江汉平原浅层含水层系统中砷释放与迁移过程研究
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摘要
砷是环境中存在的一种氧化还原活跃的微量元素,它对于人类赖以生存的自然环境及人体健康有着巨大的影响,尤其是当它处于水体环境时,它对人体健康的危害更大。目前,起源于喜马拉雅山脉的砷对于南亚和东南亚的地下水造成了严重污染,在这里有大约1亿的人正在以未经处理的砷含量比较高的不安全的地下水作为饮用水源。这些人口主要分布在印度、中国、缅甸、巴基斯坦、越南、尼泊尔及柬埔寨等国家。目前关于地下水中砷的来源、迁移转化机理及其人体健康效应的相对全面系统的研究主要在恒河流域、红河流域及湄公河流域。在2005年,江汉平原首次报道了地下水砷中毒事件,这之后长江流域的地下水砷污染问题也引起了人们的关注。
地下水中的砷原本存在于沉积物中,在特定的地球化学条件下由固相迁移到液相,砷释放机理及进入液相之后在地下水中滞留时间一直是高砷地下水研究的两个重要方面。另外,人类活动对于地下水中砷含量的影响也一直是人们关注的热点问题。本文系统揭示了江汉平原典型高砷地下水系统中水动力条件和生物地球化学过程耦合作用下的砷的地球化学行为。本文不仅可以对当地地下水资源管理提供指导,而且可以为世界高砷地下水研究提供对比。
我们整个研究是在位于江汉平原沙湖原种农场的一个高砷地下水监测场中开展。这个监测场位于东荆河北侧,面积约12km2。在这个监测场中,我们在13个点建了39口监测井,每个点布设3口监测井,分别为监测10m、25m及50m深度的地下水水位和水化学特征。除了监测井外,我们还对研究区内的地表水体的水位及水化学进行了监测。整个监测过程持续了一个水文年,其中水位观测每半月进行一次,水化学观测每一个月进行一次。通过一年的监测,我们发现在我们的研究区沉积物仍在向地下水大量释放砷,而且地下水中的砷含量有着很强的时空变异性。垂向上来看,地下水中的砷主要集中在25m的深度,水平来看砷分布极不规则,在很小的范围内地下水砷含量可以差别很大,高砷地下水出现的位置附近都分布着池塘或者灌溉渠道等地表水体。从时间上来看,在雨季地下水中砷含量大量升高,旱季大量降低。这种现象比较罕见,只在柬埔寨一些强烈的地下水补给或排泄区观察到过。
关于沉积物释放砷的机理有着各种理论,有些学说认为是由于含砷矿物的氧化还原降锌,有些认为是由于磷酸盐的竞争吸附。目前广为人们所接受的解释处是铁氧化物的还原性溶解过程中伴随的砷的释放。为了揭示研究区沉积物释放砷的潜在机理及其发生的位置,我们对研究区的沉积物进行了基于同步辐射技术的XANES分析和实验室接种培养实验,发现上覆隔水层的粘土沉积物和25m以上的含砂层沉积物在有有机碳的条件下都有释放砷的能力,而30m以下的含水砂层沉积物几乎不具有释放砷的能力,因为砷主要是以硫化物的形式存在,这种矿物在还原条件下状态非常稳定。
江汉平原高砷地下水的另一个显著特征是水中硫化物含量非常高。在我们的研究区地表水体中硫酸盐含量很高,导致地下水系统中会发生强烈的硫酸盐还原作用。为阐明水中硫化物对砷释放迁移的影响,我们采用浅层及深层的沉积物研究了流动条件下,砷在硫化物影响下的地球化学行为。研究表明,硫化物的存在可以短暂促进砷的释放,但长期来看,硫化物会促进铁氧化物向次生矿物的演变,生成的次生矿物一方面会钝化铁氧化物表面,阻止砷的进一步释放,另一方面生成的次生矿物如磁铁矿等对砷有更强的吸附控制能力,不利于砷在液相的迁移。
我们已经证实江汉平原的沉积物在有有机碳存在的条件下目前仍然具有释放砷的能力,那么目前的关键问题就在于地下水系统中目前是否有充足的有机碳供微生物作用释放砷。通过我们的分析,发现我们的系统中沉积物中的有机碳基本上已经被耗尽,地下水中的有机碳主要是通过地表水的补给来获取。这样有机碳的补给途径就会被系统的水文地质条件所控制。为了研究地下水系统的水文地质条件对地下水中砷的地球化学行为的影响,我们对研究区地下水系统的水流场进行了刻画。通过我们的野外监测和室内模拟,我们得出研究区的地下水流场为：地表补给的水体在上覆隔水层垂直流动进入含水层,然后受区域地下水流控制在含水层进行水平流动。砷在上覆粘土层被释放后垂向迁移进入砂质含水层然后被运输到地下水系统的排泄区。我们研究区的地下水系统与孟加拉及柬埔寨的地下水体统相比,垂向地下水水流非常快,而水平地下水流非常慢,而这也是导致地下水系统中砷时空变化的最主要的原因。
