Volume 3

A study of the pollution and natural purification of the Ohio river / Prepared by direction of the surgeon general.

  • United States. Public Health Service
Date:
1924-
Catalogue details

Licence: Public Domain Mark

Credit: A study of the pollution and natural purification of the Ohio river / Prepared by direction of the surgeon general. Source: Wellcome Collection.

    18/94 (page 10)
    sl ae the oxygen content below its saturation value; but as soon as thi depression occurs absorption of oxygen from the atmosphere fol lows. The sources of available oxygen in a stream are therefor mainly two—the oxygen initially dissolved in the water, with it maximum possible equilibrium value at the saturation point, and th oxygen absorbed from the atmosphere by partially deoxygenate water in the process of physical reaeration.”° Reaeration is a facto of so great importance in the biochemical oxidation of a stream tha it merits discussion in some detail. | | In a quiescent, unsaturated body of water the surface film, i immediate contact with the atmosphere, rapidly becomes saturate with oxygen, and can dissolve no more of the gas until that whicel has already passed into solution at the surface has been carried t the deeper strata by diffusion or “streaming,” or by both of thes agencies comkined. A more or less well defined concentratio. gradient is thus formed, with approximate saturation at the surfac and decreasing concentrations at increasing depths, the draft on th surface film being balanced by replenishment of the supply throug! solution from the atmosphere. Thus the rate at which a give small volume of water becomes reoxygenated is a function of it saturation deficit and its distance from the surface. In a flowing stream conditions of quiescence are, of course, modi fied greatly by the constant vertical mixing or overturning whic! the water receives as it flows along the channel. An approach t quiescence may be found in occasional pools during periods of slug gish flow, but even under these circumstances slight currents an eddies, due to movements of the water and to slight differences temperature, bring about a more or less continuous mixing of th stream throughout its cross section. The concentration gradien characteristic of quiescent waters is therefore subject to constan disturbance, but since a comparatively small proportion of the wate reaches the exact surface layer, the forces distributing the dissolve oxygen throughout the body of the stream are those of mechanica mixing and diffusion combined. Viewed as a diffusion phenomenor this distributing action may be conceived as being one in which th effect of mixing is simply to greatly decrease the effective dept. through which diffusion acts, the amount of contraction in the effec tive depth being a function of the turbulence of the stream and de ee ES ee eee UE Ore! Sayer. 10 A third possible source of available oxygen is the activity of certain aquatic plant notably the green and blue-green. algae, which occasionally find in sluggish pools a favo able environment for their propagation. This, however, is a special case which is ne ordinarily an important factor in the reaeration of flowing streams. The term “ streaming” has been applied by Adeney to the downward movement ¢ currents of water from the surface to the lower strata, due largely, as subsequently note by Phelps (Trans. Am. Soc. C. E., vol. 76, p. 1626), to slight differences in density resul ing from surface evaporation. ay