Spatial Distribution of Water Quality and Load Units in the Iinashi River Basin, Shimane Prefecture, Japan

The spatial distributions of water quality and load units were determined for each small sub basin of the Iinashi River basin as a first step in water environment management. In addition, a GIS database was established to comprehensively resolve the issue of water quality degradation. It was found the concentrations of water quality parameters in the Iinashi River basin were relatively low at almost all sampling points. The average concentrations of each parameter are 1.86 mg·L -1 of chemical oxygen demand, 0.73 mg·L -1 of dissolved organic carbon, 18.5 % of biodegradation ability, 6.58 mg·L -1 of silica, 519.8 μgN·L -1 of total nitrogen, and 27.4 μgP·L -1 of total phospho- rus. In addition, the load units discharged from the small watersheds were obtained. The load units varied greatly regardless of similar land use in the basin. Moreover, discharged loads from observation point 1-31 in the Yamasa River basin were higher than those from observation point 1-5 in the upstream of the Iinashi River basin regardless of high percentage of forest area (91.51%), low percentage of agricultural land (2.3%) and low population density (24 people·km -2 ). Finally, the discharged loads from observation point 1-3 were de- termined as 53.2 g·ha -1 ·day -1 of suspended solid, 45.4 g·ha -1 ·day -1 of chemical oxygen demand, 20.9 g·ha -1 ·day -1 of dissolved organic car- bon, 8.1 g·ha -1 ·day -1 of particulate organic carbon, 16.3 g·ha -1 ·day -1 of total nitrogen, 5.0 g·ha -1 ·day -1 of dissolved organic nitrogen, 1.3 g·ha -1 ·day -1 of particulate organic nitrogen, and 0.3 g·ha -1 ·day -1 of total phosphorus.