The Nitrate Nitrogen Concentration in River Water and the Proportion of Cropland in the Tokachi River Watershed

Abstract

It is widely known that nitrate pollution of river water and groundwater in agricultural regions has become severe in Japan and in other agricultural countries around the world. On the other hand, in the Tokachi River Watershed, where large-scale upland field cultivation with dairy farming is the main form of agriculture, few surveys and little research have addressed the impact of agriculture on water quality in the area. This study examined the relationship between the nitrogen concentration in river water at normal water level and the proportion of cropland in the Tokachi River Watershed with taking notice of both maintaining sustainable agriculture and conserving water quality environment. At 37 locations on the Tokachi River main stream and its tributaries (their downstream ends), the NO₃-N concentration in river water and the proportion of cropland in the watersheds were surveyed for three years. Nitrate nitrogen as a percent of total nitrogen in river water ranged from 84% to 91%. The maximum T-N in water from the tributaries was approx. 8.8 mg/L. There was a correlation (significance of 1%) between the NO₃-N concentration in river water and the proportion of cropland in the watersheds. The correlation coefficients ranged from 0.80** to 0.95** in the water from the main stream. It was confirmed that the impact factor, or the slope of the regression curve that represents the relationship between the two, ranged from 0.037 to 0.075, which was greater than the values reported for various other locations in Hokkaido. Future studies on conserving the water environment by reducing the nitrogen loading into the river system while maintaining the cropland in the Tokachi River Watershed in favorable condition will need to explore the feasibility of land use assessment methods based on the new viewpoint.