通过前面的分析,我们可以看出江汉平原高砷地下水的形成及分布是水动力-水化学过程耦合作用的结果,而这些过程在进行时又受区域水文地质条件尤其是沉积环境的控制和人类活动的影响。为进一步揭示砷释放迁移过程中的水动力-水化学过程耦合效应,我们采用一维及二维反应性溶质运移模拟对地下水中的砷分布进行了刻画。模拟结果进一步验证了我们通过对沉积物分析得出的砷是由上覆粘土及25m以上砂质沉积物释放进而被运移到含水层的模型。砷释放所需要的有机碳主要通过地表池塘及灌溉渠道等水体补给得到,而人类活动对地表水的污染进一步加剧了沉积物向地下水释放砷,这主要是通过地表水补给的硫酸盐还原产生大量硫化物对铁氧化物还原性溶解实现的。
本文的创新点主要体现在：(1)对研究区的沉积物进行了基于同步辐射技术的光谱学(XANES)、实验室接种培养及连续化学提取等全面的分析；(2)对地下水系统进行了长期连续的水动力水化学监测；(3)对研究区高砷地下水系统硫化物对砷地球化学行为的影响开展了针对性的专项实验进行研究；(4)将沉积物实验室分析结果与野外水化学水动力学监测数据耦合进行了反应性溶质运移模拟,系统分析了江汉平原高砷地下水成因及时空分布特征；(5)对江汉平原典型的砷污染含水层系统进行了从分子尺度(沉积物分析)到场地尺度(水文地质监测)的复合研究。
Arsenic (As) is a redox-sensitive toxin that severely impacts environmental quality and human health and it becomes most hazardous when conditions are such that it partitions to the aqueous phase. Himalayan-derived As is contaminating groundwater in South and Southeast Asia. It was estimated that there are over100million individuals exposed to unsafe As levels by drinking untreated groundwater in India, China, Myanmar, Pakistan, Vietnam, Nepal, and Cambodia. Systematic and comprehensive research on the fate and transport, as well as the health effects, of As have been taken mostly in areas of the Ganges-Brahmaputra-Meghna River system, Red River system, and Mekong River system. With the first report of groundwater arsenic poisoning in Jianghan plain,2005, Yangtze River system was added to the list.
While arsenic is native to the sediments, its mechanism of mobilization into the aqueous phase and its subsequent residence time in groundwater are the focus for science community and sustainable water management. In addition, the human-induced effects on arsenic concentrations are currently a topic of intense debate. The research presented in this thesis elucidates the coupled hydrological and biogeochemical processes controlling arsenic concentrations within aquifers of Jianghan Plain. In addition to guide water management in Jianghan Plain, this work also provides contrast to research in South and Southeast Asia where arsenic contamination of groundwater represents the largest mass poisoning in history.
