From the view point of water supply stability, lakes and reservoirs are expected to be important water resources. Lake Inawashiro is the most important water resource not only for the Aizu districts but also for Koriyama city and the surrounding area. Lake Inawashiro has 104km² of surface area, 3, 859, 000, 000m³ of volume and is classified as an oligotrophicated but acid-eutrophicated lake. The water quality of Lake Inawashiro has been at very high level. We considerthat the water quality of lake Inawashiro should be effectedby the Nagase river.
Water quality was investigated at fifteen points of the Sukawa river and Nagase river. The pHvalue of the water is about 2-5 because of sulfuric acid flow out from out abandoned mind. As ferric, aluminium and phosphorus concentration decrease with flow down travel, it had been concludedthat the river had been undergoing self purification concerning to phosphorus. Sulfuric acidwater, which flows into Lake Inawashiro from the Nagase River has possibility of dissolving ferricand aluminium which have the ability of the flocculating and sedimenting of phosphorus.
Thus, in the Nagase river, phosphorus concentration which causes eutrophication is very low, comparing to the other rivers which flow into Lake Inawashiro.