Effect of Water Conveyance to Impove Water Quality in the Barato River

The Barato River, in the northern part of Sapporo, Hokkaido, was deteriorated because of stagnated water bodies and Sapporo's wastewater inflow. To improve the water quality of the Barato River, water has been diverted from the Ishikari River and the Toyohira River into the uppermost reach and the middle stream of the Barato River since 2007. This study clarifies the water quality change by water conveyance, based on our surveys and simulations. The water quality surveys found that inorganic nitrogen (IN) and biological oxygen demand (BOD) were decreased after water conveyance. And inorganic phosphorus (IP) was increased. To estimate these water quality findings, we constructed a water quality simulation model that incorporates the freezing-over of water bodies. The constructed model shows good temporal and spatial reproducibility and enables water quality to be forecast throughout the year, including the ice-cover period. The forecasts using the model agree well with the survey results of the 2007-2010. From calculation results, it was assumed that IN and BOD decreasing was caused by dilution and phytoplankton decreasing. IP increasing assumed due to accumulation of unused phosphorus by phytoplankton. And remarkable changes seem in survey result. Blue-green algae decreased selectively with water conveyance year by year from 2007.However, blue-green algae increased from 2011, in additionally dominant species of blue-green algae change to Merismopedia punctate from Phormidium spp. These change suggest that regime sift occurred in blue-green algae selectively and BOD value of the Barato River showed to improved. But, ecosystem model parameter of phytoplankton needs to calibrate again.