| Nonpoint source (NPS) pollution has become a serious problem in China with rapid development of the country’s economy. NPS not only causes serious surface water pollution, but also results in deterioration of ground water quality. Contaminated groundwater poses a threat to drinking water safety and at the same time exacerbates surface-water pollution. It is thus important to study the mechanism of transport of non-point pollutantsin subsurface in dealing with NPS problems. The Shaying river basin in the Huai river basin was selected for this study because it is one of the major agricultural regions of China with serious NSP pollution problems. Fate and transport of nitrate, a major NPS pollutant, was simulated and its contribution to surface water in the basin was evaluated. Shaying river is the largest tributary of Huai River with a drainage area of40,000kmz, covering portion of Henan and Anhui provinces. The Shaying river basin is one of the most important grain-producing regions and livestock breeding bases. River pollution by NPSs, e.g., fertilizers, pesticides, and animal wastes is a serious problem in the basin. Besides the surface-water pollution NPS pollutants, especially nitrate, also infiltrate into soils and aquifers and pollute groundwater. Previous studies of NPS pollution control in the basin focused only on surface runoff and ignore groundwater discharge to rivers. It is thus necessary to study the interaction of groundwater and surface water and to estimate the nitrate load from groundwater to surface water.First of all, a field survey was conducted in this project which shown that the nitrate pollution in Shaying river basin is serious. The average concentration of nitrate-nitrogen (NCV-N) in the region is5mg/1with the maximum of52mg/1. Among133samples collected, the NCV-N concentrations of18samples (13.6%of the samples) are larger than10mg/1,8samples (6%) are larger than20mg/1. The most seriously polluted regions include Xinzheng, Yanling, Linying, Luohe, Xiangcheng, Zhoukou, Linquan and Jieshou. Secondly, the conceptual and numerical models of the groundwater flow in the shallow aquifer of the basin was built based on its hydrogeology conditions. The groundwater flow was simulated with MODFLOW in the GMS (Groundwater Modeling System) software. The numerical model was calibrated using observed groundwater levels. The result shows that a total of22.5x10m groundwater is discharged into the river every year, which is equivalent to9.6%of the annual rainfall and39%of the surface runoff of the basin.Thirdly, based on the results of groundwater flow modeling, a nitrate transport model was built and the changes in the groundwater nitrate concentrations were simulated for past thirty years from1981and2010and predicted for the next forty year from2011to2050. The results show that the groundwater nitrate concentration increases consistently during the last30years. At the end of2010there are0.82million tons of nitrate in groundwater, about0.027million tons per year. It is also shown that the groundwater nitrate concentration continues increasing even without additional source inputs. Based on the modeling results, it is estimated that the total nitrate discharged to the rivers is30,600tons in2010, which is about40%of the Huai river basin. The average groundwater nitrate concentration is13.5mg/1and the nitrate concentration of surface water will5.3mg/1if all the groundwater was discharged into rivers. It is also shown that the nitrate discharged to rivers will also increase during2011-2050, without additional input, the maximum of61,300tons will be reached in2037. Under different fertilization management plans, the groundwater nitrate concentrations for future80years were predicted. The simulation result demonstrates that attention should be paid to optimize the fertilization applications as soon as possible which will greatly benefit groundwater and surface-water pollution management. |
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