【作者】 李梦娣；

【导师】 王焰新；

【作者基本信息】 中国地质大学 ， 地下水科学与工程， 2013， 博士

【摘要】 地下水饮用水源中高含量的砷(As)的毒性已经引起全球范围关注。在本次研究的大同盆地,全新世含水层中天然成因的砷破坏了居民安全饮用水的长期供应。世界范围的学者针对天然环境中砷的迁移转化提出了多种不同的机制,以及详细的砷地球化学行为的反应模型。含水层的地质属性控制的氧化还原条件,金属氧化物/氢氧化物和相关的生物地球化学过程,决定性地影响着地下水中砷的浓度。最近,人为活动因素,如农业灌溉,对地下水系统中砷的富集的影响引起了越来越多的重视。本论文研究主体旨在揭示大同盆地地地下水系统中砷的含量,以及水文,地球化学和生物地球化学过程对砷迁移的影响。综合同位素学以及地下水水文地球化学揭示了氧化还原反应,吸附/解吸,和生物地球化学过程的主要控制因素。解译变量包括氧同位素(δ18O-H2O)和氢同位素(δD和3H),硝酸盐氮同位素(δ15N-NO3),硫酸盐和硫化物的硫同位素(δ34S-SO4和δ34S-sulfide),硫酸氧同位素(δ18O-SO4)和钼同位素(δ98/95Mo)。结果表明,多同位素综合研究可有效地揭示氧化还原条件变化和伴随的氧化还原反应引起的砷的释放和固定。依据地下水的水化特征,大同盆地地下水可分为四个主要类型：Na-HCO3水,Ca+Mg-HCO3水,Na-Cl和Na-Cl+SO4水。调查揭示了局部和区域尺度的地下水成分的变化特点,来自边山补给区的地下水(主要为Ca+Mg-HCO3型水)中砷的浓度水平普遍较低,而Na-HCO,水中砷含量显著。地下水地球化学的另一个显著的特征是,Mg2+浓度普遍较高,尤其在盆地中心地下水中。Mg富集被认为是矿物风化(如白云石、菱镁矿),人为输入(如化肥)和微生物的活动(如细菌作用的硫酸根还原)的综合作用。大同盆地沉积物中富含有机质,旺盛的细菌代谢过程影响着含水层中主要溶质,包括Mg, Ca, SO4, HCO3的含量。相比盐渍化过程,氧化还原条件对地下水中砷的富集有着更加决定性的作用。地下水中砷与氯没有发现明显的相关性。由此进一步推断,蒸发作用对砷的迁移不存在控制性的影响。富砷地下水中氧化还原指标测定表明砷上要富集在还原性环境中。地下水中检测到的S2-和NH4+可作为微生物活动的信号,如细菌作用的硫酸盐还原和反硝化过程。硫酸根还原作用引起的砷的迁移和固定可能同时存在于含水层中,而这两个进程的优先级被由Fe2+和S2-浓度决定。因为相关的硫化物溶解度的控制其与砷的共沉淀过程。水同位素(δ18O-H2O和δD)比值揭示了大同盆地地下水的降水来源。δ18O和δD相对于当地降水线的偏移是由孔隙水蒸发,垂直补给以及地下水的混合的综合作用导致。深层地地下水相对均一的δ18O值反映沿着地下水流径的混合作用。地下水同位素组成上要控制因素有：(1)降雨；(2)蒸发作用;(3)与不同同位素组成的灌溉回水的混合作用。老的深层地下水从深层含水层被抽取出并与相对年轻的地下水混合导致了地下水中氚浓度与深度的非相关关系。砷浓度在不同年龄的地下水中跨度较大。地下水滞留年龄与砷的富集没有直接的关联。但是比较而言,年轻的地下水中砷的浓度相对较高。通过从不同深度地下水氚浓度的解译,不同年龄和不同砷浓度的的灌溉地下水之间的混合过程对含水层中砷的分布有着重大影响。微生物的代谢过程涉及许多重要的地球化学过程,包括氧化/还原反应,沉淀/溶解和吸附/解吸过程,这些过程都能显著的影响砷的地球化学的命运。为了揭示氮循环过程对砷的影响,对地下水中硝酸盐氮同位素(δ15N-N03)进行了检测。氮同位素研究表明地下水中较高的615N-N03值(高达+28‰)可能来源于动物粪便的点输入。奶牛及羊畜牧业是大同盆地重要产业。硝酸盐浓度和δ15N-N03比值之间的负相关关系表明氮同位素明显经历了显著的反硝化过程作用所致同位素分馏。在一定条件下,含水层中反硝化过程可以由黄铁矿氧化驱动。反硝化引起的铁的(氢)氧化物转化(氧化/还原溶解),可促进吸附在其表而的砷向地下水中的释放。大同盆地地下水中,砷的浓度与(δ15,N-N03存在不明显的的正相关关系,表明反硝化过程及其引起的铁的化合物的溶解会引起砷在地下水中的富集。为了更好地理解硫的生物地球化学循环的对砷迁移的影响,对地下水中硫酸盐和硫化物的同位素特征进行了分析。大同盆地地下水中硫酸根高δ34S-S04比值伴随着高的δ18O-s04值可能来源于细菌参与的硫酸盐还原生物硫循环(BSR)和BSR中间产物的再氧化。相对高的δ34S值表明地下水中的硫酸根不太可能来源于硫化物氧化。另一方而,碳酸盐+硫酸盐(CAS)蒸发盐可能为潜在的硫酸根源。普遍偏高的δ18O-so4值,偏离于CAS蒸发和肥料的氧同位素值,可能源于来自氮循环中的硝化、反硝化和硫化物的氧化过程,将较高的δt8O大气中的氧纳入S042-。不同δ34S-So4和δ18O-so4值的灌溉水的混合也在地下水硫酸盐同位素组成控制起着重要作用。较大的硫同位素分馏Δ34Ssulfide-S04与砷的浓度成正相关。含水层沉积物中砷释放到地下水的机制过程可以解释为：由BSR产生的硫化物氧化与反硝化过程相互影响；NO3-消耗导致铁的(氢)氧化物的还原溶解(例如,针铁矿)进而释放吸附的砷。本研究首次报道了地下水中溶解Mo同位素组成((δ98/95Mo)。大同盆地地下水中钼同位素范围为-0.12‰～+2.17%o,平均比值为+1.1‰。桑干河水中(δ98Mo(+0.72‰)与文献记录的平均河流δ98Mo值相当。相比于静海相环境,地下水中的硫化物浓度显著偏低。然而,S2-检测结果显示Mo-Fe-S化合物可能缓慢形成于特定条件下,这可能会导致可以别的钼同位素分馏。S2-和δ08Mo呈现出正相关,原因可能由于在一些地下水环境中,轻Mo同位素优先沉淀形成的钼的硫化物导致溶液中的钼同位素升高。相比于锰,地下水中δ98Mo与铁的相关性更紧密。地下水δ98Mo比值的增加并伴随着钼含量的降低可能是由于铁化合物的还原溶解及对钼的再吸附循环过程。砷浓度与δ98Mo比值呈现出弱正相关,表明铁的还原溶解导致砷为释放到地下水中,而钼同位素分馏可以作为此过程的间接指示。此外,根据钼同位素分析,本论文提出了一个新的砷活化机制：由于砷和钼具有相似的形成条件,钼在硫化物的共沉淀过程中与砷形成竞争,从而促进砷向地下水中的释放。更多还原

