P L. Smedley, D G. Kinniburgh/Applied Geochemistry 17(2002 )517-568 et aL, 1992), though significant seasonal variations in As(Ill/AsT ratios varying between 0. 1-0.9 but are typ speciation as well as absolute concentration have been cally around 0.5-0.6(DPHE/BGS/MML, 1999: Smedley found Concentrations and relative proportions of As(V et al, 2001b). Ratios in reducing groundwaters from Inner and As(lIn vary according to changes in input sources, Mongolia are typically 0.6-0.9(Smedley et al., 2001a) redox conditions and biological activity. The presence of Concentrations of organic forms are generally low or As(lID may be maintained in oxic waters by biological negligible in groundwaters(e.g. Chen et al., 1995) reduction of As(V). particularly during summer month Higher relative proportions of As(lln have been found 2. 4. Impact of redox kinetics on arsenic speciation in river stretches close to inputs of As(lI-dominated industrial effluent(Andreae and Andreae, 1989)and Redox reactions are important for controlling the waters with a component of geothermal water. behaviour of many major and minor species in natural Proportions of As(lln) and As(v) are particularly waters, including that of As. However, in practice. variable in stratified lakes where redox gradients can be redox equilibrium is often achieved only slowly and the with estuarine waters, distinct changes in As speciation ments (O, C, N,S and Fe). Redox-sensitive minor and occur in lake profiles as a result of redox changes. For trace elements such asAs respond to these change example, in the stratified, hypersaline and hyperalkaline rather than control them. The slow rate of many het Mono Lake( California, USA), there is a predominance erogeneous redox reactions is supported by the studies of As(V) in the upper oxic layer and of As(lln) in the of Wersin et al. 1991) who estimated that the complete 2000). Rapid oxidation of As(im occurs as a result of Swiss lake sediment would take more than I ka. Equili- microbial activity during the early stages of lake turn- brium thermodynamic calculations predict that As(V) over(Oremland et al., 2000). The As oxidation occurs concentrations should be greater than As(lincon- before Fe(ln) oxidation. Unlike Mono Lake, speciation centrations in all but strongly reducing conditions, i.e. of As in lakes does not necessarily follow that expected where SO4 reduction is occurring. While this is indeed from thermodynamic considerations. Recent studies often found to be the case, such theoretical behaviour is have shown that arsenite predominates in the oxidised not necessarily followed quantitatively in natural waters epilimnion of some stratified lakes whilst arsenate may where different redox couples can point to different persist in the anoxic hypolimnion(Kuhn and Sigg, 1993; implied redox potentials(Eh values), reflecting thermo- Newman et aL., 1998). Proportions of As species may also dynamic disequilibrium( Seyler and Martin, 1989; Eary vary according to the availability of particulate Fe and and Schramke, 1990: Kuhn and Sigg, 1993). In Oslo- Mn oxides(Pettine et al. 1992; Kuhn and Sigg, 1993) fjord, Norway, As(lIn) was found under oxidising con- Organic forms of As are usually minor in surface ditions(Abdullah et aL, 1995). Also, in oxygenated waters. In lake waters from Ontario. Azcue and nriagu seawater, the As(V)/As(III) ratios should be of the order 1995)found As(lll) concentrations of 7-75 ug I of 105-1026(Andreae, 1979)whereas measured ratic As(V)of 19-58 ug I-I and only 0.01-1.5 ug I-I of of 0. 1-250 have been found, largely supported by bio- onganic As. Nonetheless, proportions of organic forms logical transformations (Johnson and Pilson, 1975 As can increase as a result of methylation tions Cullen and Reimer, 1989). Oxidation of As(ln by dis- catalysed by microbial activity(bacteria, yeasts, algae). solved O2, so-called oxygenation, is a particularly slow The dominant organic forms found are dimethylarsinic eaction. For example, Johnson and Pilson(1975)gave acid (DMAA: (CH3)2Aso(OH)) and mono- half-lives for the oxygenation of As(In) in seawater methylarsonic acid (MMAA; CH3AsO(OH)), where As ranging from several months to a year is present in both cases in the pentavalent oxidation Other studies have demonstrated the stability of state Proportions of these two species have been noted As(V)/As(lID) ratios over periods of days or weeks dur- to increase in summer as a result of increased microbial ing water sampling when no particular care was taken to activity(e.g. Hasegawa, 1997). The organic species may prevent oxidation, again suggesting relatively slow oxi- so be more prevalent close to the sediment-water dation rates. Andreae(1979)found stable ratios in sea- interface(Hasegawa et al, 1999) water for up to 10 days(4 C). Cherry et al. (1979)found In groundwaters, the ratio of As(lll) to As(V) can from experimental studies that the As(v)/As(lll) ratios vary greatly as a result of variations in the abundance of were stable in anoxic solutions for up to 3 weeks but redox-active solids, especially organic C, the activity of that gradual changes occurred over longer timescales microorganisms and the extent of convection and diffu Cherry et al. (1979)suggested that the measured As(V/ on of O2 from the atmosphere. In strongly reducing As(lIn ratios in natural waters, especially groundwaters, aquifers(Fe(lll) and SOa reducing aquifers), As(lll might be used as an indicator of the ambient redox(eh) typically dominates, as expected from the redox sequenc conditions as the redox changes are sufficiently rapid to Reducing As-rich groundwaters from Bangladesh have occur over periods of years. Yan et al. (2000)have alsoet al., 1992), though significant seasonal variations in speciation as well as absolute concentration have been found. Concentrations and relative