Distribution Characteristics And Risk Assessment Of Phosphorus Forms In The Sueface Sediments Of Xiangjiaba Reservoir

Hydropower development is a key strategy for energy supply,climeate change mitigation and economic development in many developing countries.Reservoir construction has brought great social and economic benefits,and is also changing the regional ecological environment.Xiangjiaba reservoir is the last stage of cascade reservoir in the lower reaches of Jinsha River,which has a very important impact on the input,migriation and transformation of nutrients in the lower reaches of the Yangtze River.However,there is a relative lack of knowledge about the phosphorous forms of sediments in this area.Taking Xiangjiaba reservoir as the research object,the surface sediments were collected in September 2020 to study the distribution characteristics and risk assessment of phosphorus in sediments.The main conclusions are as follows:(1)The extraction times of sediments were increased in the extraction process,and the recovery rate of TP was increased from 50.6～59.4%to 93.8～95.1%.Inorganic phosphorus and organic phosphorus were extracted simultaneously.The forms of inorganic phosphorus are divided into exchangeable inorganic phosphorus(Ex-P_i),aluminum bound inorganic phosphorus(Al-P_i),iron bound inorganic phosphorus(Fe-P_i),Occluded phosphorus phosphorus(Oc-P),authigenic calcium bound inorganic phosphorus(ACa-P_i)and detrital inorganic phosphorus(De-P_i).The forms of organic phosphorus are divided into exchangeable organic phosphorus(Ex-P_i),Fe/Al bound organic phosphorus(Fe/Al-P_o),calcium bound organic phosphorus(Ca-P_o)and residual organic phosphorus(Re-P_o).(2)The sediments of Xiangjiaba reservoir were analyzed with the modified sequential extraction method to classify the forms of phosphorus.The content of TP in sediments ranged from 370.6 to 616.4μg/g,The content of OP in sediments ranged from 122.2 to 210.9μg/g.The content of IP in sediments ranged from 248.5 to 416.8μg/g,accounting for 63.6～69.7%of TP,which is the main occurrence form of phosphorus in sediments.There is a good correlation between IP and TP is consistent as a whole.The spatial distribution shows a trend of high trend of high content in the upper reaches and low content in the lower reaches.(3)The concentration of different forms of phosphorous followed the following order:De-P_i(265.8μg/g,75.5%)>Oc-P(39.3μg/g,11.2%)>Al-P_i(18.5μg/g,5.1%)>ACa-P_i(12.3μg/g,3.4%)>Fe-P_i(12.2μg/g,3.3%)>Ex-P_i(6.2μg/g,1.8%).De-Pi is the main form.The concentration of different forms of phosphorous followed the following order:Re-P_o(132.9μg/g,61.8%)>Fe/Al-P_o(22.5μg/g,12.1%)>Ca-P_o(16.3μg/g,5.7%)>Ex-P_o(2.3μg/g,0.8%),Re-P_o is the main existing state of OP.(4)In terms of spatial distribution,the contents of Ex-P_i and Al-P_i vary greatly in space,and the high-value area is mainly distributed in Zhongdu river.The spatial distribution of Fe-P_i is generally high in the upstream and low in the downstream.The low-value area is mainly in Dawen river,and the high-value area is mainly in Xining river,about 6 times the content of Dawen river.The spatial distribution of Oc-P is generally uneven Uniformity,the content distribution in the main stream of Jinsha river is uneven,and Zhongdu river,Xining river and Dawen river all show the characteristics of low upstream and high downstream.ACa-P_i has significant spatial differences.The high value area mainly appears in Zhongdu river,and the low value area is in Dawen river and Jinsha river;De-P_i and TP have similar spatial distribution characteristics,that is,they show the trend of high upstream and low downstream.Jinsha River has the highest concentration compared to the three tributaries.The spatial difference of Ex-P_ois significant,and the difference is mainly reflected in Zhongdu river.Fe/Al-P_ogradually increases along the Jinsha River and Dawen River.The spatial difference between Zhongdu River and Xining river is small,and the high value area appears in the lower reaches of Jinsha River,which is mainly affected by sediment TOC.The high value area of Ca-P_o appears in Zhongdu river,where there are more endogenous biogenic organic phosphorus and Ca-P_o has homology with ACa-P_i.The spatial distribution of Re-P_o shows a trend of high upstream and low downstream,and the sources of Re-P_o and De-P_i are consistent,which is affected by terrigenous input.(5)The sum of Ex-P_i,Al-P_i Fe-P_i,Oc-P,Ex-P_o and Fe/Al-P_o is used to measure the upper limit of sediment release potential,that is,the content of bioavailable phosphorus(BAP).The content of bioavailable phosphorus ranged from 25.1 to 129.7μg/g,the mass concentration of BAP in TP is low,accounting for BAP 13.0～25.1%of TP.The release risk in Xangjiaba reservoir is low.The content of BAP in Zhongdu river is the highest.The content of BAP ranged from 128.4 to 129.3μg/g.The content of BAP in Dawenxi river is the lowest.The content of BAP ranged from 75.6 to 103.2μg/g.The N/P ratio in the study area ranged from 1.1 to 4.0,indicating that phosphorus in sediments is mainly imported from external sources.The TOC/P ratio in the study area ranged from 28.3～147.7.indicating that buried deposited phosphorus is mainly refractory phosphorus and the release risk of phosphorus in sediments is low.