The Cause of the pH Rise Downstream in Small Irrigation Streams in Toyama Prefecture

Abstract

In most small irrigation streams in Toyama prefecture, the pH values are above 7.5 which is the upper limit of for the quality standards of irrigation water. In order to clarify the cause of this phenomenon, an investigation of the water quality in an irrigation stream was carried out. In the stream, the pH rose downstream and varied diurnally to become higher in the daytime. When the water from the stream had been left in a tank for 5 hours, the pH of the water decreased from 7.7 to 7.4. The result shows that the high pH in the stream was due to the decrease in the content of carbon dioxide in the water. The highest value in the pH of the stream was observed near noon, and the lowest value, lower than RpH, was observed at night. In addition, the pH diurnal variation increased with the increasing quantity of attached algae in the stream. It was concluded from our findings that the pH diurnal variation in the irrigation stream in Toyama prefecture was due to a change in the photosynthetic activities of attached algae. Key Words: irrigation water, pH, attached algae, photosynthesis, carbon dioxide