proportions of As(V) and As(III) vary according to changes in input sources, redox conditions and biological activity. The presence of As(III) may be maintained in oxic waters by biological reduction of As(V), particularly during summer months. Higher relative proportions of As(III) have been found in river stretches close to inputs of As(III)-dominated industrial effluent (Andreae and Andreae, 1989) and in waters with a component of geothermal water. Proportions of As(III) and As(V) are particularly variable in stratified lakes where redox gradients can be large and seasonally variable (Kuhn and Sigg, 1993). As with estuarine waters, distinct changes in As speciation occur in lake profiles as a result of redox changes. For example, in the stratified, hypersaline and hyperalkaline Mono Lake (California, USA), there is a predominance of As(V) in the upper oxic layer and of As(III) in the reducing layer (Maest et al., 1992; Oremland et al., 2000). Rapid oxidation of As(III) occurs as a result of microbial activity during the early stages of lake turn￾over (Oremland et al., 2000). The As oxidation occurs before Fe(II) oxidation. Unlike Mono Lake, speciation of As in lakes does not necessarily follow that expected from thermodynamic considerations. Recent studies have shown that arsenite predominates in the oxidised epilimnion of some stratified lakes whilst arsenate may persist in the anoxic hypolimnion (Kuhn and Sigg, 1993; Newman et al., 1998). Proportions of As species may also vary according to the availability of particulate Fe and Mn oxides (Pettine et al., 1992; Kuhn and Sigg, 1993). Organic forms of As are usually minor in surface waters. In lake waters from Ontario, Azcue and Nriagu (1995) found As(III) concentrations of 7–75 mg l
1 , As(V) of 19–58 mg l
1 and only 0.01–1.5 mg l
1 of organic As. Nonetheless, proportions of organic forms of As can increase as a result of methylation reactions catalysed by microbial activity (bacteria, yeasts, algae). The dominant organic forms found are dimethylarsinic acid (DMAA; (CH3)2AsO(OH)) and mono￾methylarsonic acid (MMAA; CH3AsO(OH)2), where As is present in both cases in the pentavalent oxidation state. Proportions of these two species have been noted to increase in summer as a result of increased microbial activity (e.g. Hasegawa, 1997). The organic species may also be more prevalent close to the sediment-water interface (Hasegawa et al., 1999). In groundwaters, the ratio of As(III) to As(V) can vary greatly as a result of variations in the abundance of redox-active solids, especially organic C, the activity of microorganisms and the extent of convection and diffu￾sion of O2 from the atmosphere. In strongly reducing aquifers (Fe(III)- and SO4-reducing aquifers), As(III) typically dominates, as expected from the redox sequence. Reducing As-rich groundwaters from Bangladesh have As(III)/AsT ratios varying between 0.1–0.9 but are typi￾cally around 0.5–0.6 (DPHE/BGS/MML, 1999; Smedley et al., 2001b). Ratios in reducing groundwaters from Inner Mongolia are typically 0.6–0.9 (Smedley et al., 2001a). Concentrations of organic forms are generally low or negligible in groundwaters (e.g. Chen et al., 1995). 2.4. Impact of redox kinetics on arsenic speciation Redox reactions are important for controlling the behaviour of many major and minor species in natural waters, including that of As. However, in practice, redox equilibrium is often achieved only slowly and the redox potential tends to be controlled by the major ele￾ments (O, C, N, S and Fe). Redox-sensitive minor and trace elements such as As respond to these changes rather than control them. The slow rate of many het￾erogeneous redox reactions is supported by the studies of Wersin et al. (1991) who estimated that the complete reductive dissolution of Fe(III) oxides in an anoxic Swiss lake sediment would take more than 1ka. Equili￾brium thermodynamic calculations predict that As(V) concentrations should be greater than As(III) con￾centrations in all but strongly reducing conditions, i.e. where SO4 reduction is occurring. While this is indeed often found to be the case, such theoretical behaviour is not necessarily followed quantitatively in natural waters where different redox couples can point to different implied redox potentials (Eh values), reflecting thermo￾dynamic disequilibrium (Seyler and Martin, 1989; Eary and Schramke, 1990; Kuhn and Sigg, 1993). In Oslo￾fjord, Norway, As(III) was found under oxidising con￾ditions (Abdullah et al., 1995). Also, in oxygenated seawater, the As(V)/As(III) ratios should be of the order of 1015–1026 (Andreae, 1979) whereas measured ratios of 0.1–250 have been found, largely supported by bio￾logical transformations (Johnson and Pilson, 1975; Cullen and Reimer, 1989). Oxidation of As(III) by dis￾solved O2, so-called oxygenation, is a particularly slow reaction. For example, Johnson and Pilson (1975) gave half-lives for the oxygenation of As(III) in seawater ranging from several months to a year. Other studies have demonstrated the stability of As(V)/As(III) ratios over periods of days or weeks dur￾ing water sampling when no particular care was taken to prevent oxidation, again suggesting relatively slow oxi￾dation rates. Andreae (1979) found stable ratios in sea￾water for up to 10 days (4 C). Cherry et al. (1979) found from experimental studies that the As(V)/As(III) ratios were stable in anoxic solutions for up to 3 weeks but that gradual changes occurred over longer timescales. Cherry et al. (1979) suggested that the measured As(V)/ As(III) ratios in natural waters, especially groundwaters, might be used as an indicator of the ambient redox (Eh) conditions as the redox changes are sufficiently rapid to occur over periods of years. Yan et al. (2000) have also P.L. Smedley, D.G. Kinniburgh / Applied Geochemistry 17 (2002) 517–568 527