【作者】 陈宏；

【导师】 王定勇； 魏世强；

【作者基本信息】 西南大学 ， 农业环境保护， 2009， 博士

【摘要】 汞是一种高毒性的非必需元素,具有持久性、易迁移性和高度的生物富集性等特征。自20世纪50年代初日本发生水俣病事件以来,汞污染问题一直受到人们的广泛关注。20世纪80年代后,随着逐年增加的化石燃料燃烧、金属冶炼等人为过程不断向大气环境释放大量的汞,使得全球大气汞浓度逐年增加。由于汞具有特殊的物理性质和长距离传输特性,使国际社会对全球汞污染问题的关注达到了前所未有的高度,被联合国环境规划署等很多国际机构列为全球性优先控制污染物,具有跨国污染的属性,由此引发了新一轮汞污染问题研究的热潮。人们在研究汞污染问题时发现,在河流上构筑大坝并形成水库后,水域的环境条件将发生极大的变化。水库内的流速减缓,复氧能力改变,泥沙大量沉积,营养物质滞留,水体的初级生产力提高,氧化还原电位降低。这些物理化学条件的变化和伴随发生的各种复杂过程,将会影响进入水库中汞的浓度及其时空分布,使汞对水生生态环境的影响程度将会加剧。可见水库是一个典型的“汞敏感生态系统”,对水库汞污染问题的研究极为重要。三峡水库是目前我国最大,也是举世瞩目的特大型水库。三峡水库面积1084km²,淹没的陆地面积就达632 km²。其中,淹没的耕园地（不包含河滩地）为275.1 km²,这些耕园地大多为三峡库区最肥沃、复种指数最高的农业用地。三峡库区南岸位于西南汞矿化带东缘、川东南高汞背景区,库区上游及支流输入的汞负荷巨大。根据研究预测,三峡水库蓄水后,库区汞活化效应将增强,鱼体对汞元素的生物富集作用加剧,水库鱼体汞含量将增加0.35～1.5倍左右,存在鱼体汞含量超过食品淡水鱼汞含量标准的风险。由此可见,三峡水库汞污染问题是倍受世人关注的重要问题。三峡水库属特大型年调节水库。按照规划设计,水库正常蓄水位高程175m,坝顶高程185m,5月底降至防洪限制水位145m。这种水库调度方式使得库周形成垂直高度为30m、面积400多km²的水库消落带。对大型水库消落带,由于是库区径流的汇集地带,自然也成为环境汞的汇集区。另一方面,消落带土壤作为水库的最后一道缓冲带,它所汇集的汞等污染物最终又会影响到水体质量。因此,对水库而言,消落带既是汞的汇,又是汞的源,是汞的敏感区。三峡水库蓄水运行后,消落带土壤将长期处于干湿交替的变换过程,土壤的性质将会发生一系列变化,从而对汞的迁移转化及库区的水质产生很大的影响。目前库区消落带土壤汞赋存量及对环境的影响如何,对这一方面的系统研究相对缺乏。为此,本研究拟以我国最具特色的周期性干湿交替环境——三峡库区消落带为研究对象,开展消落带土壤汞的形态特征及其汞库存量的研究,并对消落带土壤汞对水环境的影响进行风险评估,对正确认识区域及全球大气汞的来源及释放具有重要意义。本研究以三峡库区重庆段消落带土壤为研究对象,对库区各区县消落带范围的土壤进行系统采样,研究整个消落带耕园土壤及不同高程范围土壤汞及其赋存形态与变化特征,并分析影响汞赋存状态的因素,通过分析样品总汞及各形态汞含量,估算区域内耕园土壤汞库中总汞及各形态汞库存量。通过吸附解吸试验,研究不同类型土壤的吸附解吸特征,通过对土壤汞的库存量、汞的赋存状态、吸附解吸特征的分析,评价消落带土壤汞库对水环境带来的风险。主要研究结果如下:整个消落带土壤中汞平均含量0.0839mg/kg,为重庆市土壤平均背景值的1.7倍、我国背景值（0.04mg/kg）的2.1倍,表明消落带土壤具有明显的汞富集趋势。土壤汞赋存形态以残留态为主,各形态汞的平均分配系数依此为:残留态（61.4%）＞碱溶态（16.1%）＞酸溶态（14.0%）＞过氧化氢态（6.5%）＞水溶态（2.0%）。对土壤理化性质与汞含量及形态的相关分析表明,土壤有机质与土壤总汞含量呈极显著相关（r=0.46**,n=158）,而与土壤砂粒、粘粒、CEC呈负相关关系。