Urban ponds are playing an important role not only as a nexus between nature and city residents but also as precious habitat for wild lives in urban settings. It is often the case that nutrient rich groundwater is the only available water source for the ponds. Water quality degradation due to eutrophication has often been reported in such ponds, resulting in excessive bloom of green algae accompanied by unpleasant smell during summer seasons. In order to prevent algae blooms, the nutrient concentrations in the pond water needs to be properly controlled and managed.
We evaluated sources and factors that affect nitrate and phosphate concentrations in Pond Junsai which is an urban park pond situated approximately 10km east of Tokyo in a highly populated area. The purpose is to find an effective way to manage the water quality using nutrient rich groundwater.
Pond Junsai consists of 6 small ponds having total surface area of 2.2x10^4m^2 and watershed area of 3.7x10^5m^2. Air photo analysis indicated that impermeable area in the watershed was 4.4% in 1947 and has increased up to 71.3% in 2013 due to urbanization, resulting disappearance of spring water which used to support the pond. Having no influent stream, the pond has been fed by precipitation and groundwater pumped up from multiple wells since 1979. Currently inflow to the pond consist of 67% groundwater and 33% precipitation. Approximate turn over time is 51.2 days.
The groundwater from multiple wells have various but high either nitrate or phosphate concentrations depending on the depth of the screens. Precipitation, groundwater and waterfowls are main nutrient sources to the pond and account 9%, 72% and 17% of nitrate load, and 6% 57% and 37% of phosphate load, respectively. Annual nitrate load to the pond was estimated to be 266.0kg/year, whereas phosphate load was 126.3kg/year. Budget calculation indicated at least 126.3kg of nitrate is consumed in the pond, whereas 8.3kg of phosphate is produced/released within the pond annually.
Several counter measures to reduce nutrient concentrations were experimented in 2017 and 2018 within a small part of the ponds. Draining and drying a pond reduced the phosphate concentration while it increased the nitrate concentration after rewatering. Air bubbling caused sediment disturbance, which increased phosphate concentration. Clearing the flow passage and shortening the residence time was found to be effective to reduce both nitrate and phosphate concentrations, which lead to success in preventing algae bloom during summer of 2018. An attempt to control N/P ratio which affects algae bloom by changing mixing ratio of groundwater from different depths is currently in progress.