Water Quality Assessment In The Upper Dongjiang River Basin And Non-point Source Nitrogen And Phosphorus Pollution Loads Estimation From Soil Erosion In A Small Basin

The upper Dongjiang River Basin is the main runoff zone and waterconservation district for Dongjiang River Basin. Consequently, its waterquality is of significant importance in the water quality protection of theentire Dongjiang River Basin. The fact that the basin receives plenty ofrainfall and locates in a hilly terrain subjects it to serious nitrogen andphosphorus loss from soil erosion which cannot be overlooked. Therefore,proper estimation of the non-point source nitrogen and phosphoruspollution loads from soil erosion is of crucial theoretical significance forthe pollution prevention and control in Dongjiang River Basin.In this study, the main water pollution factors in the upper DongjiangRiver Basin, nitrogen and phosphorus, were determined by wastedischarge capacity estimation and water quality evaluation. This wasfollowed by collecting soil samples from Yuancheng District, Heyuan Cityand measuring the nitrogen and phosphorus contents. Then, based onArcGIS software and Revised Universal Soil Loss Equation (RUSLE), asoil erosion estimation model for upper Dongjiang River Basin was built.Finally, based on the built soil erosion estimation model and the measuredtotal nitrogen and total phosphorus data of the soil samples, non-pointnitrogen and phosphorus pollution loads in a small basin were then calculated.The main results were as follows:(1) The pollution sources in the study area included industrial wastewater, planting drainage, livestock breeding waste water, as well as sewagedrainage. Heyuan City was the main contributor to industrial pollutionsources, while planting drainage and livestock breeding waste watermainly came from the Dingnan County, Xunwu County and AnyuanCounty.(2) Based on single factor water quality identification index method,generally, the water quality in the upper Dongjiang River Basin was good,while the contamination risk couldn’t beignored, and the risk in thedownstream was higher. Additionally, the main polutants were nitrogenand phosphorus.(3) Nitrogen and phosphorus contents in surface soil samples fordifferent land use were analyzed. The results showed that whereas forestland had the highest average total nitrogen content (2094.40mg/kg),built-up areas had the lowest (824.20mg/kg). In addition, whereas thehighest total phosphorus value was found in grass land (2162.33mg/kg),the lowest was in the forest land (114.62mg/kg).(4) Based on ArcGIS software and RUSLE, the annual soil erosionmodel was built. The results indicated that the average erosion modulus inthe upper Dongjiang River Basin was479.893t/(km2·a), and the annualtotal soil erosion area was15451.57km2, accounting for81.93%of thetotal area of the upper Dongjiang River Basin. The total soil erosion loadwas741.51million tons. The annual nitrogen and phosphorus loads in thesmall basin were19575.77t and10327.38t respectively.