土壤汞含量与土壤pH呈负相关关系,总汞和绝大部分形态的汞都和粉粒和粘粒含量呈负相关关系。研究表明三峡库区消落带土壤中的汞主要来源于外源性输入。在3个水位高程范围总汞含量的变化为145～155m（0.0856mg/kg）＞155～165m（0.0804mg/kg）＞165～175m（0.0602mg/kg）,土壤汞含量与高程呈负相关关系。同一高程范围,除在145～155m范围内右岸总汞平均含量高于左岸外,在其它两个高程层均表现为右岸总汞平均含量高于左岸,两岸平均汞含量随高程变化也表现为总汞相一致。土壤剖面汞及其形态分布存在着显著的差异,并受各种理化性质的影响,各形态汞与有机质含量几乎都表现为正相关,随有机质含量的增加而增加。土壤剖面中总汞、碱溶态汞和残留态汞在土壤各层中的分布大体呈现出高-高-低或高-低-高的变化趋势。在整个消落带土壤汞库中,各层土壤汞的形态比例和消落带表层土壤中各形态汞所占的比例关系基本一致,可以通过土壤中各形态汞的含量比例关系来近似反映土壤汞库中各形态汞的比例关系。全市消落带耕园地总汞库存量为14951.65-33492.15kg,平均汞库存量为20615.35kg。其中0-20cm层土壤汞平均库存量为7051.79kg,占总汞库存量的34.20%;20-40cm层土壤汞平均库存量为5172.70kg,占总汞库存量的25.10%;40-60cm层土壤汞平均库存量为8390.86kg,占总汞库存量的40.70%。全市消落带耕园地水溶态汞平均库存量为270.97kg。0-20cm、20-40cm、40-60cm各层土壤水溶态汞平均库存量分别为81.46、113.61和75.90kg。酸溶态汞平均库存量为2995.23kg,土壤各层平均库存量分别为442.19、1276.47和1276.57kg。碱溶态汞平均库存量为3450.45kg,土壤各层平均汞库存量分别为1559.33、1049.26和841.86kg。过氧化氢溶态汞平均库存量为650.16kg,土壤各层平均库存量分别为209.46、233.91和200.11kg。残留态汞平均库存量为13255.52kg,土壤各层平均库存量分别为4759.46、2499.55和5996.51kg。土壤pH、有机质、粒径等土壤理化性质对土壤汞吸附解吸有很明显的影响。土壤对汞的最大吸附量顺序为:紫色潮土＞中性紫色土＞灰棕潮土,对Hg²⁺的吸附作用力顺序为:中性紫色土＞紫色潮土＞灰棕潮土。3种土壤吸附汞的速率顺序为灰棕潮土＞紫色潮土＞中性紫色土。土壤对汞的吸附作用力越大,其解吸率也就越小。根据Langmuir方程拟合得出的最大解吸量顺序为灰棕潮土＞紫色潮土＞中性紫色土。3种土壤对汞的解吸率都不太高,最大解吸率分别为灰棕潮土8.74%,紫色潮土5.85%,中性紫色土2.07%,说明汞容易与土壤结合且比较稳定,在土壤中的移动性比较弱。在三峡水库消落带土壤中,中性紫色土对周边环境地表径流带来的汞的吸持能力最强,即“汇”的作用大,而灰棕潮土对汞的解吸率最大,即“源”的强度高。土壤汞释放进入水体后对水体中汞含量的影响非常小,但由于水土流失、土壤汞释放到水体中的汞会通过食物链的富集放大,使长江流域重庆段鱼肉中汞的平均含量达到0.206mg/kg,增幅达43.2%,汞的持续释放会使汞对水体和鱼类潜在风险会进一步增加。通过单因子评价、地累积指数评价和生态潜在污染指数评价,消落带土壤除个别地方受中等影响外,其它地方的汞的环境影响都在安全范围。通过土壤汞的健康风险评价,发现Hg经摄食途径进入人体的健康风险远低于可以接受的水平(10^-6),表明消落带土壤汞含量对人体基本不会产生危害。更多还原

【Abstract】 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.更多还原