Nitrogen And Phosphorus Pollution Characteristics And Ecological Restoration Of Xiang Xi River Bank Of The Three Gorges Reservoir

After the Three Gorges Project was in operation,the Xiang xi River Basin was transformed from a river ecosystem to a lake ecosystem,and its ability to self-purify pollutants was greatly reduced.As nitrogen,phosphorus and other nutrients continue to flow into the Bay,the bloom phenomenon in the Xiang xi River Basin continues to erupt.In this paper,The distribution characteristics of nitrogen and phosphorus nutrients in the water,sediments,the fluctuating zone（145¹75 m）and the upper marginal（185 m）soil of Xiang xi Bay were studied through field monitoring and indoor testing.The phosphorus characteristics of the sediments,the fluctuating zone and the upper marginal soil of Xiang xi River bank were analyzed.Based on the nitrogen and phosphorus pollution of the Xiang xi River,phytoremediation technology was used to compare the ability of aquatic plants to absorb and purify nitrogen and phosphorus in the soil,select dominant species,and combine the experimental results with the sediments to control phosphorus plants.According to the pollution status of sediments in Chen Jia Wan section of the Xiang xi River,an endogenous phosphorus controlled phytoremediation demonstration was conducted to provide theoretical and technical support for the control of endogenous phosphorus in the Xiang xi River basin of the Three Gorges reservoir area.The specific research content and results are as follows:According to the investigation of the vegetation in the falling zone and the upper margin of the Xiang xi River,the physicochemical properties of the fluctuating zone and the sediment are analyzed.The plants found and confirmed were 34 species in 34genera,Gramineae,and Compositae,all of which were dominant families in the area.Physical and chemical indicators of soil in sediments and water-falling belts have important impacts on the ecological environment.Through indoor testing,the temporal and spatial distribution characteristics of nitrogen and phosphorus nutrients in the bank of the Xiang xi River Basin were analyzed.In the five selected sections of the Xiang xi River sediments,the TN content is in the first-order section,and the TP is mostly in the third and fourth-order sections.The actual pollution of the TP in the Xiang xi River is higher than that in the TN.The content of TN,available nitrogen,and TP in the water-level fluctuation zone and the upper margin were all exceeded,among which there were many abnormal values and were strongly affected by human activities.The regional differences were significant.The contents of TN,available nitrogen,and TP in the 145 and 155 m fluctuating zone were all greater than those in the swept zone.Nitrogen and phosphorus in the water-fluctuating zones and the upper margins are likely to be diverted downward by rainwater.In December 2016,the mean value of the diffusion flux at each sampling point of the sediment was 0.069 mg?（m²·d）^-1(variation range:0.024⁰.140mg?（m²·d）^-1).Phosphorus is released from the sediment to the upper water body,which has a significant effect on the eutrophication of the water body,and the downstream release flux is higher.SMT method was used to study the phosphorus form of Xiang xi River bank and understand the specific process of phosphorus release from reservoir bank.The Fe/Al-P content of the fluctuating zone is higher than that of the upper edge,and Fe/Al-P tends to move downwards.The proportion of OP to TP in the fluctuating zone and the upper marginal soil is also relatively high,indicating that the Xiang xi River basin is affected more by humans and the proportion of non-point source pollution is greater.There was no significant difference in the ratio of Ca-P to TP in the subsidence zone and the upper margin,indicating that the Ca-P of the Xiang xi River’s fluctuating zone and the upper margin is relatively stable.The TP content in the sediments differed significantly,with a mean Fe/Al-P/TP value of 32.21%and an average OP/TP value of 11.80%.Both were active forms of phosphorus and released phosphorus into the upper water body.From the flooding test of the swept zone soil,it can be seen that the initial stage of flooding is the high-risk period of phosphorus release and the reason for the decrease of TP is mainly caused by the release of Fe/Al-P and OP to the water body.Using indoor cultivation experiments,the biomass and soil nitrogen and phosphorus abatement rates were analyzed.The biomass of 12 plants changed significantly before and after,and the net biomass increased from 0.81⁴2.15 g?pot^-1.The average net biomass was 16.44 g?pot^-1.The TP reduction rate ranges from21.42%⁴5.70%,and the average rate of TP reduction is 33.98%.The TN abatement rate ranges from 26.71%⁵8.43%with an average rate of reduction of 37.74%.By subtracting the evaluation index,the elimination effects of Nelumbo nucifera Gaertn,Typha orientalis presl,Phragmites australias,Canna indica,Hydrocotyle chinensis,and Alternanthera philoxeroides on TN and TP all belonged to the V-level strong elimination.The phytoremediation technology was used to screen aquatic plants.A demonstration project of 2500 m² was constructed in the Chen Jia Wan area of Gao wan he Bay,Xia Kou Town,Xing Shan County,Xiang xi River Basin.Form a three-dimensional control system from sediment to water and monitor and evaluate the effect of endogenous phosphorus recovery.After calculation,the average phosphorus release control rate reached 59.45%,which has far exceeded the assessment target of 15%,in line with the demonstration project requirements.The endogenous phosphorus control demonstration project provided a new approach for the treatment of endogenous phosphorus pollution in the tributaries of the Three Gorges Reservoir,and provided technical support for the treatment of water pollution in the Three Gorges reservoir area.