Distribution Characteristics, Sources And Risk Assessment Of Nitrogen, Phosphorus And Metal Elements In The Three Gorges Reservoir

As a important hydraulic engineerings with extensive comprehensive benefits,the Three Gorges Project has brought people great convenience,such as flood control,power generation,navigation,water supply,etc.Howerver,it has also caused some ecological problems,such as the occurrence of water blooms and water pollution in some areas of the reservoir area in recent years.Therefore,this paper focused on water and sediments in 10 tributaries and Maoping of the Three Gorges reservoir from Yunyang to Yichang section,and studied time-space distribution,source and pollution characteristics of inoganic ions,nutrients and heavy metals in water and sediments.,and then clarified water environment change and environmental health risk assessment in river-reservoir systems.The conclusions are as follows:?1?The majority of the inoganic ions in the study area did not exceed the surface water quality standard and had seasonal differences.In interstitial water,the inoganic ions had a tendency to release;through source analysis,it was found that Na⁺and Cl^-,Na⁺,Mg²⁺and Ca²⁺in water may have the same source,and compared to the dry season,the inoganic ions in the flooding season were more affected by agricultural activities.This research showed,the pH of the water in the study area was 7.39,which was slightly alkaline.Except two sampling sites of F^-,the other inorganic ions did not exceed the surface water quality standard.There were seasonal differences in the inorganic ions in the water and interstitial water,but there were no significant difference in the longitudinal spatial distribution,because water layer was constantly flowing and evenly mixed.Correlation analysis indicated that Na⁺and Cl^-,Mg²⁺and Ca²⁺in water may had the same source,Such as material weathered from rocks or human activities.At the same time,the relationship between NO₃^-and SO₄^2-indicated that the inorganic ions in the flooding season were more affected by agricultural activities,and in the dry season,were affected by agricultural activities and industrial activities.?2?The TOC,TDN and NH₄⁺-N water had seasonal differences,and the eutrophication was more likely to occur during the flooding season;in interstitial water,the nutrient also had a tendency to release;in the flooding,the nutrient in sediments was more abundant than in the dry season,and nitrogen was mainly composed of organic nitrogen;the source analysis showed that nutrients in sediments were more terrestrial inputs during the flooding season,and NH₄⁺-N as endogenous sources was the most likely to be released from sediments.Overall,the concentration of TDP in water reached the standard value of the National Surface Water Environmental Quality Standards?GB3838-2002??,and the concentration of TDN in water exceed the standard of ?.There were more tributaries which N/P was less than 29 in the flooding season,and it indicated that eutrophication was more likely to occur during the flooding season.The analysis of variance showed that the TOC,TDN and NH₄⁺-N in the water had seasonal fluctuations,but there was no significant difference between the water layers.Compared with the dry season,there were the higher contents of TOC,TDN and TDP in the sediments during the flooding season,and the C/N in the sediments indicated that the cause may be terrigenous materials entered the water body during the flood season.TOC content had positive correlation with TN,and the coefficient was high,indicating that the nitrogen in sediments may be mainly organic nitrogen.Through analysis,it was found that the nutrients in sediments had the possibility of releasing to the overlying water.NH₄⁺-N had significant correlation in water,interstitial water and sediments,and was most likely to be released,but this release trend was not very strong.?3?For heavy metals in water,Pb exceeded the limit value of the standards ??GB3838-2002?,while in interstitial water,Zn,Mn,Ba,Sr,Fe had a tendency to release to overlying water;meanwhile,There were accumulation of Zn and Mn in sediments;the sources identification showed that the input contribution of nutrients in the study area was greater during the flooding season,while the industrial activity contributes more during the dry season.The results showed,in water samples,the Sr concentration exceeded the mean value in the world rivers,achieved the standard of natural strontium mineral water.The Pb exceeded the drinking water quality standard and the Class ? limit of the standard.At the same times,the Pb concentration in Maoping was the highest during the dry season,and it was because of the retention effect of the dam.Therefor,attention should be paid to the control of Pb pollution in the water.In sediments,the contents of Zn and Mn were higher than the background value,indicating that the two heavy metals have cumulative effects in the sediment.The contents of Zn,Cu,Cr,Mn and Pb were highest in Maoping during the dry season,and it was also bacause retention effect of the dam.Correlation analysis shows that Mn were related to heavy metals such as Zn,Cu and Cr and so on.Principal component analysis showed that the contribution rate of nutrient input was the highest in the flooding season,and the highest was industrial activity in the dry season,which was closely related to the fertilization amount and rainfall.?4?By evaluating nutrients,heavy metals,and inorganic ions,it was known that the content of TN and TP in sediments exceeded the background value of the soil,and there was a cumulative phenomenon.The sediment quality guidelines results indicated that most of the samples were at lowest effect level,and all the samples were not seriously polluted.As far as heavy metals,both the single factor pollution index method and the Nemero comprehensive pollution index method reflected that the overall water quality was better,and the Tongzhuang River was the best.The evaluation of the cumulative index method reflected the slight pollution of Cu in sediments of Maoping and Modao River.However,except for the Tongzhuang River,the other tributaries and Maoping sediments were slightly polluted by Zn,so it is necessary to control the pollution of Zn and Cu.The health risk assessment of heavy metals on human body in the study area indicated that the risk of non-carcinogenicity of F^-,NO₃^-,Cu,Zn,Cr,Pb,Fe,Mn and Ni were not high.