Estimation and evaluation of non-point source pollutants loads in mountainous area of Sichuan

Abstract: Non-point source (NPS) pollution produced from agricultural production activity is one of the largest threats to water environment. The management and control of NPS pollutants to achieve the goals of improving surface water quality have gotten an increasing concern throughout the world. Because of the heterogeneity of key driving factors in NPS pollutions such as terrain, precipitation, and vegetation type, the management of NPS pollution in mountainous areas needs more meticulous effort than the plain areas. In this research, considering the characteristics of the spatial heterogeneity of the natural and human factors in mountainous areas, the loads of NPS pollutants, such as total nitrogen (TN) and total phosphorus (TP) are evaluated by the export coefficient model and the InVEST (integrate valuation of ecosystem services and tradeoffs tool), which is a water purification model. The excesses of NPS pollutants are analyzed according to the water environmental function zoning. The research is intended to reveal the spatial difference of NPS pollution in mountainous areas, determine the key regions for management, and provides a foundation for regional pollution control planning and environmental management efficiency promotion in mountainous areas. The method to estimate and evaluate NPS pollutants loads is developed based on the export coefficient model and InVEST water purification model, and is successfully applied to the case study of Baoxing County, showing the applicability of this method for mountainous areas. The results indicate that: 1) TN and TP loads in Baoxing County are respectively 1 156.44 and 81.41 t/a in 2010, and the spatial distribution of TN and TP loads has the characteristics of uneven distribution, local concentration, and closing to the water. 2) The contributions to pollution from various types of pollutant sources are different. The magnitude order for the contribution from various types of pollutant sources is livestock and poultry breeding > agricultural land > rural life. As above mentioned, the contribution of the livestock and poultry breeding to NPS pollution is the biggest and sustainable resource management should be prioritized in an effort to reduce the burden. Such management practices may involve building methane tanks, field ponds and the comprehensive utilization of waste facilities, which are aimed to reduce the level of NPS pollutant export into water bodies. 3) Compared with the pollutants thresholds of water quality standard at Class III at the watershed scale, the total amounts of TN and TP loads in Baoxing County do not exceed the thresholds. However, at the 30 m × 30 m grid scale, according to the pollutant load, water yield and water quality standard at Class III, TN and TP excesses in Baoxing County are calculated, and the results are respectively 763.63 and 51.16 t/a. The excesses of NPS pollutants in mountainous areas show significant spatial variation. The regions in which TN and TP loads exceed the thresholds are mainly distributed in cultivated land, grassland, residential land and the regions near the water, and these regions are the key regions for NPS pollution control and management.