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 arereduced to 10 mg l
1 . The Japanese limit for drinking water is also 10 mg l
1 while the interim maximum acceptable concentration for Canadian drinking water is 25 mg l
1 . The US-EPA limit was also reduced from 50 to 10 mg l
1 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 con￾stituents in drinking water routinely analysed by national laboratories, water utilities and non-govern￾mental organizations (NGOs) and so the body of infor￾mation 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 drinking￾water sources, and often unexpectedly so. Indeed, As and F are now recognised as the most serious inorganic con￾taminants 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 metal￾loids 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 gen￾erally 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 l
1 ) even at near-neutral pH values. Therefore the oxyanion￾forming 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 condi￾tions. Selenium is mobile as the selenate (SeO4 2
) oxyanion under oxidising conditions but is immobilized under reducing conditions either due to the stronger adsorp￾tion 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 oxi￾dising 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 l
1 range when all other oxyanion-forming elements are present in the mg l
1 range. Redox potential (Eh) and pH are the most important factors controlling As speciation. Under oxidising con￾ditions, H2AsO4
is dominant at low pH (less than about pH 6.9), whilst at higher pH, HAsO4 2
becomes domi￾nant (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 distribu￾tions 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 con￾centrations 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 geo￾chemical 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