Nutrient loading from watershed of Honjo area in Lake Nakaumi was estimated using geographical information systems (GIS) by calculation of each unit loading of point and nonpoint sources. COD, TN and TP loading were estimated at 59.5 t/y, 16.1 t/y and 1.5 t/y, respectively. Forest was the main land use in this watershed, but main loading source was domestic wastewater. The share of the domestic wastewater to total loading was estimated at 31% in COD, 37% in TN and 48% in TP. Sewage treatment plants were not developed in this area, however, if development usual treatment systems for the treatment of domestic wastewater in this area, higher nutrient loading is estimated. Therefore, advanced treatment system is need for development in this area.
The self-purification capacity in Honjo area was estimated by the difference of water quality between Honjo area and Lake Nakaumi and exchanging water volume between them. The average concentrations of COD, TN and TP in Honjo area were lower than those in Lake Nakaumi. According to the collecting data of Shimane prefecture, the differences were 1.1 mg/l in COD, 0.115 mg/l in TN and 0.004mg/l in TP during 1996 to 1998. Removal amounts of COD, TN and TP in Honjo area were estimated at 2,132kg/d, 250kg/d and 11.4kg/d, respectively. These values the almost equal to the loading amounts of 20% in COD, 6.7% in TN and 3.4% in TP from the watershed of Lake Nakaumi.