Organic pollution in dammed river water in a low-precipitation region of Japan

Organic pollution in eutrophic river water dammed by estuary weirs was studied in the Shin and Kasuga rivers of the Shikoku region, Japan. Both rivers are located in a low-precipitation region of Japan. The investigation was performed at intervals of 3-14 days from 12 July 2002 to 13 July 2003. The annual averages of chlorophyll a concentration were approximately 150 and 40 mu g L-1 in the Shin and Kasuga rivers, respectively. The concentration often exceeded 500 mu g L-1 in the Shin River in winter. Dissolved oxygen saturation exhibited marked variation. High and low values, i.e., > 150% in the surface water layer and 0% in the bottom water layer, were occasionally observed. Dissolved methane concentrations > 2,000 nM were often observed from spring to fall. In both rivers, the serious organic substance pollution was confirmed through 1 year.