2025 Volume 48 Pages 175-182
In this study, we evaluated the characteristics of the days of the week and parts of the day that sampled river water for which environmental quality standards are available for the conservation of the living environment based on the analysis of regular water quality monitoring data collected from FY2013 to FY2022 at environmental reference points in Japanese rivers. We also evaluated days-of-the-week- and parts-of-the-day-specific variations of each water quality parameter. The days-of-the-week sampling of river water concentrated in the middle of the week, particularly on Wednesdays. In addition, most river water samplings were conducted in the morning. There were no apparent days-of-the-week-specific variations in water quality parameters. However, the values for pH, biochemical oxygen demand, suspended solids, dissolved oxygen, and total zinc tended to be on average slightly higher in the afternoon than in the morning on many days of the week. The potential biases resulting from the timing of samplings and parts-of-the-day-specific variations in water quality parameters should be considered when discussing the environmental quality and effluent standards for these parameters.
