Transboundary nitrogen pollution from continental China to a river that flows into the Sea of Japan

We sampled water from a point in the downstream part of the Hii River every day from July 1, 2010, to June 30, 2011. The Hii River flows from Shimane Prefecture into the Sea of Japan. The total flow of the river was 1.51 × 109 m3 during the sampling period. The mean, minimum, and maximum total nitrogen (TN) concentrations were 0.60 mg L-1, 0.24 mg L-1, and 1.34 mg L-1, respectively. The mean, minimum, and maximum nitrate (NO3-N) concentrations were 0.50 mg L-1, 0.14 mg L-1, and 1.06 mg L-1, respectively. Dissolved inorganic nitrogen (DIN) accounted for most of the TN. More than 95% of the DIN was NO3-N. The NO3-N concentration was higher between December and March than at other times. We compared the results of this study with the results of a survey conducted in 2001-2002, and found that the mean nitrate concentration in the river water had increased from 0.38 mg L-1 to 0.50 mg L-1 in about 10 years. Anthropogenic pollution in the Hii River drainage basin was not found to increase between 2002 and 2011, so we believe that transboundary nitrogen pollution from continental China increased over that period and that it is influencing the nitrogen concentrations in the Hii River.