Applied Geochemistry PERGAMON Applied Geochemistry 17(2002)517-568 www.elsevier.com/locate/apgeochem Review a review of the source behaviour and distribution of arsenic in natural waters P L. Smedley*, D.G. Kinniburgh on OX10 8BB UK Received 1 March 2001; accepted 26 October 2001 Editorial handling by R. Fuge Abstract The range of As concentrations found in natural waters is large, ranging from less than 0.5 ug I-I to more than 5000 I-I. Typical concentrations in freshwater are less than 10 Hg l-l and frequently less than 1 ug l-l. Rarely, much higher concentrations are found, particularly in groundwater. In such areas, more than 10% of wells may be ' affected (defined as those exceeding 50 ug 1-)and in the worst cases, this figure may exceed 90%. Well-known high-As groundwater areas have been found in Argentina, Chile, Mexico, China and Hungary, and more recently in West Bengal(India), Bangladesh and Vietnam. The scale of the problem in terms of population exposed to high As con- entrations is greatest in the Bengal Basin with more than 40 million people drinking water containing'excessive'As These large-scale 'natural As groundwater problem areas tend to be found in two types of environment: firstly, inland or closed basins in arid or semi-arid areas, and secondly, strongly reducing aquifers often derived from alluvium. Both environments tend to contain geologically young sediments and to be in flat, low-lying areas where groundwater flow is sluggish. Historically, these are poorly flushed aquifers and any As released from the sediments following burial has been able to accumulate in the groundwater. Arsenic-rich groundwaters are also found in geothermal areas and, on a more localised scale. in areas of mining activity and where oxidation of sulphide minerals has occurred. The As content of the aquifer materials in major problem aquifers does not appear to be exceptionally high, being normally in the range 1-20 mg kg. There appear to be two distincttriggers' that can lead to the release of s on a large scale. The first is the development of high pH(>8.5) conditions in semi-arid or arid environments usually as a result of the combined effects of mineral weathering and high evaporation rates. This pH change leads either to the desorption of adsorbed As(especially As(V) species) and a range of other anion-forming elements (v, B, F, Mo, Se and u) from mineral oxides, especially Fe oxides, or it prevents them from being adsorbed. The second trigger is the development of strongly reducing conditions at near-neutral pH values, leading to the desorption of As from mineral oxides and to the reductive dissolution of Fe and Mn oxides, also leading to As release. Iron (II) and As(lll are relatively abundant in these groundwaters and SO4 concentrations are small(typically 1 mg I-I or less). Large concentrations of phosphate, bicarbonate, silicate and possibly organic matter can enhance the desorption of As because of competition for adsorption sites. A characteristic feature of high groundwater As areas is the large degree of spatial variability in As concentrations in the groundwaters. This means that it may be difficult, or impossible, to predict reliably the likely concentration of As in a particular well from the results of neighbouring wells and means that there is little alternative but to analyse each well. Arsenic-affected aquifers are restricted to certain environments and appear to be the exception rather than the rule. In most aquifers, the majority of wells are likely to be unaffected, even when, for example, they contain high concentrations of dissolved Fe C 2002 Published by elsevier Science Ltd. All rights reserved nding author. Fax: +44-1491-692345 ddress. pls(@ bgs ac uk(P L. Smedley) 0883-2927/02/. see front matter C 2002 Published by Elsevier Science Ltd. All rights reserved. PII:S0883-2927(02)00018-5
Review A review of the source, behaviour and distribution of arsenic in natural waters P.L. Smedley*, D.G. Kinniburgh British Geological Survey, Wallingford, Oxon OX10 8BB, UK Received 1March 2001; accepted 26 October 2001 Editorial handling by R. Fuge Abstract The range of As concentrations found in natural waters is large, ranging from less than 0.5 mg l1 to more than 5000 mg l1 . Typical concentrations in freshwater are less than 10 mg l1 and frequently less than 1 mg l1 . Rarely, much higher concentrations are found, particularly in groundwater. In such areas, more than 10% of wells may be ‘affected’ (defined as those exceeding 50 mg l1 ) and in the worst cases, this figure may exceed 90%. Well-known high-As groundwater areas have been found in Argentina, Chile, Mexico, China and Hungary, and more recently in West Bengal (India), Bangladesh and Vietnam. The scale of the problem in terms of population exposed to high As concentrations is greatest in the Bengal Basin with more than 40 million people drinking water containing ‘excessive’ As. These large-scale ‘natural’ As groundwater problem areas tend to be found in two types of environment: firstly, inland or closed basins in arid or semi-arid areas, and secondly, strongly reducing aquifers often derived from alluvium. Both environments tend to contain geologically young sediments and to be in flat, low-lying areas where groundwater flow is sluggish. Historically, these are poorly flushed aquifers and any As released from the sediments following burial has been able to accumulate in the groundwater. Arsenic-rich groundwaters