Abstract
Oxidation and reduction of iron occurs relatively easily in natural water. Ferrous ion is the dominant form under reduced conditions. Ferric iron is the dominant ionic form under oxidized conditions. Under neutral pH value, ferrous iron is transformed to ferric iron by oxidation, joining with the hydroxide ion in surface water to form reddish-brown ferric hydroxide or ocher. Ocher is insoluble and either settles to the river bed or remains in suspension, leading stream water to turbidity. Such stream water degradation is predominant in the upper basin of the Kabamuta River and is attributable to ferric iron contained in spring water inflows. Thus, the purpose of this paper is to demonstrate the chemical processes of water degradation and its improvement methods from a view point of erosion control engineering.
Ferrous iron concentration, soluble iron concentration, and turbidity were measured from 1995 to 1997 at various sampling points chosen according to geomorphological features of the stream. Altogether, eleven experiments were conducted. Most of the ferrous iron was transformed to ferric iron by oxidation in the first 50m from the spring water inlet point and ferric iron was hydrated instantaneously. The maximum level of water turbidity was observed at a point about 600 m downstream from the inlet point, where is the just before injection of Takachiho River. The maximum level of turbidity decreased as the amount of oxidized ferrous iron became greater.
Since the ferrous iron oxidation rate proved to be the function of residence time and dissolved oxygen concentration of the stream water, it is reasonable to assume that the model based on convection-diffusion equation incorporated these variables in order to predict the value of maximum turbidity. Theoretical values calculated by the model agree well with the actual values, and it is suggested that maximum turbidity is decreased less than one-fourth of the ordinary value when atmospheric conditions accelerate the oxidation of ferrous iron due to long residence time in the first 100m of the stream.