【Abstract】 Elevated levels of arsenic (As) in groundwater has caused worldwide attention due to the risk of As toxicity from drinking water sources. In Datong Basin for this study, naturally occurring arsenic (As) in Holocene aquifers has undermined the success of supplying the residents with safe drinking water. Diverse mechanisms governing arsenic mobilization in natural environments have been scrutinized by worldwide scholars to contribute profound perspectives and detailed reacting models of arsenic geochemical behavior. As concentrations in groundwater is predominantly managed by the redox conditions, presence of metal oxides/hydroxides and related biogeochemical processes, which are governed by the geological attributes of the aquifers. More recently, anthropogenic influences, such as agricultural irrigation, has caught increasingly greater attention to the As enrichment in groundwater systems.The research work contained within this dissertation serves to shed light onto the occurrence of high levels of As and the hydrogeological. geochemical and biogeochemical processes which affect As mobility within groundwater system in Datong Basin. An integrated approach by combining multiple isotopes with hydrogeochemistry of groundwater was tested to improve the understanding of governing redox reactions and adsorption/desorption as well as biogeochemical process. The explanatory variables included oxygen isotope (δ18O-H2O) and hdrogen isotopes (δD and3H), nitrogen isotope of nitrate (δ15N-NO3), sulfur isotopes of sulfate and sulfide (δ34S-SO4and δ34S-Sulfide), oxygen isotope of sulfate (δ18O-SO4) and molybdenum isotope (δ98Mo). Integrated interpretation of multiple isotopes proves to be useful to provide effective evidences about As release and sequestration resulting from shifts in redox conditions and subsequent redox reactions.Detailed investigation on the hydrogeochemistry of groundwater in Datong basin indicates that the groundwater can be divided into four main groundwater types:Na-HCO3water, Ca+Mg-HCO3water. Na-Cl water and Na-Cl+SO4water. The study reveals that the local and regional scale variations in groundwater composition, and the levels of As concentrations are generally low in the groundwater abstracted from the basin’s marginal areas where Ca+Mg-HCC3water prevails. High As groundwater is commonly Na-HCO3water. Another remarkable characteristic of the groundwater geochemistry is that the Mg2-concentration is commonly high, especially that in the central basin. The Mg enrichment was considered as a result of coupled processes of minerals weathering (such as dolomite and magnesites). anthropogenic (such as fertilizer application) and microbial activities (such as BSR). In the organic rich sediment in Datong Basin, major chemicals including Mg2+,Ca2+,SO42-and HCO3-contents relied on vigorous bacterial metabolisms. As enrichment was revealed fatally related to the redox conditions in groundwater other than the salinization process since no clear correlation was observed between As and CI’. To enlarge this sequitur, evapotranspiration may not be a significant controlling factor of As mobilization. Redox proxies determinations from the arsenic enriched groundwaters indicated that high arsenic