Studying On The Exchange Flux Of Inorganic Nitrogen And The Influence Factors At Sediment-Water Interface In Dongjiang Main Stream-Huizhou Section

Dongjiang river is an important source of drinking water which supplying for Guangdong province and HK Special Administrative Region, and its quality is directly related to the civil life in Guangzhou 、 Shenzhen and HK cities and economic development. In recent years, with the rapid economic development and urbanization,water quality of its main stream has been seriously polluted by domestic sewage and industrial and agricultural wastewater with nitrogen. Excessive emission of nitrogen nutrients into the water would result in the decrease of water quality and eutrophication.At the same time, most pollutants accumulate in the sediment after flowing into river,and internal pollution become increasingly prominent, which has aroused the attention of scholars at home and abroad. Therefore, it’s meaningful to know the content of nitrogen and the distribution of these nutrients in Dongjiang river, discuss the migration and release rule of nitrogen and study the exchange flux and factors in sediment-water interface to control the nitrogen pollution in the main stream and protect the Dongjiang river water quality.In this paper, main stream of Dong Jiang river(Huizhou section) was investigated.Based on the research of natural environment, six sampling points were chosen during June in 2013 to March in 2014, and the lab incubation was adopted to simulate and measure the nitrogen exchange flux and migration under different conditions in order to study the effect of related factors.The main conclusions are as follows:a).The water quality in Dongjiang main stream(Huizhou section) was in good condition for most of the year. Total nitrogen pollution was serious, which can basically meet class of Ⅳ~Ⅴ(lake and reservoir, according to the concentration of N),and some point were even inferior the fifth class. Ammonia nitrogen concentration was generally low,with the values less than 0.5mg/L for most of the time,which can basically meet the second class requirement. The concentration of nitrate nitrogen ranged from 1.28 to2.22mg/L, and the concentration of nitrite nitrogen ranged from 0.001 to 0.071mg/L which was comparably low. To evaluate the nitrogen pollution in the water using Nemerow index showed that the main pollutant was nitrate nitrogen reaching slight pollution, and the concentration of ammonia nitrogen and nitrite nitrogen were pretty clean, especially the conceration of nitrite nitrogen.b).The annual exchange fluxes of ammonia nitrogen were positive values all year around in the main stream of Dongjiang river(Huizhou section), which indicated that the sediments in the main stream of Dongjiang river(Huizhou section) were the source of ammonia nitrogen. The exchange fluxes of nitrate nitrogen and nitrite nitrogen were pretty low. In the exchange simulation experiment, ammonia nitrogen of the sedimentswere released into overlying water quickly at the first stage. As time gone by, the process of exchange between sediments and overlying water was slowed down and exchange flux decreased gradually. The change of exchange flux of nitrate nitrogen was different from that of ammonia nitrogen. The exchange flux of nitrate nitrogen was pretty low. Moreover, the sediments were a dynamic transformation between sink and source of nitrate nitrogen, not only the sink or the source.c).The simulation experiments of overlying and sediment system exchange flux has been studied: The rise of temperature would help the mineralization and consuming dissolved, preventing the nitration. The release rate of ammonia nitrogen under anaerobic circumstances was higher than the rate under aerobic conditions, which was conducive to the release of ammonia nitrogen in the sediments. Nitrification and denitrification were the main exchange process in the sediment-water interface, which was limited by dissolved oxygen. The lower p H values were conducive to exchange interaction, leading more ammonia released into the interstitial water. By contrast, the exchange flux of ammonia nitrogen would decrease. Under alkaline conditions, the concentration of ammonia nitrogen was less resulting in a greater concentration difference between the sediments and water system, which accelerated the release of ammonia nitrogen in the sediments.In conclusion, nitrate nitrogen was the main pollutant in the overlying water and the sediment was an important internal source of ammonia nitrogen. With the condition of dissolved oxygen deficiency, the disoxidation of nitrate nitrogen would help increasing the concentration of ammonia nitrogen. The higher temperature, the lower dissolved oxygen, too peracid and too alkaline conditions were conducive to the release of ammonia nitrogen from sediment.