Evaluation of River Water Quality in Agricultural Watershed with the Environmental Standards

Abstract

Japan has general environmental standards for organic matter in its public water bodies. However, there are no environmental standards for nutrient salts such as phosphorus (P) and nitrogen (N) in river water. In this study, the water quality of rivers at normal water level was evaluated in agricultural watersheds by focusing on water quality standards. The investigated sites are in the Tokachi area (24 watersheds; upland and dairy farming) and the Nemuro area (11 watersheds; large-scale dairy farming), which are in Eastern Hokkaido. The investigations were carried out in the two agricultural areas in 2005, 2006 and 2012. Electrical conductivity (EC) and water temperature were measured at the same time as river water samples were collected in the two areas. Water quality was analyzed for potential of hydrogen (pH), biochemical oxygen demand (BOD), suspended solids (SS), total phosphorus (T-P), total nitrogen (T-N) and other values. Most of the values associated with organic matter, such as pH, BOD and SS, in river water were within the thresholds for the standards of The Environmental Standards Concerning the Conservation of the Living Environment (Rivers). In contrast, some values of nutrient salts (N and P) exceeded the thresholds (T-P≦0.1 mg/L, T-N≦1.0 mg/L). Many of the watersheds in the Tokachi area had T-N concentration in excess of 1.0 mg/L. This suggests that the concentrations of nutrient salts (P and N) increase with increases in agricultural land development in a watershed. For the evaluation of river water quality in agricultural watersheds, it was found to be necessary to measure both organic matter and nutrient salts as indices of water quality. And it was found that controlling the runoff of P and N from agricultural land is essential for water quality conservation in public water bodies.