Mercury Pool In Soils Of Water Level Fluctuating Zone Of Three Gorges Reservior Region And Its Risk Assessment

Mercury is a highly toxic non-essential element with significant features of durable,easy mobility and a high degree of biological enrichment.Since the 20th century,due to,Minamata disease took place in Japan,mercury pollution problem has been the focus of global attention.After 20th century 80s,with the annual increasing of fossil fuel combustion,metal smelting and other anthropogenic processes to keep the atmospheric release of large amount of mercury,the global mercury concentrations increases year after year.Due to the special physical properties and long-distance transmission characteristics of mercury,the international community's concerns about the global mercury pollution have reached an unprecedented height.Furthermore,United Nations Environment Program and many other international organizations categorize mercury as a global priority pollutant,which leads a new round of studies about mercury pollution in the globe.While scholar studied the mercury pollution,the dam built in the river and formed reservoirs would radically affect the environmental conditions of this watershed,which caused the sequent results such as slowing down water flow rate,changing reoxygenation,deposition of large number of sediments, retention of nutrient matters,enhancement of primary productivity and lower redox potentials.Thus thses physical-chemical changes occurred with various others complicated processes,made a significant impact on temporal and spatial distribution of mercury in certain reservoir,and intensified the environmental influence of mercury in the aquatic eco-environment.All above indicated the reservoir is a typical mercury-sensitive ecosystem,and it's so necessary to study the mercury pollution problem.The Three Gorges Reservoir is the largest,but also world-renowned large reservoirs.The Three Gorges Reservoir area of 1084km²,submerged land area amount to 632 km²,which submerged cultivation lands(not including flood lands) are 275.1 km² with most high fecundity and multiple-crop index.The south bank of the Three Gorges reservoir area is located in the eastern margin of southwest mineralization zone of mercury,which belongs to high mercury background value zone of southeast of Sichuan province with a large mercury load from reservoir upstream and tributaries.According to research forecasts,the reservoir will increase the activation effect of mercury,which also enhances mercury concentrations by 0.35-1.5 times in fish body through intensified bio-enrichment.There exists a risk that mercury content exceeds food safety standard for fish.Thus it's necessary to focus on the mercury pollution in Three Gorges reservoir.The Three Gorges Reservoir is a large reservoir with one year period for adjustment.Accordance with the planning and design,the normal water level of reservoir on operation is 175m,peak height is 185m,and the limitation water level is 145m in lately of march for flood control.Because of this way of reservoir operation,Water level fluctuating zones in the reservoir form with 30m vertical height and more than 400km² areas.For the water level fluctuation zones of large reservoir,the reservoir is not only pool for runoff,but also a natural pool for mercury in environment.On the other hand,the lands of water level fluctuation zones are final buffer zones for reservoir,which affect on water quality due to water pool for mercury.Thus,reservoir is so important environmental condition for pollutants and water level fluctuation zones are both pool and source of mercury,which are mercury-sensitive zones.After operation of reservoir,the land(especially soils) of water level fluctuation zones experience the long-term alternation of wet and dry processes,the characteristics of soils will change sequentially,which make a significant impact on transformation and transfer of mercury,as well as water quality in reservoir.Currently,however there is lack of systematical researches about mercury load and inventory,meanwhile few reports presented the corresponding environmental effect.Thus,this research used the water level fluctuation zones of Three Gorges Reservoir,which most typical water fluctuating zone with distinctive alternation of wet and dry processes,as investigated subject.It investigated the forms characteristics and inventory of mercury,also assessed the environmental risk of mercury on soil of water level fluctuation zones,which showed a important significance to currently understand the source and release of mercury in global and regional range.In this research,soils from typical water level fluctuation zones were used as investigation subject, and soil of different districts and counties from the zones were sampled systematically.It discussed forms characteristics and transformation of mercury in soil according to different heights in whole crop fields, and then analyzed the factors,which would affect mercury formations in soils.Meanwhile,of analyze the contents of total mercury and various forms mercury,estimation was processed to know the inventory of total and various forms mercury in the investigated fields.Furthermore,it studied the adsorption and desorption characteristics from mercury pool of various types soils on mercury through batch adsorption and desorption experiments in lab.Finally,by the data sourced from all above processes,environmental risk assessment was processed to assess the water quality and environmental risk caused by mercury pool of water level fluctuation zones.The results were all as follows:Residual mercury was the dominated form in average concentration of mercury of all water level fluctuation zones soils.The average concentration of mercury in soils is 0.0839mg/kg,the result not only preponderate over 1.7 times of background value of