are also found in geothermal areas and, on a more localised scale, in areas of mining activity and where oxidation of sulphide minerals has occurred. The As content of the aquifer materials in major problem aquifers does not appear to be exceptionally high, being normally in the range 1–20 mg kg1 . There appear to be two distinct ‘triggers’ that can lead to the release of As on a large scale. The first is the development of high pH (>8.5) conditions in semi-arid or arid environments usually as a result of the combined effects of mineral weathering and high evaporation rates. This pH change leads either to the desorption of adsorbed As (especially As(V) species) and a range of other anion-forming elements (V, B, F, Mo, Se and U) from mineral oxides, especially Fe oxides, or it prevents them from being adsorbed. The second trigger is the development of strongly reducing conditions at near-neutral pH values, leading to the desorption of As from mineral oxides and to the reductive dissolution of Fe and Mn oxides, also leading to As release. Iron (II) and As(III) are relatively abundant in these groundwaters and SO4 concentrations are small (typically 1mg l1 or less). Large concentrations of phosphate, bicarbonate, silicate and possibly organic matter can enhance the desorption of As because of competition for adsorption sites. A characteristic feature of high groundwater As areas is the large degree of spatial variability in As concentrations in the groundwaters. This means that it may be difficult, or impossible, to predict reliably the likely concentration of As in a particular well from the results of neighbouring wells and means that there is little alternative but to analyse each well. Arsenic-affected aquifers are restricted to certain environments and appear to be the exception rather than the rule. In most aquifers, the majority of wells are likely to be unaffected, even when, for example, they contain high concentrations of dissolved Fe. # 2002 Published by Elsevier Science Ltd. All rights reserved. 0883-2927/02/$ - see front matter # 2002 Published by Elsevier Science Ltd. All rights reserved. PII: S0883-2927(02)00018-5 Applied Geochemistry 17 (2002) 517–568 www.elsevier.com/locate/apgeochem * Corresponding author. Fax: +44-1491-692345. E-mail address: pls@bgs.ac.uk (P.L. Smedley)
518 P L. Smedley, D.G. Kinniburgh/ Applied Geochemistry 17(2002 )517-568 Contents 1. Introduction 2. Arsenic in natural waters 520 2.2. Abundance and distribution 2.2.1. Atmospheric precipitation 2.2.2. River water 2.2.3. Lake water 2. 24. Seawater and estuaries 2.2.5. Groundwater 2.2.6. Mine drainage 2.2.7. Sediment porewaters. 2.2.8. Oilfield and other brines 2.3. Distribution of arsenic species in water bodies 2.4. Impact of redox kinetics on arsenic speciation 3.1 Minerals 3.1.1. Major arsenic minerals 3. 2. Rock-forming mineral 3.2. Rocks sediments and soils 3.2.1. Igneous rocks. 3.2.2. Metamorphic rocks 530 3.2.3. Sedimentary rocks 530 3.2.4. Unconsolidated sediments 3.2.5. Soils 3.2.6. Contaminated surficial deposits 3.3. The atmosphere 4. Mineral-water interactions 41. Controls on arsenic mobilisation 4.2 Arsenic associations in sediments 4.3. Reduced sediments and the role of iron oxides 4.4. Arsenic release from soils and sediments following reduction.... 4.5. Speciation of elements in sediments and the role of selective extraction techniques 4.6. Transport of arsenic 539 5. Groundwater environments with high arsenic concentrations 5.1. World distribution of groundwater arsenic problems 5.2. Reducing environments 5.2.1. Bangladesh and India (West Bengal 5.2.2. Taiwa 5.2.3. Northern china 233177 5.2.4.Ⅴ letan 5.2.5. Hungary and Romania.…… 5.3. Arid oxidising environments. 5.3. 2. Chile 5.3.3. 549 5.4. Mixed oxidising and reducing environments 5.4. 1. South-western USA 5.5. Geothermal sources 5.6. Sulphide mineralisation and mining-related arsenic problems 5.6.1. Thailand
Contents 1. Introduction ........................................................................................................................................................... 519 2. Arsenic in natural waters ....................................................................................................................................... 520 2.1. Aqueous speciation........................................................................................................................................ 520 2.2. Abundance and distribution.......................................................................................................................... 520 2.2.1. Atmospheric precipitation ................................................................................................................. 521 2.2.2. River water........................................................................................................................................ 523 2.2.3. Lake water......................................................................................................................................... 524 2.2.4. Seawater and estuaries....................................................................................................................... 525 2.2.5. Groundwater ..................................................................................................................................... 525 2.2.6. Mine drainage.................................................................................................................................... 