groundwater mainly occurs in the reducing environment. Detectable S2-and NH4+in groundwater were further considered as markers for the contribution of bacteria such as sulfate reducing bacteria and denitrifying bacteria. Both As mobilization and immobilization caused by BSR were present in the aquifer in Datong Basin, while the priority of these processes was circumstantially controlled by the Fe2+and S2-concentrations which were related to the Fe-sulfides solubilities.Water isotope (δ18O-H2O and δD) data indicate a local precipitation origin of groundwater. Shifting of δ18O and δD apart from the LMWL was proposed as a result of the mixing of direct vertical recharge and evaporation of shallow groundwater which can lead to a parallel shift away from the LMWL. The relatively homogenized δ18O values in deep groundwater reflect the effect of mixing of groundwater along the flow path. The primary factors affecting groundwater isotopic compositions are:(1) precipitation;(2) evapotranspiration;(3) mixing of groundwaters with various isotopes signatures with irrigation return water. Non-linear correlation between tritium concentration (3H) and depth might result from pumping of aged deep groundwaters from deep aquifers. Generally, arsenic concentrations in groundwaters with different ages ranged widely. Groundwater residence time showed neglectful control on arsenic enrichment. Relatively higher As contents occurred in the young groundwaters. As interpreted by the3H concentrations of groundwater from different depths, mixing of irrigation groundwaters with various ages and As concentrations played important role in arsenic distribution in the aquifer.Microorganisms’ metabolisms involve many important geochemical processes including reduction/oxidation, precipitation/dissolution, and adsorption/desorption which significantly modify the geochemical fate of As. To gain insights into nitrogen cycling influencing arsenic mobility, nitrogen isotope of nitrate (δ15N-NO3) was detected. Nitrogen isotope investigation suggested that the highest δ15N-NO3values in groundwater (up to+28.0%o) might be contributed from point input of animal manure since cow and sheep husbandry are important industries in Datong basin. Negatively correlated nitrate concentration and δ15N-NO3ratio suggested that N isotope of nitrate has experienced significant isotopic fractionation from denitrification. Pyrite oxidation driving denitrification might be present in the aquifer. Co-precipitated As onto the Fe-oxyhydroxides might desorbed into groundwater due to Fe transformations during denitrification process. The weak positive correlation between As and δ15N-NO3ratio within the groundwaters carrying moderate δ15N-NO3values was consistent with this assumption.In an effort to better understand sulfur biogeochemical cycling affecting As mobilization, isotope signatures of dissolved sulfate and sulfide were analyzed. High δ34S-SO4, along with elevated δ18O-SO4of