pH fluctuation of Sawano-ike Pond in Kyoto accompanied by water level changes

Abstract

The supply and consumption of alkalinity in Sawano-ike Pond was investigated to reveal the mechanisms of pH fluctuation in the pond. Alkalinity in the bottom layer at the deepest point of the pond increased in summer, but in-pond generation of alkalinity was estimated to be very small because of the low depth of water in the pond. According to the calculations of water budget and solute balance, subsurface water, the alkalinity of which is moderately high, accounted for 45% of the total influx during a period of relatively high water level. The predicted concentrations of major ions and alkalinity in the pond calculated from the mass balance coincided well with measured concentrations. The relatively high pH values of the pond water during a period of high water level are considered to be mainly caused by alkalinity supplied by influxes, especially subsurface water. During a period of relatively low water level, predicted alkalinity exceeded measured alkalinity by 29%, indicating the occurrence of in-pond consumption of alkalinity with a resultant decrease in pH.