Behaviors And Controls Of Nitrogen Pollution In A Mountainous Reservoir

The Tianzhuang Reservoir is a typical mountainous reservoir, which is situated in the mountain area of central south of Shandong province. It is also the potential water source for Yiyuan County. But the nitrogen in the reservoir water has exceeded the environmental quality standards for surface water under the influences of production and use of nitrogen fertilizer. With the development of agriculture and industry and population growth, the contradiction between water demand and water supply are being aggravating. Therefore, it is urgent to study the behaviors and controls of nitrogen pollution in the Tianzhuang Reservoir in order to provide the foundation for the prevention and treatment of the water pollution.Based on the natural environment investigation in the area of the Tianzhuang Reservoir, the nitrogen loadings were determined through the monitor of water quality and water quantity of the inflows, the experiments of leaching and nitrogen release from the sediments, and the nitrogen analysis in the porewaters in surface sediment. In addition, the feasibility of sediments capping by natural zeolite to prevent the ammonium release were assessed via the comparison experiments; In these experiments the ammonium in water near the capping-water interface and in overlying water were monitored; The interfacial flux of ammonium and the time of ammonium breakthrough the capping layer were also calculated. At last, a plane 2D flow-water quality was developed to simulate the spatial-temporal distribution of nitrogen in the Tianzhuang Reservoir. The treatment effects of local sediment capping by zeolite, the fly ash move and agricultural diffuse control were also predicted. The major conclusions can be summarized as follows.⑴The Tianzhuang Reservoir is a polymictic revervoir. The concentrations of nitrate nitrogen and ammonium nitrogen are higher than the normal levels of the Chinese reservoirs. The water quality of the Tianzhuang Reservoir is inferior to class Ⅴ(GB3838-2002) under the influence of exceeding standard of ammonium nitrogen.⑵The release flux of ammonium nitrogen from sediments ranges from 80 to 175 mg/ (m2·d), and the sediments are the important source for ammonium nitrogen of the reservoir. The influx of nitrate nitrogen is about 40mg/(m2·d), and the denitrification in surface sediments is the most thorough pathway for nitrogen removal.⑶Agriculture diffuse is the largest source of nitrate nitrogen, which (604.8t/a)accounts for 96.9% of the total loadings. The sediments are the largest source (128.8t/a) of ammonium nitrogen, which occupies 76.0% of the total loadings.⑷The results of water quality simulation indicate that the concentrations of nitrate nitrogen range from 5.00 to 8.00 mg/L, and the concentrations in the same period of different hydrological years are close. Ammonium can be up to the standard only in wet period of wet years. Ammonium in dry period of dry years is below the standard seriously, and its exceeding criterion multiple is in the range of 1.00-1.67. The ammonium concentrations in dry period are higher 1.30-1.50 mg/L than values in the wet period of the same hydrological year.⑸The leachates from fly ash at the north bank of the Tianzhuang Reservoir influence the quality of ambient water. The concentration difference of nitrate nitrogen in the ambient water can be up to 0.20-0.50 mg/L between before and after the fly ash removal. The control of agricultural diffuse is the key factor to improve the water quality with respect to nitrate. The concentration of nitrate nitrogen will decrease about 20% if the loadings of agricultural diffuse decrease 30%.⑹The breakthrough time of ammonium through the same capping material is positive to the square of the layer thickness. The breakthrough time through zeolite- capping is longer than the time through sandy layer remarkably. The steady release flux of ammonium from the sediments is negative to the thickness of capping layer. The complex capping of 15 cm zeolite layer and 15 cm sand layer is more effective than the 50 cm sand layer.⑺The water quality with regard to ammonium will improve obviously after the sediments in intake area are capped by zeolite, and the ammonium nitrogen can be up to the standard in wet periods of all hydrological years. Ammonium nitrogen of the water in every part of the reservoir may be up to the standard when the internal loadings are decreased 80%.