Distribution Characteristics Of Mercury And Organophosphorus Ester In Water,Soil/Sediment And Fish In The Three Gorges Reservoir

Mercury?Hg?and organophosphate ester?OPEs?,both of which are long-distance transported,highly biotocic,degradation-resistant and bioaccumulative in food chain,potentially affecting their global distribution.Once entering the water environment,Hg and OPEs are readily to be enriched in fish.Generally,they have extrenmely high bioaccumulation factors?BAFs?,being up to 10⁶-10⁷.Consumption of Hg and OPEs contaminated fish would pose a serious threat to human health.Therefore,it is of practical significance to investigate the distribution of Hg and OPEs in water environment and the enrichment characteristics of fish bodies.The Three Gorges Reservoir?TGR?is one of an extremely large reservoir worldwide.Due to its special annual operation mode of"storage in winter and discharge in summer",a fluctuation zone of 349 km² will be formed with a length of 557.8km and a latitude difference of 30 m from April to September every year.The fluctuation zone,as a cross zone of the interaction of water and land in the basin of the reservoir area,is a convergence zone of the runoff in the reservoir area,which is naturally easy to become a convergence zone of environmental pollutants.Moreover,the three gorges reservoir area is located in an area with intensive human activities,and its ecological environment is getting relatively fragile.Thus,it is very important to study the biogeochemical behavior of pollutants in the TGR area.Therefore,two kinds of pollutants,inorganic Hg and organic OPEs,were chosen as the research objects.Fuling Zhenxi and Zhongxian Shibaozai on mainstream as well as Kaixian Haifeng lake/Qukou and Zhongxian Tujing on tributaries in Yangtze River?Chongqing section?were selected as the sampling sites.We investigated the levels and distribution characteristics of Hg and OPEs in different depth?0 m,10 m,30 m and 50 m?of water and various water-level elevation?>175 m,165 m,155 m,145 m and inundated soil?and depth?0-5 cm,5-10 cm,10-20 cm and>20 cm?of soil,and further explored the Hg and OPEs levels of fish and their bioaccumulation characteristics with throphic levels.Base on this,we evaluated their potential ecological risk to the TGR area.The research results are as follows:?1?The mean concentration of total mercury?THg?in water samples from the TGR was8.71±4.17 ng L^-1,significantly below the surface water environment quality standard(50 ng L^-1).The level of total methylercury?TMeHg?was 0.37±0.32 ng L^-1,obviously lower than some contaminated areas(1.8-7.2 ng L^-1).Both THg and TMeHg presented the higher concentrations in warm season than those in cold,as well as the greater concentrations in mainstream?S1 and S2?than those in tributaries?S3 and S4?.The concentrations of all Hg forms increased with the water depth?except for reactive mercury?.The average total concentration(?₁₀OPEs)in the TGR is 289±158 ng L^-1(108-865 ng L^-1),in which Tris?2-chloroethyl?phosphate?TCEP?,Tris?1,3-dichloro-2-propyl?phosphate?TDCP?,Tris?2-chloroisopropyl?phosphate?TCPP?,Tri-n-butyl phosphate?TNBP?and Tri-iso-butyl phosphate?TIBP?are primary OPEs monomers.But there is no significant differences among sampling sites?p>0.74?.The OPEs mainly exist in the dissolved phase,and the Log K_p valueranged 1.84 to 4.87.However,there is no significant correlation between Log K_ow and LogK_oc?R²=0.04,p=0.28?.?2?The mean concentrations of THg and TMeHg in soils/sediments from the TGR were51.3±35.6 ng g^-11 and 0.34±0.27 ng g^-1,repectively.The THg concentration was higher than the background value of soil mercury in China(38.0 ng g^-1),but the level of MeHg was really low.No significant seasonal difference?p>0.05?were also exhibited.THg content increased with the increase of soil depth,while MeHg concentrations decreased with the elevation of soil depth?except for>175m soil?.Additionally,water-level fluctuation and soil organic matter?SOM?are important factors controlling the distribution of THg and MeHg.The average total content of?₁₀OPEs in the soils/sediments was 205±135 ng g^-1(16.3-553 ng g^-1),in which TDCP?40.7%?,TCPP?14.2%?,TIBP?13.0%?,Triphenyl phosphate?TPHP,11.6%?and 2-Ethylhexyl diphenyl phosphate?EHDPP,10.1%?were the main monomers,but there was no significant differences among sampling locations?p>0.05?.The content of OPEs in the unsubmerged soil?>175 m?was lower than those in the fluctuation zone,which indicated that the water level change played an important influence on the distribution of OPEs in the TGR.In addition,except for TCEP?r=0.296,p<0.05?,TPRP?r=0.234,p<0.05?,TNBP?r=0.241,p<0.05?and TIBP?r=0.295,p<0.05?,other OPEs monomers and?