Nitrogen pollution in the Hangzhou section of the Qiantang River was studied from 2000 to 2001. The researchers analyzed NH3-N, NC2-N, and NO3-N concentrations in different years, seasons, and river sections. They found that NH3-N levels were higher in low flow seasons, exceeding water quality standards. Nitrogen pollution mainly came from sewage and livestock waste, with industrial sources in some areas. NO3-N concentrations increased from 1991 to 2001 due to excessive nitrogen fertilizer use in the river basin.