Our research work was carried out in a typical high-arsenic groundwater system in a field monitoring site (about12km2) along Dongjing River which is a tributary of Hanjiang River. In this study area, we constructed a field monitoring nest including39monitoring wells at three depths (10m,25m and50m) in the groundwater and some other sites in adjacent surface water bodies like rivers, channels and ponds. With this monitoring nest, we conducted hydrogeological monitoring of the groundwater and adjacent surface water for a whole hydrological year. And from our field monitoring results, arsenic release is still going on at current time. And most importantly, groundwater As exhibits large spatial and temporal variations. Spatially high arsenic concentrations were located in25m wells at sites adjacent to surface water bodies of ponds and irrigation channels. Temporally, there is obvious arsenic increase during rainy season and decrease during dry season in groundwater.
Various theories have been put forth regarding the modes of arsenic release to the aqueous phase, ranging from the oxidative or reductive degradation of arsenic-bearing solids to competitive ligand displacement by phosphate. Reductive dissolution of Fe(III)(hydr)oxides and concomitant arsenic release has become the most widely accepted explanation of high arsenic groundwater concentrations. In order to evaluate the potential mechanisms of arsenic desorption to groundwater and at where sediment is releasing arsenic, spectroscopic and laboratory batch incubation experiments were conducted with aquifer sediments from the study area. Our results indicated that both the upper aquitard clay sediments and the lower aquifer sand sediments have great potential to release arsenic if sufficient fresh organic carbon is supplied.
Another significant feature of the high-arsenic groundwater in the study area is the high dissolved sulfide. In our study area, the recharging surface water often contains high sulfate leading to great sulfate reduction in the groundwater system. To elucidate the influence of high dissolved sulfide on arsenic release and retention, we examined arsenic release and adsorption under high sulfide condition in flow system with natural sediments. The results indicate that, although dissolved sulfide is a strong reductant it can only promote arsenic release for a transient time. Over longer time, it will promote arsenic retention to more stable minerals via promoting iron minerals secondary transformation.
With the evidence that the above25m sediment has the potential to release arsenic currently, an important question is whether there is sufficient organic carbon to be recharged to fuel microbially-mediated arsenic releasing. Organic carbon in the aquifer sediments is almost depleted according to our analysis. The major source of organic carbon should be the recharging surface water. The possible way for the infiltration of organic carbon into the aquifers is via the leaky aquitards. To advance understanding of hydrological influences on As behavior within groundwater of Jianghan Plain, the flow system of an As-rich aquifer in Jianghan Plain along Yangtze River was examined. From our results, groundwater flow in this system is vertically through the upper clay layers and horizontally through the sandy aquifer material below. Arsenic released via reductive dissolution within the upper clay layers will transport downward into the aquifer and then horizontally toward a point of discharge. However, the particularity of this system is that the vertical flow is very fast and the horizontal flow is pretty slow, leading to the significant spatial and temporal variations of arsenic concentrations in groundwater.
The current distribution of arsenic in aquifers of Jianghan Plain is the result of coupled biogeochemical and hydrologic process, which vary depending on the sedimentology of a specific area and local human perturbations. To reveal the impact of conductive the biogeochemical and hydrogeologic conditions on arsenic release in this study area, we used one and two dimensional reactive transport modeling calibrated with biogeochemical and hydrogeologic field data to simulate the subsurface distribution of arsenic. The results of our simulations support the concept model of arsenic release from upper clay layers and below sand aquifer, where horizontal transport of arsenic is extremely weak, leading to the erratic spatial distribution of arsenic. Constructed ponds and channels provide sufficient source of fresh organic matter and pollutants via leaky aquitards to facilitate arsenic release by sulfidization in the aquifers.
