P L. Smedley, D G. Kinniburgh/Applied Geochemistry 17(2002 )517-568 porewaters extracted from tailings in Ontario(Table 1). species for later laboratory analysis. Alternatively, pre In such cases, high porewater As concentrations are servation with HCl and ascorbic acid has been success most likely to be linked to the strong redox gradients ful although this may destroy monomethylarsonic acid that occur below the sediment-water interface, often (MMAA)if present. over depth scales of centimeters. Burial of fresh organic In rain water, oxidation states of As present will vary matter and the slow diffusion of o, through the sedi according to the source. This is likely to be dominantly ment leads to reducing conditions just below the sed As(In2O3 when derived from smelters, coal burning ment-water interface. This encourages the reduction of and volcanic sources, although organic species may be As(V) and desorption from Fe and Mn oxides, as well as derived by volatilization from soils, arsine(As(-IlDH3) reductive dissolution of these minerals. There is much may derive from landfills and reducing soils such as evidence for cycling of As between shallow sediment peats, and arsenate may be derived from marine aero- porewaters and overlying surface waters in response to sols. Reduced forms will undergo oxidation by O2 in the temporal variations in redox conditions. atmosphere and reactions with atmospheric SO2 or O3 Sullivan and Aller(1996)carried out an elegant study are likely( Cullen and reimer, 1989). of the cycling of As in shallow sediments from the off- In oxic seawater, the As is typically dominated by shore shelf of the Amazon situated far from population As(V), though some As(lln) is invariably present and centres. They measured porewater As and Fe con- becomes of increasing importance in anoxic bottom entration profiles as well as sediment As and Fe(l waters. Ratios of As(V)/As(Ill are typically in the concentrations. There was frequently a well-correlated ange 10-100 in open seawater(Andreae, 1979; Peterson peak in dissolved As and Fe concentrations some 50- and Carpenter, 1983; Pettine et al., 1992). Arsenic(V) 150 cm beneath the surface, with As concentrations in should exist mainly as HAsO and H,Asoa in the ph the peak averaging about 135 ug I-I and reaching a range of seawater (pH around 8.2; Figs. I and 2)and aximum of 300 ug l-l, much greater than from marine As(lID) mainly as the neutral species H3AsO3 Relatively coastal environments. The dissolved As/Fe molar ratio high proportions of H3AsO3 are found in surface ocean varied but was typically about 1: 300. Dissolved As var- waters( Cullen and Reimer, 1989; Cutter et al., 2001) ied inversely with easily-leachable(6 M HCI) As in the These coincide with zones of primary productivity. sediment and increased directly with solid-phase Fe(In). Increases in organic As species have also been recorded In these sediments. Fe oxides were believed to be in these zones as a result of methylation reactions by much more important source of As than Mn oxides phytoplankton. The relative proportions of As species are more variable 2. 2.8. Oilfield and other brines n estuarine waters because of variable redox and salinity. Only limited data are available for As in oilfield and and terrestrial inputs(Howard et al, 1988; Abdullah et al other brines, but some published accounts suggest that 1995). However, they are still dominated by As(v) concentrations can be very high. White et al. (1963) Andreae and Andreae(1989)found As(V/As(lIn ratios reported a dissolved As concentration of 230 ug I-in a varying between 5-50 in the Schelde Estuary of Belgiu Na-HCO3 groundwater from a 1000 m deep oilfield well with the lowest ratios in anoxic zones where inputs of from Ellis Pool, Alberta, Canada. They also reported a industrial effluent had an impact. Increased proportions concentration of 5800 ug I-I As in a Na-Cl-dominated of As(ln also result from inputs of mine effluent 可 n Searles Lal6n1s rom Tisakurt variations in As fornia, have As concentrations up to 243 mg 1-l(Na 119 aries(Riedel, 1993). In seasonally anoxic estuarine I-l White et al., 1963: Table 1) waters, variations in the relative proportions of As(n) and As(V) can be large. Peterson and Carpenter(1983) 2.3. Distribution of arsenic species in water bodies found a distinct crossover in the proportions of the two species with increasing depth in response to the onset of Many studies of As speciation in natural waters have anoxic conditions in the estuarine waters of Saanich been carried out. Most attempt to separate the inorganic Inlet of British Columbia. Arsenic(IIn) represented only species into As(lID) and As(V), usually by chromato- 5%(0.10 ug l-)of the dissolved As above the redox graphic separation or by making use of the relatively front but 87%(1.58 ug l-)below it. In marine and low reduction of As(V) by Na borohydride. Some studies estuarine waters, organic forms are usually less abun lso measure the organic As species. The sampling and dant but are nonetheless often detected(e.g. Riedel, analytical techniques required are not trivial and not yet 1993: Howard et al., 1999). Concentrations of these will ell-established (Edwards et aL., 1998). Both oxidatio depend on abundance and species of biota present and f As(lID and reduction of As(v) may occur during sto- on temperature. rage(Hall et al., 1999). Separation of species may be car- In lake and river waters, As(V) is also generally the ried out in the field to avoid the problem of preserving dominant species(e.g Seyler and Martin, 1990; Pettineporewaters extracted from tailings in Ontario (Table 1). In such cases, high porewater As concentrations are most likely