₁₀OPEs have no significant correlations with soil OM?p>0.05?,suggesting that OM is not the main factor affecting the distribution of OPEs in theTGR area.?3?The average THg and MeHg concentrations in fish muscle from the TGR were 54 ng g^-1(0.50-272 ng g^-1)and 23 ng g^-1(0.10-199 ng g^-1),respectively,far below than the world health organization?WHO?maximum allowable intake limit(THg,500 ng g^-1,w.w.)and the US EPA water quality control standards(MeHg,300 ng g^-1,w.w.).They also performanced as higher in perdatory fish than non-predatory fish.Moreover,the concentrations of Hg in most fish samples was not significantly correlated with their size,which might be caused by:1)the large increase of terrestrial nutrients induced by the impoundment of the TGR;2)the phenomenon of"growth dilution"existed,but it needs to be further confirmed.The average?₁₀OPEs concentration of fish muscle samples was812±490 ng g^-1(45.2-2887 ng g^-1),in which TCEP(203±176 ng g^-1),TDCP(153±123ng g^-1),TNBP(336±227 ng g^-1)and TPHP(122±152 ng g^-1)were the main contaminating monomers.The median values of TCEP,TDCP,TNBP,Cl-OPEs,Alkyl-OPEs and?₁₀OPEs performed as increasing tends with tropical levels.Moreover,the concentrations of?₁₀OPEs and OPEs monomers had no significant correlation with the size of the fish body?p>0.05?,indicating that OPEs would not enlarge with the growth of fish.?4?The LogBAF values of THg and MeHg in fish muscle from the TGR were 3.89?2.04-4.76?and 4.83?2.51-6.11?,respectively,increasing with tropical levels.The LogBAF values of TCEP,TDCP,TCPP,TNBP,TIBP,Tris?2-ethylhexyl?phosphate?TEHP?,Tris?2-butoxyethyl?phosphate?TBOEP?,EHDPP,and TPHP were 3.34?1.99-4.07?,3.59?2.22-4.43?,2.56?2.56-2.56?,3.94?2.81-4.61?,3.37?1.66-4.43?,3.05?2.46-3.52?,3.23?2.29-3.90?,4.51?3.79-5.22?and 4.05?2.74-5.40?,respectively,in which OPEs monomers?except for TPHP,TBOEP and Aryl-OPEs?and?₁₀OPEs increased with tropical levels.There was no statistically correlations between LogBAF and Log K_ow;but the LogBAF values increased with the increasing of Log K_ow when Log K_ow<7,and there was a significant correlation between LogBAF and Log K_ow?R²=0.52,p<0.05?.The range of?¹³C value in fish muscle?n=71?was-26.45^-17.24‰,showing no significant trend with the trophic level,whereas the range of?¹⁵N value was 5.72¹5.34‰,increasing with trophic levels.However,a significant negative correlation between the values of?¹³C and?¹⁵N was observed?R²=0.30,p<0.01?,indicating that the food sources of different trophic levels varied greatly and a certain degree of pollutants were accumulated.The limited variation range?2.83?1.21⁴.04??of trophic position?TP?proved that the food chain was relatively short in the TGR.The trophic magnify factors?TMFs?of THg,MeHg and inorganic mercury?IHg?in fish muscle were 0.09,0.11 and0.09,respectively,which were at low levels.The?¹⁵N values showed no significant correlations with the concentrations of 6 major OPEs monomers?detection rate>50%?and the?OPEs,indicating that OPEs did not magnify with trophic levels.?5?This study found that the concentrations of THg and MeHg in fish muscles in the TGR did not increase obviously during the period of 1986 to 2015,indicating that there was no significant"reservoir effect"at the initial impoundment stage in the TGR.The possible reasons as follows:1)low dissolved organic matter?DOC?content and high value of pH were adverse to mercury methylation;2)the organic matter content of submerged soil was low and the methylation activity was weak;3)submerged plants have a low degradation rate and release less mercury and nutrients into water,and meanwhile,submerged plants may play a role in absorbing and fixing mercury;4)the short and simple food chain structure,as well as the low trophic magnification factor?TMFs?of mercury in the TGR.Despite the low levels of THg and MeHg in fish muscle were determined in the TGR,mercury exposure may also be a risk for surrounding fishermen who frequently fed on fish.Besides,the risk factor value?RQ?was used to evaluate the hazards of OPEs in water to aquatic organisms.The results showed that the risk factors of the main OPEs pollutants?TCEP,TDCP,TCPP,TNBP and TIBP?were all less than 0.1,implying no toxic risk.Moreover,the daily intake?DI?was introduced to study the adverse effects of the content level of OPEs in fish on human health.The results showed that the daily intake of TCEP,TDCP,TNBP,TIBP,TPHP and TBOEP were far less than the daily reference dose?RfD?.The RQ values of TCEP?0.0000-0.0095?,TDCP?0.0000-0.0113?,TNBP?0.0000-0.0125?and TPHP?0.0000-0.0045?were all far less than 1,suggesting no risks to humans.