Origin of particulate organic matter in a river with remarkable water pollution in Shikoku Island, Japan

This study on the origin of particulate organic matter (POM) in a river under efficient water use for irrigation in a basin was carried out on the Shin River in Kagawa Prefecture, Japan. The organic pollution in the basin where water resources are scarce was studied. The level of POM in the Shin River was significantly influenced by the natural and artificial hydrological environment in the river basin. The degree of organic pollution in the Shin River was high, and most of the organic matter was algae. The δ 18 O of water of the middle and lower reaches of the Shin River was high, and water subjected to evaporation for a lengthy time period in the basin flows into the river. The concentration of organic matter in the Shin increases along with the increase in δ 18 O. Algae also increases in the stay waters in the basin of the middle–lower reaches and flows into the Shin. It was thought that the irrigation ponds in the basin were the main sources of water and organic matter for the river. This tendency was notable during the nonirrigation season in that the volume of water in the basin decreased. On the other hand, the proportion of algae in organic matter in the river decreased when there was considerable precipitation.