525 2.2.7. Sediment porewaters.......................................................................................................................... 525 2.2.8. Oilfield and other brines.................................................................................................................... 526 2.3. Distribution of arsenic species in water bodies.............................................................................................. 526 2.4. Impact of redox kinetics on arsenic speciation.............................................................................................. 527 3. Sources of arsenic................................................................................................................................................... 528 3.1. Minerals......................................................................................................................................................... 528 3.1.1. Major arsenic minerals ...................................................................................................................... 528 3.1.2. Rock-forming minerals...................................................................................................................... 529 3.2. Rocks, sediments and soils ............................................................................................................................ 530 3.2.1. Igneous rocks..................................................................................................................................... 530 3.2.2. Metamorphic rocks ........................................................................................................................... 530 3.2.3. Sedimentary rocks ............................................................................................................................. 530 3.2.4. Unconsolidated sediments ................................................................................................................. 532 3.2.5. Soils ................................................................................................................................................... 533 3.2.6. Contaminated surficial deposits......................................................................................................... 533 3.3. The atmosphere ............................................................................................................................................. 533 4. Mineral-water interactions ..................................................................................................................................... 533 4.1. Controls on arsenic mobilisation................................................................................................................... 533 4.2. Arsenic associations in sediments.................................................................................................................. 534 4.3. Reduced sediments and the role of iron oxides ............................................................................................. 534 4.4. Arsenic release from soils and sediments following reduction....................................................................... 537 4.5. Speciation of elements in sediments and the role of selective extraction techniques ..................................... 538 4.6. Transport of arsenic ...................................................................................................................................... 539 5. Groundwater environments with high arsenic concentrations ............................................................................... 542 5.1. World distribution of groundwater arsenic problems ................................................................................... 542 5.2. Reducing environments ................................................................................................................................. 543 5.2.1. Bangladesh and India (West Bengal)................................................................................................. 543 5.2.2. Taiwan............................................................................................................................................... 547 5.2.3. Northern china .................................................................................................................................. 547 5.2.4. Vietnam ............................................................................................................................................. 547 5.2.5. Hungary and Romania...................................................................................................................... 548 5.3. Arid oxidising environments.......................................................................................................................... 548 5.3.1. Mexico............................................................................................................................................... 548 5.3.2. Chile .................................................................................................................................................. 548 5.3.3. Argentina........................................................................................................................................... 