Aso-kai or Asonoumi is an inland sea called kaisekiko surrounded by Miyazu City and Yosano-cho Yosano-gun Kyoto Prefecture. It is formed by Amanohashidate, which partitions it from Miyazu Bay of the Japan Sea. In the closed-section of the bay the Noda-gawa River (Kyoto Prefecture) flows out, and the sea water of Miyazu Bay flows in and out only through a narrow aqueduct.
Although the scenery of Amanohashidate is entrancingly beautiful, the quality of water in the Aso-kai and Miyazu Bay across the Amanohashidate has been increasingly polluted due to the delay of sewage work in Noda-gawa River system, which flows into Aso-kai. The mean value of fiscal year 2005 was 2 -3 mg/l in Aso-kai and 1 -1.5 mg/l in Miyazu Bay, both of which tend to increase year by year. The fiscal year 2005 investigation showed that the value of chemical oxygen demand did not meet the requirement. Since it is a lagoon, eutrophication and organic contamination are the most serious problem in the sea area of Wakasa Bay, Kyoto Prefecture where waste water standards for nitrogen and phosphorus are regulated.