Spatial And Temporal Variations And Source Identification Of Nitrogen Pollution In The Dongjiang River Basin

Dongjiang river basin as the most representative type drinking water river,such as Hong Kong and Macao to Hong Kong,Guangzhou,Shenzhen,Dongguan and other large bay area core cities as an important area for the water supply,Dongjiang river basin water network density,population distribution and water distribution is highly overlap,water quality greatly influenced by artificial factors,revealed a typical compound pollution,water quality security face greater risk.Among them,the excessive nitrogen is an important factor affecting the water quality of Dongjiang River,and the frequent blooms caused by nitrogen pollution.Compared with the status of regional important drinking water source,the monitoring ability of water pollution in the Dongjiang River Basin is very weak.In view of the nitrogen pollution problem in the Dongjiang River Basin,water quality samples were collected and tested in the whole Dongjiang River Basin from the Xinjangbei sample point at the source area to the Shiziyangkou sample point at the north main stream of the Dongjiang River.Based on the spatial and temporal distribution of pollution and combined with the nitrogen-oxygen bistable isotope values of nitrate nitrogen(δ¹⁵N,δ¹⁸O),the status and causes of nitrogen pollution in each region were statistically analyzed,and the distribution maps of nitrogen and oxygen isotopes of typical pollutants were drawn,so as to accurately trace the sources of nitrogen pollution and identify the risk of nitrogen pollution in the whole running water of Dongjiang River.In view of the lower reaches of the Dongjiang river area（Li zhouwu continent water intake pumping station of the river）under the influence of the upstream drainage pollution problem,through the synchronous monitoring water quality of hydrology data,accounting from the upstream drainage into the downstream Dongjiang river north main stream of the total amount of pollutants,analysis influence liu house the flow state and the main stream of water pumping stations of the pollutants and flux and flux pollution contribution rate,provide strong support for the prevention and control of Dongjiang.In addition,simulated experiments were carried out between the sediment from Zengcheng District of Dongjiang River and the domestic sludge from Xintang Sewage Treatment Plant.The experimental groups with different sediment under different light conditions were set to monitor the nitrogen index in the overburden water and explore the removal and transformation rules of trinitrogen in the water after the release of sediment nitrogen.The results show that:（1）the pollution of nitrogen from the source of Dongjiang River is serious.In space,the total nitrogen concentration in Dongjiang River is higher in tributary area and source area,and the main stream is gradually increasing in the upper,middle and lower reaches,and the whole is mainly nitrate nitrogen.Theδ¹⁵N（NO₃^-）andδ¹⁸O（NO₃^-）in dry season and high season in Dongjiang River Basin showed obvious differences.Theδ¹⁵N（NO₃^-）in dry season was lower than that in high season,but theδ¹⁸O（NO₃^-）was higher than that in high season.The main sources of nitrogen in the source area of Dongjiang River in dry season are domestic sewage,farmland fertilization is the main source of upstream,the main source of middle reaches is domestic sewage,farmland fertilization and soil,the downstream is domestic sewage and soil,Sewage and field fertilization are the main sources of nitrogen.Domestic sewage,fertilization and soil are the main sources of nitrate nitrogen in the high tide period in Dongjiang River area.The main sources of nitrate nitrogen were in the upper,middle and lower reaches of soil and tributaries,including farmland fertilization and soil.while the nitrate nitrogen in the middle reaches and tributaries comes from domestic sewage.Therefore,the control of nitrogen source pollution in Dongjiang River Basin,the source area,the middle reaches and the tributaries are mainly to control the direct discharge of domestic sewage,and the upstream and downstream areas mainly reduce agricultural non-point source pollution.（2）In terms of time distribution,the average water period of nitrogen concentration in the main stream of Jiangbei is larger than that in the dry season and higher than that in the high water period.It is not difficult to see from the spatial distribution that the pollution of the main tributaries is more serious,because it is affected by the living pollution of the coastal residents,but it also has little effect on the overall water quality of the Dongjiang north main stream.In addition,the main composition of nitrogen in the main stream of Jiangbei,mainly nitrate nitrogen.According to the analysis of nitrogen and oxygen isotope traceability results,the main sources of nitrogen in the water in the dry season of Jiangbei main stream are domestic sewage and soil nitrogen.The nitrogen source is mainly domestic sewage,soil nitrogen and farmland nitrogen source,among which domestic sewage accounts for a large proportion.Nitrogen sources are mainly domestic sewage,soil nitrogen and farmland nitrogen sources,among which nitrogen sources in soil are relatively large.The annual fluxes of Shilong section,Dadun section and Zengjiang section are 1017.32t/a,1180.75t/a and 144.55 respectively t/a,and the annual fluxes of ammonia nitrogen are125.50 t/a,177.32t/a and 14.93 t/a,respectively.The annual fluxes of total phosphorus are52.12 t/a,64.06t/a and 4.2 t/a.,The changes of water quality in upper,middle,lower reaches and tributaries including farmland fertilization and soil were analyzed respectively.Shilong in the entrance section had the greatest impact on water quality in Dadun in the exit section.so it is necessary to ensure the water quality of the large pier section or the Liuwuzhou pumping station to meet the standard,and to ensure the water quality of the Shilong section to meet the standard.（3）Under different conditions,the conversion rule of total nitrogen concentration was mainly nitrate.After the domestic sludge entered the water body,the self-purification ability of the water body was weakened.In the light group,due to the occurrence of algae assimilation,the concentration of ammonia nitrogen could still be controlled even with the entry of a large amount of domestic sludge.However,in the dark group,when a large amount of domestic sludge was mixed into the bottom mud,the concentration of ammonia nitrogen was significantly increased compared with the control group.The two peaks of dissolved oxygen appeared under light conditions,which may be caused by the growth and death of algae,and the death of algae caused the decrease of dissolved oxygen.In the absence of light,nitrification and denitrification played a major role in nitrogen conversion.Under light conditions,the assimilation of algae plays a major role.From the perspective of kinetic process,the assimilation effect of algae is obviously higher than that of nitrification and denitrification in water for nitrogen removal.