549 5.4. Mixed oxidising and reducing environments................................................................................................. 549 5.4.1. South-western USA........................................................................................................................... 549 5.5. Geothermal sources ....................................................................................................................................... 550 5.6. Sulphide mineralisation and mining-related arsenic problems ...................................................................... 551 5.6.1. Thailand ............................................................................................................................................ 551 518 P.L. Smedley, D.G. Kinniburgh / Applied Geochemistry 17 (2002) 517–568
P L. Smedley, D.G. Kinniburgh/ Applied Geochemistry 17(2002 )517-568 5. 6.2. Ghana 5.6.3. United States 5.6. 4. Other areas 6. Common features of groundwater arsenic problem areas 6. 1. A hydrogeochemical perspective 6.2. The source of arsenic 6.3. Arsenic mobilisation--the necessary geochemical trigger .3.1. Desorption at high ph under oxidising conditions 6.3.2. Arsenic desorption and dissolution due to a change to reducing conditions 6.3.3. Reduction in surface area of oxide minerals 6.3.4. Reduction in binding strength between arsenic and mineral surfaces 6.3.5. Mineral dissolution 6.4. Transport--historical groundwater flows 6.5. Future developments in arsenic research 6.6. Identification of at riskaquifers 7. Concluding remarks 559 Acknowledgements… References 1. ntroduction a variety of sources depending on local availability: sur- face water(rivers, lakes, reservoirs and ponds), ground- The recent finding that groundwaters from large areas water(aquifers) and rain water. These sources are very of West Bengal, Bangladesh and elsewhere are heavily variable in terms of As risk. Alongside obvious point enriched with As has prompted a reassessment of the sources of As contamination, high concentrations are factors controlling the distribution of As in the natural mainly found in groundwaters. These are where the environment and the ways in which As may be mobilised. greatest number of, as yet unidentified, sources are likely Arsenic is a ubiquitous element found in the atmosphere to be found this review therefore focuses on the factors ils and rocks, natural waters and organisms. It is mobi- controlling As concentrations in groundwaters. Hoy ed through a combination of natural processes such ever. the authors also review the occurrence of s weathering reactions, biological activity and volcanic broad range of natural waters since these may indirectly f anthropogenic be involved in the formation of As-rich groundwaters and activities. Most environmental As problems are the result can also provide a useful background against which to of mobilisation under natural conditions. However, view groundwater As concentrations. Furthermore, many man has had an important additional impact through of the processes involved in the uptake and release of A mining activity, combustion of fossil fuels, the use of are common to a wide range of natural environments rsenical pesticides, herbicides and crop desiccants and Following the accumulation of evidence for the the use of As as an additive to livestock feed particularly chronic toxicological effects of As in drinking water for poultry. Although the use of arsenical products such recommended and regulatory limits of many authorities as pesticides and herbicides has decreased significantly in are being reduced. The Who guideline value for As in the last few decades, their use for wood preservation drinking water was provisionally reduced in 1993 from still common. The impact on the environment of the use 50 to 10 ug l-l. The new recommended value was based of arsenical compounds, at least locally, will remain for on the increasing awareness of the toxicity of As, parti some cularly its carcinogenicity, and on the ability to measure Of the various sources of as in the envir nt, it quantitatively(WHO, 1993). If the standard basis for drinking water probably poses the greatest threat to risk assessment applied to industrial chemicals were human health. Airborne As, particularly through occu- applied to As, the maximum permissible concentration pational exposure, has also given rise to known health would be lower still. The ec maximum admissible con problems in some areas. Drinking water is derived from centration (MAC)for As in drinking water has been
1. Introduction The recent finding that groundwaters from large areas of West Bengal, Bangladesh and elsewhere are heavily enriched with As has prompted a reassessment of the factors controlling the distribution of As in the natural environment and the ways in which As may be mobilised. Arsenic is a ubiquitous element found in the atmosphere, soils and rocks, natural waters and organisms. It is mobilised through a combination of natural processes such as weathering reactions, biological activity and volcanic emissions as well as through a range of anthropogenic activities. Most environmental As problems are the result of mobilisation under natural conditions. However, man has had an important additional impact through mining activity, combustion of fossil fuels, the use of arsenical pesticides, herbicides and crop desiccants and the use of As as