The investigation of Lake Narutaki Dam

Abstract

Lake Narutaki dam is situated in the Kibi Plateau, middle of the smooth hilly regions of Okayama prefecture. This dam has special water quality problems as it supplies city water. These problems are; (1) Coloring is caused by dissolved Mn and Fe from the bottom of the lake where oxygen is almost decreased to zero, (2) A nasty smell and bad taste of city water is originated from vegatative planktons. Therefore, the investigation into the actual condition of this lake was carried out from June to December of 1991. Mn and Fe were always found to be dissolved at the bottom of the lake in high concentration. In the middle of the lake, Fe concentration decreased but Mn had almost the same concentration as that of the bottom. At the surface of the lake, Fe concentration was 0.1-0.3 mg/L while Mn concentration was 0.02-0.4 mg/L. The vertical profile of the vegetative plankton in the water made no great difference until 4 m depth, but it radically decreased in deeper places more than 6 m depth. The following treatment methods are proposed from these data; (1) In order to avoid the influence and transport of Fe and Mn in the filter and subsequent back washing, the income pipe position should be varied in depth according to the season, (2) Longer contact with manganese powder for removing the color and prolonged contact with charcoal for removing nasty smell and bad taste, (3) Introducing advanced treatment for example Ozone treatment for upgrading the taste. With the use of above treatment methods, the problems are thought to alleviate. However, the measure of the origin of pollution is desirable to protect the water quality of this lake, because water is scarece in this region and the only source of water supply is this dam.