The research presented in this thesis seeks to discern the dominant factors controlling arsenic concentrations in Jianghan Plain. The distinguish features of this research include:(1) Synchrotron based XANES, batch incubation and sequential extraction are integrated to analyze the sediment;(2) Long-term continued hydrodynamic-hydrochemical monitoring was conducted in the field;(3) Contraposing the sulfide influence of arsenic release and retention, specific column experiment was carried out;(4) Reactive transport modeling coupling solid phase results and field hydrodynamic-hydrochemical monitoring results was conducted to illustrate the special and temporal distribution of groundwater arsenic;(5) The research was carried out in the typical arsenic contaminated aquifer system, probing a range of spatial scales, from the molecular level to the field scale.
地下水中的砷原本存在于沉积物中,在特定的地球化学条件下由固相迁移到液相,砷释放机理及进入液相之后在地下水中滞留时间一直是高砷地下水研究的两个重要方面。另外,人类活动对于地下水中砷含量的影响也一直是人们关注的热点问题。本文系统揭示了江汉平原典型高砷地下水系统中水动力条件和生物地球化学过程耦合作用下的砷的地球化学行为。本文不仅可以对当地地下水资源管理提供指导,而且可以为世界高砷地下水研究提供对比。
我们整个研究是在位于江汉平原沙湖原种农场的一个高砷地下水监测场中开展。这个监测场位于东荆河北侧,面积约12km2。在这个监测场中,我们在13个点建了39口监测井,每个点布设3口监测井,分别为监测10m、25m及50m深度的地下水水位和水化学特征。除了监测井外,我们还对研究区内的地表水体的水位及水化学进行了监测。整个监测过程持续了一个水文年,其中水位观测每半月进行一次,水化学观测每一个月进行一次。通过一年的监测,我们发现在我们的研究区沉积物仍在向地下水大量释放砷,而且地下水中的砷含量有着很强的时空变异性。垂向上来看,地下水中的砷主要集中在25m的深度,水平来看砷分布极不规则,在很小的范围内地下水砷含量可以差别很大,高砷地下水出现的位置附近都分布着池塘或者灌溉渠道等地表水体。从时间上来看,在雨季地下水中砷含量大量升高,旱季大量降低。这种现象比较罕见,只在柬埔寨一些强烈的地下水补给或排泄区观察到过。
关于沉积物释放砷的机理有着各种理论,有些学说认为是由于含砷矿物的氧化还原降锌,有些认为是由于磷酸盐的竞争吸附。目前广为人们所接受的解释处是铁氧化物的还原性溶解过程中伴随的砷的释放。为了揭示研究区沉积物释放砷的潜在机理及其发生的位置,我们对研究区的沉积物进行了基于同步辐射技术的XANES分析和实验室接种培养实验,发现上覆隔水层的粘土沉积物和25m以上的含砂层沉积物在有有机碳的条件下都有释放砷的能力,而30m以下的含水砂层沉积物几乎不具有释放砷的能力,因为砷主要是以硫化物的形式存在,这种矿物在还原条件下状态非常稳定。
江汉平原高砷地下水的另一个显著特征是水中硫化物含量非常高。在我们的研究区地表水体中硫酸盐含量很高,导致地下水系统中会发生强烈的硫酸盐还原作用。为阐明水中硫化物对砷释放迁移的影响,我们采用浅层及深层的沉积物研究了流动条件下,砷在硫化物影响下的地球化学行为。研究表明,硫化物的存在可以短暂促进砷的释放,但长期来看,硫化物会促进铁氧化物向次生矿物的演变,生成的次生矿物一方面会钝化铁氧化物表面,阻止砷的进一步释放,另一方面生成的次生矿物如磁铁矿等对砷有更强的吸附控制能力,不利于砷在液相的迁移。
我们已经证实江汉平原的沉积物在有有机碳存在的条件下目前仍然具有释放砷的能力,那么目前的关键问题就在于地下水系统中目前是否有充足的有机碳供微生物作用释放砷。通过我们的分析,发现我们的系统中沉积物中的有机碳基本上已经被耗尽,地下水中的有机碳主要是通过地表水的补给来获取。这样有机碳的补给途径就会被系统的水文地质条件所控制。为了研究地下水系统的水文地质条件对地下水中砷的地球化学行为的影响,我们对研究区地下水系统的水流场进行了刻画。通过我们的野外监测和室内模拟,我们得出研究区的地下水流场为：地表补给的水体在上覆隔水层垂直流动进入含水层,然后受区域地下水流控制在含水层进行水平流动。砷在上覆粘土层被释放后垂向迁移进入砂质含水层然后被运输到地下水系统的排泄区。我们研究区的地下水系统与孟加拉及柬埔寨的地下水体统相比,垂向地下水水流非常快,而水平地下水流非常慢,而这也是导致地下水系统中砷时空变化的最主要的原因。
通过前面的分析,我们可以看出江汉平原高砷地下水的形成及分布是水动力-水化学过程耦合作用的结果,而这些过程在进行时又受区域水文地质条件尤其是沉积环境的控制和人类活动的影响。为进一步揭示砷释放迁移过程中的水动力-水化学过程耦合效应,我们采用一维及二维反应性溶质运移模拟对地下水中的砷分布进行了刻画。模拟结果进一步验证了我们通过对沉积物分析得出的砷是由上覆粘土及25m以上砂质沉积物释放进而被运移到含水层的模型。砷释放所需要的有机碳主要通过地表池塘及灌溉渠道等水体补给得到,而人类活动对地表水的污染进一步加剧了沉积物向地下水释放砷,这主要是通过地表水补给的硫酸盐还原产生大量硫化物对铁氧化物还原性溶解实现的。
