The quality of irrigation water in Fukuoka prefecture [Japan]

From 1986 to 1988, chemical properties of irrigation waters in Fukuoka prefecture were examined. The average pH was 7.4 and that of Electric Conductivity (EC) was 178 micro S/cm. Both figures were within the quality standard values set for agricultural water. They were similar to the values taken on the 1st examination conducted during the period 1986 through 1988 and the 2nd examination conducted during the period in 1991 through 1993. Average COD was 6.4 mg/L,exceeding the allowable quality standard value. The values taken at the 2nd examination showed a rapid deterioration over those taken at the 1st examination. In many areas, values higher than 8 mg/L were registed, which could cause stunting in vegetative growth of rice. Average total nitrogen (T-N), which had been already over the standard value in the 1st and 2nd examinations have risen higher. As inorganic nitrogen did not increase during these periods, it was thought that an increase was caused by the increase of Org-N. It was suggested that irrigation water with high COD was caused by the pollution of organic matters. There were 2 types of changes in COD. One type showed high changes at the puddling and young ear formation stage. The other type showed high changes at the puddling and ripening stage. The former is prevalent in irrigation waters in rural areas and lower reaches, while the latter occuurs in irrigation waters near houses and roads which were liable to the influenced by the discharge of waste water.