an additive to livestock feed, particularly for poultry. Although the use of arsenical products such as pesticides and herbicides has decreased significantly in the last few decades, their use for wood preservation is still common. The impact on the environment of the use of arsenical compounds, at least locally, will remain for some years. Of the various sources of As in the environment, drinking water probably poses the greatest threat to human health. Airborne As, particularly through occupational exposure, has also given rise to known health problems in some areas. Drinking water is derived from a variety of sources depending on local availability: surface water (rivers, lakes, reservoirs and ponds), groundwater (aquifers) and rain water. These sources are very variable in terms of As risk. Alongside obvious point sources of As contamination, high concentrations are mainly found in groundwaters. These are where the greatest number of, as yet unidentified, sources are likely to be found. This review therefore focuses on the factors controlling As concentrations in groundwaters. However, the authors also review the occurrence of As in a broad range of natural waters since these may indirectly be involved in the formation of As-rich groundwaters and can also provide a useful background against which to view groundwater As concentrations. Furthermore, many of the processes involved in the uptake and release of As are common to a wide range of natural environments. Following the accumulation of evidence for the chronic toxicological effects of As in drinking water, recommended and regulatory limits of many authorities are being reduced. The WHO guideline value for As in drinking water was provisionally reduced in 1993 from 50 to 10 mg l1 . The new recommended value was based on the increasing awareness of the toxicity of As, particularly its carcinogenicity, and on the ability to measure it quantitatively (WHO, 1993). If the standard basis for risk assessment applied to industrial chemicals were applied to As, the maximum permissible concentration would be lower still. The EC maximum admissible concentration (MAC) for As in drinking water has been 5.6.2. Ghana................................................................................................................................................ 551 5.6.3. United States ..................................................................................................................................... 551 5.6.4. Other areas ........................................................................................................................................ 551 6. Common features of groundwater arsenic problem areas...................................................................................... 552 6.1. A hydrogeochemical perspective ................................................................................................................... 552 6.2. The source of arsenic..................................................................................................................................... 552 6.3. Arsenic mobilisation—the necessary geochemical trigger ............................................................................. 552 6.3.1. Desorption at high pH under oxidising conditions ........................................................................... 553 6.3.2. Arsenic desorption and dissolution due to a change to reducing conditions .................................... 554 6.3.3. Reduction in surface area of oxide minerals ..................................................................................... 555 6.3.4. Reduction in binding strength between arsenic and mineral surfaces ............................................... 555 6.3.5. Mineral dissolution............................................................................................................................ 556 6.4. Transport—historical groundwater flows...................................................................................................... 556 6.5. Future developments in arsenic research....................................................................................................... 558 6.6. Identification of ‘at risk’ aquifers .................................................................................................................. 558 7. Concluding remarks ............................................................................................................................................... 559 Acknowledgements...................................................................................................................................................... 560 References ................................................................................................................................................................... 560 P.L. Smedley, D.G. Kinniburgh / Applied Geochemistry 17 (2002) 517–568 519