本文的创新点主要体现在：(1)对研究区的沉积物进行了基于同步辐射技术的光谱学(XANES)、实验室接种培养及连续化学提取等全面的分析；(2)对地下水系统进行了长期连续的水动力水化学监测；(3)对研究区高砷地下水系统硫化物对砷地球化学行为的影响开展了针对性的专项实验进行研究；(4)将沉积物实验室分析结果与野外水化学水动力学监测数据耦合进行了反应性溶质运移模拟,系统分析了江汉平原高砷地下水成因及时空分布特征；(5)对江汉平原典型的砷污染含水层系统进行了从分子尺度(沉积物分析)到场地尺度(水文地质监测)的复合研究。
Arsenic (As) is a redox-sensitive toxin that severely impacts environmental quality and human health and it becomes most hazardous when conditions are such that it partitions to the aqueous phase. Himalayan-derived As is contaminating groundwater in South and Southeast Asia. It was estimated that there are over100million individuals exposed to unsafe As levels by drinking untreated groundwater in India, China, Myanmar, Pakistan, Vietnam, Nepal, and Cambodia. Systematic and comprehensive research on the fate and transport, as well as the health effects, of As have been taken mostly in areas of the Ganges-Brahmaputra-Meghna River system, Red River system, and Mekong River system. With the first report of groundwater arsenic poisoning in Jianghan plain,2005, Yangtze River system was added to the list.
While arsenic is native to the sediments, its mechanism of mobilization into the aqueous phase and its subsequent residence time in groundwater are the focus for science community and sustainable water management. In addition, the human-induced effects on arsenic concentrations are currently a topic of intense debate. The research presented in this thesis elucidates the coupled hydrological and biogeochemical processes controlling arsenic concentrations within aquifers of Jianghan Plain. In addition to guide water management in Jianghan Plain, this work also provides contrast to research in South and Southeast Asia where arsenic contamination of groundwater represents the largest mass poisoning in history.
Our research work was carried out in a typical high-arsenic groundwater system in a field monitoring site (about12km2) along Dongjing River which is a tributary of Hanjiang River. In this study area, we constructed a field monitoring nest including39monitoring wells at three depths (10m,25m and50m) in the groundwater and some other sites in adjacent surface water bodies like rivers, channels and ponds. With this monitoring nest, we conducted hydrogeological monitoring of the groundwater and adjacent surface water for a whole hydrological year. And from our field monitoring results, arsenic release is still going on at current time. And most importantly, groundwater As exhibits large spatial and temporal variations. Spatially high arsenic concentrations were located in25m wells at sites adjacent to surface water bodies of ponds and irrigation channels. Temporally, there is obvious arsenic increase during rainy season and decrease during dry season in groundwater.
