Assessment of long-term changes in nitrogen pollution load potential from sewage treatment water in the Tedori River Alluvial Fan Area, Japan

Long-term changes (1974–2007) in the nitrogen pollution load potential (NPLP) arising from sewage treatment water were assessed in the Tedori River Alluvial Fan Area of Japan. The total NPLP from sewage treatment systems (STS) during the 34 year period was 439 t (10 3 kg) year −1 from about 260,000 users in 1974 increasing to a peak of 793 t year −1 in 1992 from about 363,000 users, and then decreasing to 676 t year −1 from about 400,000 users in 2007. The NPLP outflow into the area increased from 356 t year −1 in 1974 to a peak of 596 t year −1 in 1985 followed by a rapid decrease to 98 t year −1 in 2007. The NPLP outflow from the public STS to the Japan Sea began in 1979 and rapidly increased to 575 t year −1 in 2007 from about 362,000 users. This represents 85.5% of the total NPLP. The NPLP from septic tanks in the area was 356 t year −1 from about 107,000 users in 1974 gradually increasing to a peak of 587 t year −1 from about 177,000 users in 1985 before rapidly decreasing to 60 t year −1 from about 15,000 users in 2007. Although the current NPLP is about 98 t year −1 in the study area, the average NPLP during 34 years was very different at 424 t year −1 . NPLP assessments affecting groundwater and closed water bodies should consider long-term processes of nitrogen pollution from STS over time periods compatible with the residence time of the receiving waters.