P L. Smedley, D G. Kinniburgh/Applied Geochemistry 17(2002 )517-568 reduced to 10 ug I. The Japanese limit for drinking clay or organic matter. In contrast, most oxyanions water is also 10 ug I-I while the interim maximum including arsenate tend to become less strongly sorbed acceptable concentration for Canadian drinking water is as the ph increases (dzombak and Morel, 1990). Under 25 ug l-l. The US-EPA limit was also reduced from 50 some conditions at least, these anions can persist in to 10 ug I-I in January 2001 following prolonged debate solution at relatively high concentrations (tens of ug 1-) over the most appropriate limit. However, this rule is even at near-neutral ph values. Therefore the oxyanion now(September 2001) being reconsidered given the high forming elements such as Cr, As, U and Se are some of cost implications to the US water industry, estimated at the most common trace contaminants in groundwaters S200 million per year. Whilst many national authorities Relative to the other oxyanion-forming elements, As is are seeking to reduce their limits in line with the WHo among the most problematic in the environment because guideline value, many countries and indeed all affected of its relative mobility over a wide range of redox condi- developing countries, still operate at present to the 50 ug tions. Selenium is mobile as the selenate(seo4 )oxyanion I-I standard, in part because of lack of adequate testing under oxidising conditions but is immobilized under facilities for lower concentrations reducing conditions either due to the stronger adsorp- Until recently, As was often not on the list of con- tion of its reduced form, selenite (Seo3 ), or due to its ituents in drinking water routinely analysed by reduction to the metal. Chromium can similarly be national laboratories, water utilities and non-govern mobilized as stable Cr(vi)oxyanion species under oxi- menta organizations(NGOs)and so the body of info dising conditions, but forms cationic Cr(lll) species in mation about the distribution of As in drinking water is reducing environments and hence behaves like other trac not as well known as for many other drinking-water cations (i.e. is relatively immobile at near-neutral pH constituents. In recent years, it has become apparent values). Other oxyanions such as molybdate, vanadate, water sources, and often unexpectedly so. Indeed, Asa g- de yI and rhenate also appear to be less mobile under that both the WHo guideline value and current nationalura standards are quite frequently exceeded in drink cing conditions. In S-rich, reducing environment many of the trace metals also form insoluble sulphides F are now recognised as the most serious inorganic con- Arsenic is distinctive in being relatively mobile under taminants in drinking water on a worldwide basis. In reduced conditions. It can be found at concentrations in areas of high As concentrations, drinking water provides the mg I-I range when all other oxyanion-forming a potentially major source of As in the diet and so its elements are present in the ug l-range arly detection is of considerable importance. Redox potential (Eh) and ph are the most important factors controlling As speciation. Under oxidising con- 2. Arsenic in natural waters H 6.9). whilst at higher ph, Hasha becomes domi- nant(H3AsO? and AsOa- may be present in extremely 2.1. Aqueous speciation acidic and alkaline conditions respectively). Under reducing conditions at pH less than about pH 9.2, the Arsenic is perhaps unique among the heavy metal- uncharged arsenite species H3 AsOg will predominate loids and oxyanion-forming elements(e. g. As, Se, Sb, Mo,(Fig. 1; Brookins, 1988; Yan et al., 2000). The distribu- V, Cr, U, Re) in its sensitivity to mobilisation at the ph tions of the species as a function of ph are given in values typically found in groundwaters(pH 6.5-8.5)and Fig. 2. In practice, most studies in the literature report under both oxidising and reducing conditions. Arsenic can speciation data without consideration of the degree of ccur in the environment in several oxidation states(3, protonation. In the presence of extremely high con centrations of reduced s, dissolved As-sulphide specie inorganic form as oxyanions of trivalent arsenite can be significant. Reducing, acidic conditions favour LAs(IDi or pentavalent arsenate [As(v]. Organic As precipitation of orpiment (As S3), realgar(AsS)or other ulphide minerals containing coprecipitated As(Cullen surface waters, but are rarely quantitatively important. and Reimer, 1989). Therefore high-As waters are not Organic forms may however occur where waters are expected where there is a high concentration of free significantly impacted by industrial pollution. sulphide(moore et al., 1988) Most toxic trace metals occur in solution as cations (e.g. Pbt, Cu+, Ni2+, Cd+, Co2+, Zn+)which gen- 2. 2. Abundance and distribution erally become increasingly insoluble as the pH increases. At the near-neutral ph typical of most groundwaters, the Concentrations of As in fresh water vary by more solubility of most trace-metal cations is severely limited than four orders of magnitude (table 1)depending on the by precipitation as, or coprecipitation with, an oxide, source of As, the amount available and the local geo- hydroxide, carbonate or phosphate mineral, or more chemical environment. Under natural conditions, the likely by their strong adsorption to hydrous metal oxides, greatest range and the highest concentrations of As are