Various theories have been put forth regarding the modes of arsenic release to the aqueous phase, ranging from the oxidative or reductive degradation of arsenic-bearing solids to competitive ligand displacement by phosphate. Reductive dissolution of Fe(III)(hydr)oxides and concomitant arsenic release has become the most widely accepted explanation of high arsenic groundwater concentrations. In order to evaluate the potential mechanisms of arsenic desorption to groundwater and at where sediment is releasing arsenic, spectroscopic and laboratory batch incubation experiments were conducted with aquifer sediments from the study area. Our results indicated that both the upper aquitard clay sediments and the lower aquifer sand sediments have great potential to release arsenic if sufficient fresh organic carbon is supplied.
Another significant feature of the high-arsenic groundwater in the study area is the high dissolved sulfide. In our study area, the recharging surface water often contains high sulfate leading to great sulfate reduction in the groundwater system. To elucidate the influence of high dissolved sulfide on arsenic release and retention, we examined arsenic release and adsorption under high sulfide condition in flow system with natural sediments. The results indicate that, although dissolved sulfide is a strong reductant it can only promote arsenic release for a transient time. Over longer time, it will promote arsenic retention to more stable minerals via promoting iron minerals secondary transformation.
With the evidence that the above25m sediment has the potential to release arsenic currently, an important question is whether there is sufficient organic carbon to be recharged to fuel microbially-mediated arsenic releasing. Organic carbon in the aquifer sediments is almost depleted according to our analysis. The major source of organic carbon should be the recharging surface water. The possible way for the infiltration of organic carbon into the aquifers is via the leaky aquitards. To advance understanding of hydrological influences on As behavior within groundwater of Jianghan Plain, the flow system of an As-rich aquifer in Jianghan Plain along Yangtze River was examined. From our results, groundwater flow in this system is vertically through the upper clay layers and horizontally through the sandy aquifer material below. Arsenic released via reductive dissolution within the upper clay layers will transport downward into the aquifer and then horizontally toward a point of discharge. However, the particularity of this system is that the vertical flow is very fast and the horizontal flow is pretty slow, leading to the significant spatial and temporal variations of arsenic concentrations in groundwater.
The current distribution of arsenic in aquifers of Jianghan Plain is the result of coupled biogeochemical and hydrologic process, which vary depending on the sedimentology of a specific area and local human perturbations. To reveal the impact of conductive the biogeochemical and hydrogeologic conditions on arsenic release in this study area, we used one and two dimensional reactive transport modeling calibrated with biogeochemical and hydrogeologic field data to simulate the subsurface distribution of arsenic. The results of our simulations support the concept model of arsenic release from upper clay layers and below sand aquifer, where horizontal transport of arsenic is extremely weak, leading to the erratic spatial distribution of arsenic. Constructed ponds and channels provide sufficient source of fresh organic matter and pollutants via leaky aquitards to facilitate arsenic release by sulfidization in the aquifers.
The research presented in this thesis seeks to discern the dominant factors controlling arsenic concentrations in Jianghan Plain. The distinguish features of this research include:(1) Synchrotron based XANES, batch incubation and sequential extraction are integrated to analyze the sediment;(2) Long-term continued hydrodynamic-hydrochemical monitoring was conducted in the field;(3) Contraposing the sulfide influence of arsenic release and retention, specific column experiment was carried out;(4) Reactive transport modeling coupling solid phase results and field hydrodynamic-hydrochemical monitoring results was conducted to illustrate the special and temporal distribution of groundwater arsenic;(5) The research was carried out in the typical arsenic contaminated aquifer system, probing a range of spatial scales, from the molecular level to the field scale.
引文
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