A primer on trace metal-sediment chemistry
A primer on trace metal-sediment chemistry
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DOI：
10.3133/ofr84709
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发表时间：
1984
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影响因子：
--
通讯作者：
A. Horowitz
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文献类型：
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作者：
A. Horowitz
在大多数水生系统中，悬浮沉积物和底层沉积物顶部几厘米处的痕量金属浓度远远大于水柱中溶解的痕量金属浓度。因此，不能智能地评估这些组分的分布、运输和可用性，也不能仅通过对溶解相的采样和分析来确定或预测它们对环境的影响。这本入门读物旨在让读者熟悉管理与海底和悬浮沉积物有关的痕量金属的浓度和分布的基本原理。悬浮物和底泥的取样和分析对监测研究非常重要，这不仅是因为与它们相关的痕量金属浓度比溶解阶段高出几个数量级，而且还因为其他几个因素。河流中痕量金属的迁移主要以泥沙为主。此外，海底沉积物是悬浮泥沙的来源，可以提供化学条件的历史记录。这一记录将有助于建立区域基线金属水平，以便与现有条件进行比较。许多物理和化学因素影响沉积物收集和浓缩痕量金属的能力。物理因素包括颗粒大小、比表面积、表面电荷、阳离子交换容量、组成等。金属浓度的增加与颗粒尺寸的减小和比表面积、表面电荷、阳离子交换容量的增加以及铁和锰氧化物、有机质和粘土矿物浓度的增加密切相关。化学因素也同样重要，特别是在区分具有相似整体化学成分的样品以及推断或预测环境可用性方面。化学因素涉及相关联(与沉积组分，如间隙水、硫化物、碳酸盐和有机物)和金属被沉积物夹带的方式(如吸附、络合和矿物晶格内)。导言大多数以化学为导向的水质研究的基本目标是描述或评价现有的环境条件，并试图确定被调查成分的一个或多个来源。一个同样重要的目标是试图预测或确定潜在的影响。这一标题可以包括生物利用度、成分运输量、化学汇的位置、最终命运和潜在的毒性影响等主题。从历史上看，美国地质调查局曾试图通过分析水样来评估水生系统中的痕量金属。这项评估需要分别通过收集和分析未经过滤和经过过滤的水来确定总的和溶解的元素和化合物的浓度。与悬浮泥沙(颗粒物、沉积物)相关的浓度由总浓度和溶解浓度之间的差异间接确定。人们认识到，这种方法使人对报告的悬浮物化学分析的可靠性产生怀疑。因此，人们倾向于根据溶液中各种成分的种类和浓度来评价水质(Feltz，1980)。然而，在大多数水生系统中，悬浮沉积物和底层沉积物顶端几厘米处的痕量金属浓度远远大于水体中溶解的痕量金属浓度。大量痕量金属(例如，As、Cd、Hg、Pb、Zn)与沉积物和海底沉积物之间的强相关性意味着，仅通过对溶解相的采样和分析不能智能地评估这些组分的分布、迁移和有效性。此外，由于海底沉积物可以作为许多痕量金属的储存库，出于几个原因，在任何水质研究的规划和设计中都必须认真考虑它们。首先，未受干扰的沉淀池包含化学条件的历史记录。如果能够找到并研究足够大和稳定的下沉，它将使调查者能够研究随时间的变化，并可能建立区域基线水平，以便与现有条件进行比较和对比。其次，在不断变化的环境或物理化学条件下(如pH、Eh、溶解氧、细菌作用)，沉积物结合的痕量金属可以溶解到水体中，可能进入食物链，并对环境产生重大影响。第三，几种相对惰性或对环境无害的无机成分可以降解或与其他无机成分反应，形成可溶的和潜在的有毒形式(例如，将元素汞转化为甲基汞)。最后，底泥应被视为悬浮泥沙的主要来源，如果不是主要来源的话。因此，必须对它们进行调查，以确定运输潜力。在不断变化的水文条件下(如暴雨或春季径流)，局部污染问题可能会突然变得普遍，并导致重大的环境影响。上述讨论表明，关于悬浮物和底层沉积物以及溶解相的数据是全面了解痕量金属对水质影响的必要条件。通过使用这类补充数据，有可能开始查明有毒或对环境有必要的金属的来源和汇以及命运和潜在影响。同样，沉积物化学数据对于迁移建模、估算地球化学循环以及推断生态系统中各种痕量金属的有效性也是必不可少的。
In most aquatic systems, concentrations of trace metals in suspended sediment and the top few centimeters of bottom sediment are far greater than concentrations of trace metals dissolved in the water column. Consequently, the distribution, transport, and availability of these constituents can not be intelligently evaluated, nor can their environmental impact be determined or predicted solely through the sampling and analysis of dissolved phases. This Primer is designed to acquaint the reader with the basic principles that govern the concentration and distribution of trace metals associated with bottom and suspended sediments. The sampling and analysis of suspended and bottom sediments are very important for monitoring studies, not only because trace metal concentrations associated with them are orders of magnitude higher than in the dissolved phase, but also because of several other factors. Riverine transport of trace metals is dominated by sediment. In addition, bottom sediments serve as a source for suspended sediment and can provide a historical record of chemical conditions. This record will help establish area baseline metal levels against which existing conditions can be compared. Many physical and chemical factors affect a sediment's capacity to collect and concentrate trace metals. The physical factors include grain size, surface area, surface charge, cation exchange capacity, composition, and so forth. Increases in metal concentrations are strongly correlated with decreasing grain size and increasing surface area, surface charge, cation exchange capacity, and increasing concentrations of iron and manganese oxides, organic matter, and clay minerals. Chemical factors are equally important, especially for differentiating between samples having similar bulk chemistries and for inferring or predicting environmental availability. Chemical factors entail phase associations (with such sedimentary components as interstitial