dissolved sulfate in groundwater in Datong Basin was considered to result from biological sulfur cycling of bacteria sulfate reduction (BSR) and re-oxidation of sulfide and the intermediates produced by BSR. Relatively high δ34S-Sulfide values excluded the sulfate contribution from sulfide oxidation. On the other hand, CAS evaporites were proposed to be a potential sulfate source. High δ18O-SO4values, which deviated from both CAS evaporites and fertilizers, may derive from nitrogen cycling involving nitrification, denitrification and sulfide oxidation, as atmospheric oxygen with comparatively high δ18O was incorporated into SO42-Mixing of irrigation water with different δ34S-SO4and δ18O-SO4values may also play important roles in governing isotopic compositions of sulfate in groundwater. Larger sulfur isotope fractionations△34SSulfide-SO4are associated with high As concentrations. Mechanisms of As release from the aquifer sediments into groundwater could be explained as:oxidation of sulfide produced by BSR could fuel denitrification; with the consumption of NO3-, reductive dissolution of Fe-oxyhydroxides (e.g., goethite) was facilitated; consequently, arsenic was desorbed into groundwater.An effort was made for the first time to validate the correlation between As concentration and Mo isotope of groundwater. Isotopic composition of dissolved Mo in groundwater (δ98/95Mo) was first reported in this study, ranging from-0.12‰to+2.17‰. with an average ratio of+1.1‰, displaying a relatively heavier ratios than those in freshwaters. Sanggan River was detected with a comparable δ98Mo ratio of+0.72‰to the documented mean riverine898Mo value of+0.7‰. Compared to the euxinic environment, sulfide concentrations in groundwater were significantly lower. However, S2-measured data demonstrated that a slowed formation of Mo-Fe-S might take place in Datong Basin, which might cause notable Mo fractionation. The partially positively correlated S2-and δ98Mo in some groundwaters might result from the preferential precipitation of light Mo by the formation of thiomolybdates.δ98Mo in groundwater was more related to Fe than Mn. The elevated δ98Mo in groundwater accompanied with progressive Mo decrease was considered as a consequence of reductive ferrihydrite dissolution and re-adsorption of Mo. This process is depicted as re-adsorption of isotopically light Mo from the groundwater. Arsenic concentration was found weakly positively correlated with δ98Mo ratio in Datong Basin, indicating that large Mo fractionation resulted from reductive dissolution of Fe oxyhydroxides could be used as a signal of As release into groundwater. In addition, a new mechanism controlling arsenic mobility was proposed in this study, based on the results of Mo isotopic data interpretation:Mo played as a competitor with As in co-precipitation with Fe-sulfides. The similarity of formation conditions for As-Fe-S and Mo-Fe-S could be delineated as the dynamo of this mechanism.更多还原