface in this pond may be due to the fact that the reservoir is comparatively small (thirtysix acres) and well sheltered from the wind by hills on both sides. It may also be true that the decomposition of the organic matter in this pond is particularly rapid.
DISSOLVED OXYGEN IN GLEN LEWIS POND, LYNN.
June 26, 1891.
The odor of the water at ten feet below the surface was disagreeable, and that at thirteen feet very offensive. It is interesting to note in this case the abrupt transition, within a distance of three feet, from a water fully saturated with oxygen to one in which the oxygen has been entirely exhausted. This is doubtless owing to the fact that the upper seven feet of water were in circulation through the effect of the wind, and thus exposed to the atmosphere at the surface; because we must assume that at this high temperature decomposition must also be active in the upper layers.
Walden Pond, Lynn, is of the same general character as Glen Lewis Pond. Its depth where samples were taken (June 26, 1891) was ten feet.
At this depth the oxygen was only 12.84 per cent. of that required for saturation, and at the surface there was 83.1
DISSOLVED OXYGEN IN Scott RESERVOIR, FITCHBURG.
June 29, 1891.
The sample from the bottom had a strong odor of sulphuretted hydrogen ; that from the depth of thirty feet, a faint odor. The samples were taken near the gate-house.
DISSOLVED OXYGEN IN LYNDE BROOK RESERVOIR, WORCESTER.
July 15, 1891.
The samples at thirty and thirty-five feet had a strong odor of sulphuretted hydrogen.
The odor of the samples which contained no oxygen was not offensive, simply earthy.
DISSOLVED OXYGEN IN RESERVOIR NO. 3, Boston WATER Works.
August 20, 1891.
DISSOLVED OXYGEN IN RESERVOIR No. 4, BOSTON WATER WORKS.
The contrast in the condition of the water in these two reservoirs is very striking. Reservoir No. 3, in which the oxygen is exhausted at a depth of fourteen feet, receives a not inconsiderable amount of direct pollution from the towns of Marlborough and Southborough, while the drainage area of Reservoir No. 4, as has been already said, is very sparsely populated.
In connection with the determination of dissolved oxygen in the Ludlow reservoir of Springfield (which contains at all seasons of the year an excessively large number of organisms in suspension in the water) an attempt was made to ascertain whether the amount of dissolved oxygen decreased in the water in its passage through the pipes. It was found that the amount of oxygen was somewhat less after a passage of six miles in the pipes than in the reservoir, and still less after three miles additional passage in the pipes. But the differences are not very great, and it is possible that the loss of oxygen may in part be due to other causes than the decomposition of organic matter.
The reservoir is only ten or thirteen feet deep at the points where the samples were taken, and the comparatively large amounts of oxygen
found in the bottom layers of the water indicate, probably, that the water in the reservoir was in circulation by means of the wind.