reduced to 10 mg l1 . The Japanese limit for drinking water is also 10 mg l1 while the interim maximum acceptable concentration for Canadian drinking water is 25 mg l1 . The US-EPA limit was also reduced from 50 to 10 mg l1 in January 2001following prolonged debate over the most appropriate limit. However, this rule is now (September 2001) being reconsidered given the high cost implications to the US water industry, estimated at $200 million per year. Whilst many national authorities are seeking to reduce their limits in line with the WHO guideline value, many countries and indeed all affected developing countries, still operate at present to the 50 mg l 1 standard, in part because of lack of adequate testing facilities for lower concentrations. Until recently, As was often not on the list of constituents in drinking water routinely analysed by national laboratories, water utilities and non-governmental organizations (NGOs) and so the body of information about the distribution of As in drinking water is not as well known as for many other drinking-water constituents. In recent years, it has become apparent that both the WHO guideline value and current national standards are quite frequently exceeded in drinkingwater sources, and often unexpectedly so. Indeed, As and F are now recognised as the most serious inorganic contaminants in drinking water on a worldwide basis. In areas of high As concentrations, drinking water provides a potentially major source of As in the diet and so its early detection is of considerable importance. 2. Arsenic in natural waters 2.1. Aqueous speciation Arsenic is perhaps unique among the heavy metalloids and oxyanion-forming elements (e.g. As, Se, Sb, Mo, V, Cr, U, Re) in its sensitivity to mobilisation at the pH values typically found in groundwaters (pH 6.5–8.5) and under both oxidising and reducing conditions. Arsenic can occur in the environment in several oxidation states (3, 0, +3 and +5) but in natural waters is mostly found in inorganic form as oxyanions of trivalent arsenite [As(III)] or pentavalent arsenate [As(V)]. Organic As forms may be produced by biological activity, mostly in surface waters, but are rarely quantitatively important. Organic forms may however occur where waters are significantly impacted by industrial pollution. Most toxic trace metals occur in solution as cations (e.g. Pb2+, Cu2+, Ni2+, Cd2+, Co2+, Zn2+) which generally become increasingly insoluble as the pH increases. At the near-neutral pH typical of most groundwaters, the solubility of most trace-metal cations is severely limited by precipitation as, or coprecipitation with, an oxide, hydroxide, carbonate or phosphate mineral, or more likely by their strong adsorption to hydrous metal oxides, clay or organic matter. In contrast, most oxyanions including arsenate tend to become less strongly sorbed as the pH increases (Dzombak and Morel, 1990). Under some conditions at least, these anions can persist in solution at relatively high concentrations (tens of mg l1 ) even at near-neutral pH values. Therefore the oxyanionforming elements such as Cr, As, U and Se are some of the most common trace contaminants in groundwaters. Relative to the other oxyanion-forming elements, As is among the most problematic in the environment because of its relative mobility over a wide range of redox conditions. Selenium is mobile as the selenate (SeO4 2) oxyanion under oxidising conditions but is immobilized under reducing conditions either due to the stronger adsorption of its reduced form, selenite (SeO3 2), or due to its reduction to the metal. Chromium can similarly be mobilized as stable Cr(VI) oxyanion species under oxidising conditions, but forms cationic Cr(III) species in reducing environments and hence behaves like other trace cations (i.e. is relatively immobile at near-neutral pH values). Other oxyanions such as molybdate, vanadate, uranyl and rhenate also appear to be less mobile under reducing conditions. In S-rich, reducing environments, many of the trace metals also form insoluble sulphides. Arsenic is distinctive in being relatively mobile under reduced conditions. It can be found at concentrations in the mg l1 range when all other oxyanion-forming elements are present in the mg l1 range. Redox potential (Eh) and pH are the most important factors controlling As speciation. Under oxidising conditions, H2AsO4 is dominant at low pH (less than about pH 6.9), whilst at higher pH, HAsO4 2 becomes dominant (H3AsO4 0 and AsO4 3 may be present in extremely acidic and alkaline conditions respectively). Under reducing conditions at pH less than about pH 9.2, the uncharged arsenite species H3AsO3 0 will predominate (Fig. 1; Brookins, 1988; Yan et al., 2000). The distributions of the species as a function of pH are given in Fig. 2. In practice, most studies in the literature report speciation data without consideration of the degree of protonation. In the presence of extremely high concentrations of reduced S, dissolved As-sulphide species can be significant. Reducing, acidic conditions favour precipitation of orpiment (As2S3), realgar (AsS) or other sulphide minerals containing coprecipitated As (Cullen and Reimer, 1989). Therefore high-As waters are not expected where there is a high concentration of free sulphide (Moore et al., 1988). 2.2. Abundance and distribution Concentrations of As in fresh water vary by more than four orders of magnitude (Table 1) depending on the source of As, the amount available and the local geochemical environment. Under natural conditions, the greatest range and the highest concentrations of As are 520 P.L. Smedley, D.G. Kinniburgh / Applied Geochemistry 17 (2002) 517–568
