The formation potentials of 24 disinfection by-products, six indicators of organic matter and the number of algae were measured in raw water once a month for one year from October 2006 to September 2007 in order to investigate the quality of water in the two water purification plants on the Ogasawara Islands. It was estimated that the main source of organic matter in raw water on the Ogasawara Islands was not autochthonous organic matter but allochthonous organic matter that flows from streams into reservoirs, as determined from the correlations between the parameters of water quality, the relationship between rainfall and water quality, UV absorbance: TOC ratio and other factors. The mean annual formation potentials of disinfection by-products in filtered raw water on Chichijima Island and Hahajima Island were as follows: total haloacetic acid: 286, 194 μg·L-1, total trihalomethane: 227, 190 μg·L-1, total haloacetonitrile: 13 μg·L-1, and chloral hydrate: 13, 9 μg·L-1, respectively. The total trihalomethane formation potentials were at the maximum levels in our country and the formation potentials of other disinfection by-products were also presumed to be their maximum levels. The formation potentials of the brominated disinfection by-products in most months were higher than those of the nonbrominated disinfection by-products. The correlations of total trihalomethane formation potential (mole concentration) with color, TOC, KMnO4 consumption and UV absorbance were lower than those of the formation potentials of total haloacetic acid, total haloacetonitrile and chloral hydrate in raw water. KMnO4 consumption and UV absorbance were effective indicators of disinfection by-product formation potential for these high correlations (P<0.01).
