Study On The Non-point Source Pollution’s Characteristics And Control Technology In Small Watershed

The problems of water resource shortage and environmental pollution inChina have become increasingly prominent, and agricultural nonpoint sourcepollution has become an important reason for water quality deterioration andeutrophication. To this end, the control of agricultural nonpoint source pollutionhas important theoretical and practical significance of ensuring food security,fully coordination of resources and environment, and building up a beautifulChina.Taking the Jilin section of Liao River as an example, the thesis combinesthe techniques of in situ field investigation, field observation with modelsimulation, to analyze the spatial and temporal distribution of nonpoint sourcepollution and major pollutants’ load, to simulate non-point source pollution inDongliao watershed and identify the critical source areas of non-point sourcepollution control by applying GIS technology and AnnAGNPS models, and tostudy nonpoint source pollution control technology for sloping field andvegetation filter strip technology as well. The research work will provide atheoretical basis and practical nonpoint source pollution control techniques forhilly areas in north China. The main contents and conclusions are listed asfollowing:1. By applying the equal standard pollution loading method, the thesis hasanalyzed11-year data from1999to2009, and found that TN and TP are themain nonpoint source pollutants in the Jilin section of Liao River Basin, and thetotal pollution loads are14,426.20t/a and5,423.44t/a, while equal standardpollution load ratio are27.55%and51.79%, respectively. The contributedloads of4-type nonpoint source pollution from high to low are: livestock＞rural life source＞farming＞aquaculture, and the livestock breeding is themain source of nonpoint source pollution for the river basin, whose pollutionload ratio is93.15%. The heaviest load of TN, TP, COD and ammonia nitrogenis distributed in Lishu, followed by Gongzhuling, Dongliao and ShuangliaoCounty. The Jilin section of Liao River is divided into three nonpoint sourcepollution control areas. Wherein the first class area (high risk area) is mainly distributed in the eastern basin, lands are used as arable field, covering15.19%of the watershed. The second class area (medium risk area) is mainlydistributed in the basin plain and hilly areas in the central and southeasternwatershed, namely Liaoyuan city, western Siping city, western and central ofLishu county and southern Gongzhuling county, covering71.67%of thewatershed. The third class area (low-risk area) is mainly distributed in thewestern region of the watershed, covering13.15%of it. Within the watershed,the southern Lishu county, eastern and southern Gongzhuling city are the keyareas for nonpoint pollution control. Livestock pollution prevention and slopingland governance are crucial for preventing the water pollution in watershedarea.2. The thesis has simulated the nonpoint source pollution in Dongliaowatershed by using GIS technology and AnnAGNPS model, the results showthat, after proper parameter corrections, the AnnAGNPS model is suitableto be used to simulate the nonpoint source pollution in Dongliao watershed,better fitting to the TN load in the small watershed (less than100km2), whileless better fitting to the TP load, nevertheless, the simulation accuracy is wellwithin the acceptable extent. In the small watershed, the loss of nitrogen ismuch higher than that of phosphorus, the loss of nitrogen is dominantly in adissolving form, while that of phosphorus is in an absorbing form. In2010, theoutput of nonpoint source pollution is364.7t for TN,21.5t for TP, respectively,which mainly come from the pollution caused by chemical fertilizer. Thenonpoint source pollutant loads show a characteristics of seasonal variation,rainfall and chemical fertilizer application are the main driving forces, theoutput of TN and TP from July to August account for52%and74%,respectively, of the annual output. The spatial distributions of TN and TP loadsshow a certain similarity, while they are different from one area to another, e.g.,pollutant output per unit area is larger in slope farmland and smaller inwoodland. Critical source areas include perennial cultivated agricultural areas,riparian and sloping land. Slope, runoff and erosion are the main driving forces.3. An experiment of the small runoff collection zone was carried out from2009to2011, the results show that, the retention effects on pollutants byon-way nonpoint source pollution control are similar to the trapping effect on rainfall runoff by various control technologies, the effects in a descending orderare bare land (vegetation natural growth)＞ridge plants＞level terrace＞transverse ridge＞longitudinal ridge (down slope). The losses of ammonianitrogen and COD per unit area in transverse ridges reduce32%and26%,respectively as compared to the longitudinal ridges. For the longitudinalridged on sloping farmlands, the implementation of water and soil conservationmeasure (terraces), biological measures (ridge vegetation zones) can reducethe loss of ammonia nitrogen and COD by72%and77%, respectively, inaddition, reduce runoff by30%. For the two technologies of source-controlling,pesticide reduction and fertilizer reduction have no obvious difference from thebackground test condition in terms of reducing nonpoint source pollutant loss.The results prove that, the implementation of protective cultivation modes,such as returning sloping farmland to tree land and grasslands, could reducethe discharge of nonpoint source pollutants into rivers efficiently.4. Observation of two years (2009-2010) on vegetation filter strip in theexperimental area demonstrates that, different configurations of vegetationfilter strips show some certain removal effects of COD, ammonia nitrogenand TP. The average COD removal rate can be sorted from the maximum tothe minimum as follows, tree-shrub-grass complexes＞tree-Grasscomplexes＞shrub-grass complexes＞grass filter strips; that of ammonianitrogen, grass filter strips＞Tree-shrub-grass complexes＞shrub-grasscomplexes＞tree-grass complexes; that of TP, shrub-grass complexes＞tree-shrub-grass complexes＞tree-grass complexes＞grass filter strips.Under the experimental conditions,10-m-width vegetative filter strip with nativeplant configuration have good removal effect on nitrogen and phosphorus inoverland flow of farmland, the average removal rate is within32%-57%.5. Based on the in situ field investigation, the thesis develops two nonpointsource pollution control technology. One is the intergrated nonpoint sourcepollution control technology for sloping land. Considering the terrain, slope,sloping length and landuse of the sloping land, an intergrated technology isapplied. it is supposed that, transverse ridge with level terrace for the slopingland with an angle larger than8degree, ridge plants for the sloping land with an angle between3degree and8degree, and transverse ridge instead oflongitudinal ridge. River locust is recommended as the ridge plant for itscapability in fixing soil. The second technology is the vegetation filter striptechnology. It is suggested to layout a10-m-width the vegetation filter stripalong the watercourse without hardening, covered by native vegetation. On theriver side, tree or shrubs are configurated, and the priority is given to willowthat is strong in germination and root system and erosion resistence. On thefarmland side, grass is configurated, and the priority is to the perennial herbArtemisia.