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ON THE AMOUNT OF DISSOLVED OXYGEN CONTAINED IN

WATERS OF PONDS AND RESERVOIRS AT

DIFFERENT DEPTHS.

The interesting changes in the character of the water of deep ponds, consequent on the stagnation and circulation of the water at different seasons of the year, was discussed in the special report on the Examination of Water Supplies, 1890 (pages 749-767). Investigations of the temperature and composition of the water from different depths in deep ponds showed the presence, in the warmer months, of a stagnant layer of water below a depth of twenty feet from the surface. If the surface of the pond is undisturbed by winds it may reach even to a greater height, say ten feet from the surface. This stagnant layer begins to form in the early part of April, and mingles with the surface water when the circulation of the water takes place in the autumn. This mingling begins in the early part of October and is completed some time in November.

It was stated in the account of this special investigation of deep ponds that the composition of this stagnant water depended upon the amount and character of the impurities in the water, and on the character of the bottom of the pond, whether or not it contained much decomposable organic matter. Thus of seven deep ponds or reservoirs examined it was found that the amount of free ammonia, which may be taken as a measure of the amount of decomposition going on in the stagnant layer, varied from 0.4720 parts per 100,000 in Jamaica Pond, Boston, to 0.0008 parts in the bottom of Reservoir No. 4 of the Boston water supply in Ashland. That the results in these two cases are not accidental is proved by repeated examinations of the deep layers in both of these bodies of water, extending over several years.

The freedom from products of decomposition in the latter case, namely, Reservoir No. 4, is due to the fact that the water is derived from an unpolluted water-shed; that the organic matter which it

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.†

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.

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* 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.

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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.

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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 per cent.

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The sample from the bottom had a strong odor of sulphuretted hydrogen; that from the depth of thirty feet, a faint odor. samples were taken near the gate-house.

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