P L. Smedley, D G. Kinniburgh/Applied Geochemistry 17(2002 )517-568 1200 20 (a) Arseni HA pH 02468101214 As-Or-H,O at 25C and I bar total pressure. found in groundwaters as a result of the strong influence HAsO 2- of water-rock interactions and the greater tendency in aquifers for the physical and geochemical conditions be favourable for As mobilization and accumulation (b)Arsenate The range of concentrations for many water bodies is arge and hence ' typical values are difficult to derive 3- Many studies of As reported in the literature have also preferentially targeted known problem areas and hence aSo reported ranges are often extreme and unrepresentative of natural waters as a whole. Nonetheless, the following compilation of data for ranges of As concentrations found in various parts of the hydrosphere and lithosphere gives a 34567891011 broad indication of the expected concentration ranges and their variation in the environment Fig. 2.(a) Arsenite and(b)arsenate speciation as a function of H (ionic strength of about 0.01 M). Redox conditions have 2.2.1. Atmospheric precipitation been chosen such that the indicated oxidation state dominates Arsenic enters the atmosphere through inputs from the speciation in both cases wind erosion, volcanic emissions, low-temperature volatilisation from soils, marine aerosols and pollutie d is returned to the earths surface by wet and dry smelter operations, coal burning and volcanic emissions deposition. The most important anthropogenic inputs are generally higher. Andreae(1980) found rainfall are from smelter operations and fossil-fuel combustion. potentially affected by smelting and coal burning to The As appears to consist of mainly As(In2O3 dust have As concentrations of around 0.5 ug I(Table 1), particles(Cullen and Reimer, 1989). Nriagu and Pacyna although higher concentrations(average 16 ug l-)have (1988) estimated that anthropogenic sources of atmo- been found in rainfall collected in Seattle some 35 km spheric arsenic(around 18, 800 tonnes a-)amounted to downwind of a Cu smelter(Crecelius, 1975). values around 70% of the global atmospheric As flux. While it given for Arizona snowpacks(Table 1; Barbaris and is accepted that these anthropogenic sources have an Betterton, 1996) are also probably slightly above base important impact on airborne As compositions, their line concentrations because of potential inputs of air infuence on the overall As cycle is not well established. borne As from smelters, power plants and soil dust. In Baseline concentrations of As in rainfall and snow in general however, sources of airborne As in most indus al areas are invariably low at typically less than 0.03 trialized nations are limited as a result of air-pollution ug I-I Table 1). Concentrations in areas affected by control measures. Unless significantly contaminated
found in groundwaters as a result of the strong influence of water-rock interactions and the greater tendency in aquifers for the physical and geochemical conditions to be favourable for As mobilization and accumulation. The range of concentrations for many water bodies is large and hence ‘typical’ values are difficult to derive. Many studies of As reported in the literature have also preferentially targeted known problem areas and hence reported ranges are often extreme and unrepresentative of natural waters as a whole. Nonetheless, the following compilation of data for ranges of As concentrations found in various parts of the hydrosphere and lithosphere gives a broad indication of the expected concentration ranges and their variation in the environment. 2.2.1. Atmospheric precipitation Arsenic enters the atmosphere through inputs from wind erosion, volcanic emissions, low-temperature volatilisation from soils, marine aerosols and pollution and is returned to the earth’s surface by wet and dry deposition. The most important anthropogenic inputs are from smelter operations and fossil-fuel combustion. The As appears to consist of mainly As(III)2O3 dust particles (Cullen and Reimer, 1989). Nriagu and Pacyna (1988) estimated that anthropogenic sources of atmospheric arsenic (around 18,800 tonnes a1 ) amounted to around 70% of the global atmospheric As flux. While it is accepted that these anthropogenic sources have an important impact on airborne As compositions, their influence on the overall As cycle is not well established. Baseline concentrations of As in rainfall and snow in rural areas are invariably low at typically less than 0.03 mg l1 (Table 1). Concentrations in areas affected by smelter operations, coal burning and volcanic emissions are generally higher. Andreae (1980) found rainfall potentially affected by smelting and coal burning to have As concentrations of around 0.5 mg l1 (Table 1), although higher concentrations (average 16 mg l1 ) have been found in rainfall collected in Seattle some 35 km downwind of a Cu smelter (Crecelius, 1975). Values given for Arizona snowpacks (Table 1; Barbaris and Betterton, 1996) are also probably slightly above baseline concentrations because of potential inputs of airborne As from smelters, power plants and soil dust. In general however, sources of airborne As in most industrialized nations are limited as a result of air-pollution control measures. Unless significantly contaminated Fig. 1. Eh-pH diagram for aqueous As species in the system As–O2–H2O at 25 C and 1bar total pressure. Fig. 2. (a) Arsenite and (b) arsenate speciation as a function of pH (ionic strength of about 0.01M). Redox conditions have been chosen such that the indicated oxidation state dominates the speciation in both cases. P.L. Smedley, D.G. Kinniburgh / Applied Geochemistry 17 (2002) 517–568 521