Effects of water transfer on water quality and estimation of the pollutant fluxes from different sources into West Lake, Hangzhou City, China

Water transfer from the Qiantang River to the West Lake, Hangzhou City, China, has been performed since 1985 to improve the lake water quality. This study was conducted to assess the water quality and estimate the pollutant fluxes from different sources into the lake to enable better water quality management. Monthly variations in water quality variables, namely chemical oxygen demand (COD), 5-day biochemical oxygen demand (BOD5), total phosphorus (TP), total nitrogen (TN), ammonium nitrogen (NH4+-N), and nitrate nitrogen (NO3−-N), and flow were analyzed for 1-year period from April 2012 to March 2013. The pollutant fluxes into the lake were mainly contributed by the water transferred from the Qiantang River, followed by runoff and rainfall. The results showed that the annual influxes of COD, BOD5, TP, TN, NO3−-N, and NH4+-N into the lake were 1,595.3, 255.8, 8.2, 576.0, 360.3, and 84.2 t, respectively, while the annual fluxes out of the lake were 2,204.6, 301.8, 7.1, 336.5, 191.6, and 36.1 t, respectively. The annual net flux of pollutants in decreasing order was TN > NO3−-N > NH4+-N > TP > BOD5 > COD, and nitrogen was found to be the major pollutant in the West Lake. It is recommended that the water transferred from the Qiantang River should be treated by chemical precipitation as well as denitrification technology to reduce the nitrogen concentration.