Seasonal Variation of Water Qualities in Lake Shinsei in Relation to the Release of Iron and Manganese from Bottom Sediments

Abstract

The release of iron and manganese in hypolimnion water from bottom sediments was observed in Lake Shinsei, Kanagawa Pref., in 1973. Its mechanism and the conditions of the releasing process were investigated by using of laboratory experiments. The results were summarized as follows :
1) The hypolimnion water of Lake Shinsei has been stable under conditions of water temperature 8°12°C, pH 6.77.0, and DO 0.00mg/l in the period ranging from the end of May to the beginning of November. In the period, total iron increased rapidly from the beginning of July and manganese from the beginning of September toward the beginning of November. These concentrations attain to the maximum, total iron 18mg/l, and manganese 6.9mg/l, respectively, in the beginning of November. Accompanying with these increases, the nutrients, conductivity, and total alkalinity simultaneously increased to the maximum at the same time, COD also increased from 3mg/l in May to the maximum, 910mg/l in the beginning of November. Redox potential conversely decreased from October to the beginning of November to the level of 0.100.15V accompanying with the slight increase between the end of May and the beginning of August. The condition which caused the release of iron and manganese was considered to be the fact that the bottom sediments had become anaerobic and eutrophic.
2) Laboratory experiments under anaerobic condition suggested that the high concentrations of these elements emerged in combination with the formation of chelated substances of the elements with tannin and the decrease of redox potential caused by the microbiological reduction process.
3) The increase of total iron and redox potential in the hypolimnion water from July to August suggests that the release of iron from the mud has been affected by the chelation of iron with an organic substance such as tannin which derives from decayed wood on the bottom. The abrupt increase of manganese from September to the beginning of November supposes the influence of some reductive substances released from the mud, since the decline of redox potential and the increase of COD were observed in the same period.
4) In company with the disappearance of thermocline in the beginning of December, the concentration of iron and manganese in the hypolimnion water decreased rapidly to less than 0.1mg/l. These phenomena would be caused by accelerating of oxidation of the hypolimnion water due to the increase of dissolved oxygen and the dilution with epilimnion water in that period of overturn.