Characteristics of the River Water Quality Under Base Flow Condition in the Tokachi River Basin, Japan

Abstract

The Tokachi River basin has an important role as a food base of Japan. It is one of the significant challenges that achieve a good balance between a food production and water quality conservation for the sustainable agriculture in this basin. Here, we carried out the river water monitoring in the Tokachi River basin and evaluated the river water quality under base flow condition. 37 sampling points in the main stream and the tributaries were monitored in late June, late August or early September, and late October 2007 to 2011. Five-year mean values and standard deviation of pH, BOD, SS and EC were evaluated. The water quality of the Tokachi River showed variations at each investigation period. However, there were no change trends of seasons or years. The mean pH values of the Tokachi River basin were the range of 7.1-7.5. The river water quality was neutrality and stable at each sampling point. The mean BOD showed comparatively low values in the Tokachi River basin (0.9-1.8 mg/L). However, the BOD values increased gradually with the main stream flow. The mean SS values were less than 25 mg/L at all sampling points. Also, the SS values tended to increase basically with the main stream flow. The mean EC values increased from upstream to downstream of the main stream (6.8-12.4 mg/L). Also, the EC values in 13 of 20 sampling points of the tributaries were higher than the main stream values (5.0-22.2 mg/L). There were significant correlations between the EC values and the proportion of the agricultural land or forest land. From these results, it was considered that the dissolved matter in the river water increased with a high proportion of agricultural land in the Tokachi River basin.