water, sulfides, carbonates, and organic matter) and ways in which the metals are entrained by the sediments (such as adsorption, complexation, and within mineral lattices). INTRODUCTION The basic goal of most chemically oriented water-quality studies is to describe or evaluate existing environmental conditions and to attempt to identify the source or sources of the constituents under investigation. An equally important goal is to attempt to predict or determine potential impacts. This heading could accommodate such subjects as bioavailability, amount of constituent transport, location of chemical sinks, ultimate fate, and potential toxic effects. Historically, the U.S. Geological Survey has attempted to assess trace metals in aquatic systems by analyzing water samples. This assessment has entailed determining concentrations of total and dissolved elements and compounds through the collection and analysis, respectively, of unfiltered and filtered water. Concentrations associated with suspended sediment (particulates, seston) are determined indirectly by the difference between total and dissolved concentrations. It is recognized that this approach casts doubt on the reliability of reported suspended-sediment chemical analyses. As a result, water quality tends to be evaluated on the kinds and concentration of various constituents found in solution (Feltz, 1980). However, in most aquatic systems, the concentration of trace metals in suspended sediment and the top few centimeters of bottom sediment is far greater than the concentration of trace metals dissolved in the water column. The strong association of numerous trace metals (for example, As, Cd, Hg, Pb, Zn) with seston and bottom sediments means that the distribution, transport, and availability of these constituents can not be intelligently evaluated solely through the sampling and analysis of the dissolved phase. Additionally, because bottom sediments can act as a reservoir for many trace metals, they must, for several reasons, be given serious consideration in the planning and design of any water-quality study. First, an undisturbed sediment sink contains a historical record of chemical conditions. If a sufficiently large and stable sink can be found and studied, it will allow the investigator to study changes over time and, possibly, to establish area baseline levels against which existing conditions can be compared and contrasted. Second, under changing environmental or physicochemical conditions (like pH, Eh, dissolved oxygen, bacterial action), sediment-bound trace metals can dissolve into the water column, possibly enter the food chain, and have a significant environmental impact. Third, several relatively inert or otherwise environmentally harmless inorganic constituents can degrade, or react with others, to form soluble and potentially toxic forms (for example, the conversion of elemental mercury to methyl-mercury). Finally, bottom sediments should be regarded as a major, if not the major, source of suspended sediment. Therefore, they must be investigated to determine transport potential. Under changing hydrologic conditions (such as a heavy storm or spring runoff), a localized pollution problem can suddenly become widespread and result in significant environmental impact. The foregoing discussion indicates that data on suspended and bottom sediments, as well as on the dissolved phase, are a requisite for the development of a comprehensive understanding of the impact of trace metals on water quality. Through the use of such additional data, it may be possible to begin to identify sources and sinks and the fate and potential effects of toxic or environmentally necessary metals. Similarly, sediment-chemical data are a requisite for transport modeling, for estimating geochemical cycles, and for inferring the availability of various trace metals in an ecological system.