Monitoring and estimating non-point source pollution on typical sections along the Weihe River

The water quality of the Weiher River is heavily affected by non-point source pollution (NSP).It is there-fore necessary to quantify NSP loading.We monitored both water flow and water quality at two cross-sections along the Guanzhong river reach near the cities of Xianyang and Lintong for two flooding and non-flooding seasons of 2009 and 2010.The measured parameters include chemical oxygen demand (COD),dissolved orthophosphate,total phosphor-us (TP),TP,NH₃-N,NO₂-N,NO₃-N and total nitrogen (TN).NSP loadings are calculated using the modified hydrological division method and the mean concentration method.The latter is proven to be a well-established and widely-used method.The results show that COD,NH₃-N and TN constitute the majority of pollutants in the Guanzhong reach.Their mean concentrations are lower but more variable during flooding seasons compared to that of non-flooding seasons.The concentration is inversely proportional to the scale of the floods in the reach.The NSP sim-ulations using the two methods agree well each other,indicating the applicability of the hydrological division method when data is scarce.The two simulations reveal that NSP accounts for about 20% to 30% of the total pollutant loading at the two cross-sections in 2009 (a dry year with P=68%).Thus,point source pollution (pollutants discharged from the cities of Xianyang and Lintong)is the main contributor to the water pollution problem in the Guanzhong river reach,as it contributes more than 80% of the total loading.However,compared to 2006 other dry year with P=69%,the point source loadings of COD,NH₃-N and TN are actually reduced by 11 937 t,791 t,and 29 t in 2009. This shows the effectiveness in controlling point source of pollution in the Weiher River.In addition,the overall com-position of pollution remains almost unchanged in 2006 and 2009,and NSP gives a substantial contribution to water pollution,which cannot be ignored.