Chongqing,but also preponderate over 2.1 times of China(0.040mg/kg).Also these showed that Mercury enrichment evidence in soils of water fluctuation areas.The average distribution coefficients were as the below order:residual mercury(61.4%)＞alkali-soluble mercury(16.1%)＞acid-soluble mercury(14.0%)＞H₂O₂-soluble mercury(6.5%)＞water-soluble mercury(2.0%).By analysis of effect of soil physic-chemistry properties on concentrations and forms of mercury,it indicated soil organic matter and soil total mercury content was highly significantly correlated(r=0.46^**,n = 158),but with soil sand,clay,CEC negative correlation.In addition,mercury concentration in soil was negatively correlated with soil pH,meanwhile most forms of mercury showed a significant negative correlation with silt and clay content in soil,which further suggested the mercury in water level fluctuation zones,was mainly from exogenous input.The changes of total mercury content from three water levers were 145-155m(0.0856mg/kg)＞155-165m(0.0804mg/kg)＞165-175m(0.0602mg/kg).There was a negative correlation between mercury content in soil and water level.In the same range of water heights,besides that mean mercury contents in right-side bank were higher than left-side in 145-155m,the other water heights showed a significant phenomena that right was higher than left.In both of banks,mean mercury contents changed with the identical trend of total mercury.On the other hand,there was a significant difference between mercury in soil layer and forms distribution.Various forms of mercury showed a positive relationship with organic matters due to different kinds of physic-chemical properties,while mercury increased with increasing of organic matters.In soil profile,total mercury,alkali-soluble mercury and residual mercury in different soil layers showed a high-high-low or high-low-high change trend.In mercury inventory of all water level fluctuation zones investigated,the ratios of different mercury forms in soil layers was identical with ratios of mercury forms in soil surfaces of water level fluctuation zones,which indicated the proportion of different mercury forms in soil mercury inventory by concentrations ratios of various mercury forms in soils. Total mercury inventory were 14951.65-33492.15kg in crop lands of water level fluctuation zones of Chongqing,with mean mercury inventory was 20615.35kg.Meanwhile mean soil mercury inventory was 7051.79kg in 0-20cm soil layer,which amounted 34.20%of all inventory.For 20-40cm soil layer,mean mercury inventory was 5172.70kg,which was 25.10%of all inventory.The mean mercury inventory was 8390.86kg,contributed 40.70%of all inventory.On the other hand,data indicated mean water-soluble mercury inventory was 270.97kg,which showed 81.46,113.61and 75.90kg of mean values in 0-20cm, 20-40 and40-60cm respectively.For acid-soluble mercury,mean value was 2995.23kg with 1559.33,1049.26 and 841.86kg respectively in three types soil layers.In addition,mean mercury inventory of alkali-soluble mercury was 3450.45kg,with 1559.33,1049.26and 841.86kg of above mentioned soil layers respectively.For H₂O₂-soluble mercury,in three depths soil layers the mean mercury inventory were 209.46,233.91and 200.11kg respectively,Which showed 650.16kg of total mean value.The rest form mercury was residual mercury,and mean inventory was 13255.52kg,which showed 4759.46,2499.55 and 5996.5 1kg in 0-20cm,20-40cm and 40-60cm soil layers respectively.Adsorption and desorption of mercury in soil have been significantly affected by properties of soil,as pH,organic matter and particle size.The maximum adsorption of mercury in soil follow the order of purple alluvial soil＞neutral purplish soil＞grey-brown alluvial soil.adsorption of Hg²⁺ in soil follow the order of:neutral purplish soil＞purple alluvial soil＞grey-brown alluvial soil.The adsorption rate of mercury in three soil follow the order of grey-brown alluvial soil＞purple alluvial soil＞neutral purplish soil.Moreover the desorption rate of mercury in soil gradually decreased with the raise of the adsorption. Meanwhile according to the result described by Langmuir model,the maximum desorption follow the order of grey-brown alluvial soil＞purple alluvial soil＞neutral purplish soil.The mercury desorption rate of the three types of soil were not too high,which the maximum desorption rate were 8.74%for gray-brown tide soil,for purple soil tide 5.85%,2.07%for neutral purple soil,respectively.Thus it indicated the mercury easily bound with soil to form into a steady state,and decreased mobility of mercury in soil.In soils of the investigated areas,neutral purple soil showed the maximum retention capacity of mercury brought by runoff from surrounding environment,which suggested the "sink" role of the soil is strongest,while mercury desorption of gray-brown tide soil mercury was the maximum,which meant the strength of as a "source" was highest.Soil mercury released from soil into water contributed a little to mercury concentration in water body. However,because of soil erosion,soil mercury released into water would be enriched and amplified mercury concentrations of water by food chain,which increased 43.2%average mercury concentration to 0.206mg/kg in fish from Chongqing part of Yangtze River.Meanwhile the sustained release of mercury further enhanced the mercury pollution potential risk of water body and fish.Through single-factor assessment,geo-accumulation index assessment and the ecological potential pollution index assessment,it indicated,except for a few places were affect by medium impact,the environmental impact of other parts of places were in safety range.Furthermore,this paper unveiled the health risk of mercury into human body by ingestion was far below the accepted limitation level(10^-6),indicating mercury in water level fluctuation zones showed little or no harm on human body.