抄録
The judgment of river water quality to the environmental standard is made based on 75% probability of BOD. The method recommended now by Japan Environment Agency is a simple one but is not accurate from statistical viewpoint. In this work, the following four analytical methods were examined with observed BOD in the report for environmental water quality of 1993 in Aichi prefecture : normal distribution, log normal distribution, and log normal distributions for the values subtracted the lower limit (estimated by Iwai method or constant) from observed values. All data conformed well to either normal distribution or log normal distribution types. Although one third of all points were satisfactory to the normal distribution, log normal distribution was assumed to be the most acceptable one because of their largest average correlation coefficient with smallest standard deviation. In comparing Japan Environment Agency method and other methods, it was indicated that the differences between 75% probabilities of BOD estimated by Japan Environmental Agency method and those by statistical analysis were not insignificant.