to be linked to the strong redox gradients that occur below the sediment-water interface, often over depth scales of centimeters. Burial of fresh organic matter and the slow diffusion of O2 through the sedi￾ment leads to reducing conditions just below the sedi￾ment-water interface. This encourages the reduction of As(V) and desorption from Fe and Mn oxides, as well as reductive dissolution of these minerals. There is much evidence for cycling of As between shallow sediment porewaters and overlying surface waters in response to temporal variations in redox conditions. Sullivan and Aller (1996) carried out an elegant study of the cycling of As in shallow sediments from the off- shore shelf of the Amazon situated far from population centres. They measured porewater As and Fe con￾centration profiles as well as sediment As and Fe(II) concentrations. There was frequently a well-correlated peak in dissolved As and Fe concentrations some 50– 150 cm beneath the surface, with As concentrations in the peak averaging about 135 mg l
1 and reaching a maximum of 300 mg l
1 , much greater than from marine coastal environments. The dissolved As/Fe molar ratio varied but was typically about 1:300. Dissolved As var￾ied inversely with easily-leachable (6 M HCl) As in the sediment and increased directly with solid-phase Fe(II). In these sediments, Fe oxides were believed to be a much more important source of As than Mn oxides. 2.2.8. Oilfield and other brines Only limited data are available for As in oilfield and other brines, but some published accounts suggest that concentrations can be very high. White et al. (1963) reported a dissolved As concentration of 230 mg l
1 in a Na–HCO3 groundwater from a 1000 m deep oilfield well from Ellis Pool, Alberta, Canada. They also reported a concentration of 5800 mg l
1 As in a Na–Cl-dominated brine from Tisaku¨rt, Hungary. Composite brines from the interstices of salt deposits from Searles Lake, Cali￾fornia, have As concentrations up to 243 mg l
1 (Na 119 g l
1 ; White et al., 1963; Table 1). 2.3. Distribution of arsenic species in water bodies Many studies of As speciation in natural waters have been carried out. Most attempt to separate the inorganic species into As(III) and As(V), usually by chromato￾graphic separation or by making use of the relatively slow reduction of As(V) by Na borohydride. Some studies also measure the organic As species. The sampling and analytical techniques required are not trivial and not yet well-established (Edwards et al., 1998). Both oxidation of As(III) and reduction of As(V) may occur during sto￾rage (Hall et al., 1999). Separation of species may be car￾ried out in the field to avoid the problem of preserving species for later laboratory analysis. Alternatively, pre￾servation with HCl and ascorbic acid has been success￾ful although this may destroy monomethylarsonic acid (MMAA) if present. In rain water, oxidation states of As present will vary according to the source. This is likely to be dominantly As(III)2O3 when derived from smelters, coal burning and volcanic sources, although organic species may be derived by volatilization from soils, arsine (As(-III)H3) may derive from landfills and reducing soils such as peats, and arsenate may be derived from marine aero￾sols. Reduced forms will undergo oxidation by O2 in the atmosphere and reactions with atmospheric SO2 or O3 are likely (Cullen and Reimer, 1989). In oxic seawater, the As is typically dominated by As(V), though some As(III) is invariably present and becomes of increasing importance in anoxic bottom waters. Ratios of As(V)/As(III) are typically in the range 10–100 in open seawater (Andreae, 1979; Peterson and Carpenter, 1983; Pettine et al., 1992). Arsenic(V) should exist mainly as HAsO4 2- and H2AsO4 - in the pH range of seawater (pH around 8.2; Figs. 1and 2) and As(III) mainly as the neutral species H3AsO3. Relatively high proportions of H3AsO3 are found in surface ocean waters (Cullen and Reimer, 1989; Cutter et al., 2001). These coincide with zones of primary productivity. Increases in organic As species have also been recorded in these zones as a result of methylation reactions by phytoplankton. The relative proportions of As species are more variable in estuarine waters because of variable redox and salinity, and terrestrial inputs (Howard et al., 1988; Abdullah et al., 1995). However, they are still dominated by As(V). Andreae and Andreae (1989) found As(V)/As(III) ratios varying between 5–50 in the Schelde Estuary of Belgium with the lowest ratios in anoxic zones where inputs of industrial effluent had an impact. Increased proportions of As(III) also result from inputs of mine effluent (Klumpp and Peterson, 1979). Seasonal variations in As concentration and speciation have been noted in estu￾aries (Riedel, 1993). In seasonally anoxic estuarine waters, variations in the relative proportions of As(III) and As(V) can be large. Peterson and Carpenter (1983) found a distinct crossover in the proportions of the two species with increasing depth in response to the onset of anoxic conditions in the estuarine waters of Saanich Inlet of British Columbia. Arsenic(III) represented only 5% (0.10 mg l
1 ) of the dissolved As above the redox front but 87% (1.58 mg l
1 ) below it. In marine and estuarine waters, organic forms are usually less abun￾dant but are nonetheless often detected (e.g. Riedel, 1993; Howard et al., 1999). Concentrations of these will depend on abundance and species of biota present and on temperature. In lake and river waters, As(V) is also generally the dominant species (e.g. Seyler and Martin, 1990; Pettine 526 P.L. Smedley, D.G. Kinniburgh / Applied Geochemistry 17 (2002) 517–568