P L. Smedley, D G. Kinniburgh/Applied Geochemistry 17(2002 )517-568 Nriagu(1995)found that concentrations increased witI depth(up to 10 m) in lake waters from Ontario, prob- quoted in the literature show a very large range from ably because of an increasing ratio of As(lln to As(V <0.5 to 5000 ug l-I(i.e. four orders of magnitude). This with depth and an influx of mining-contaminated sedi- range occurs under natural conditions. High concentra ment porewaters at the sediment-water interface. The tions of As are found in groundwater in a variety of concentrations were higher in summer when the pro- environments. These include both oxidising(under condi- portion of As(lIn) was observed to be higher. Depleted tions of high pH) and reducing aquifers and areas affected O, levels in the bottom lake waters as a result of biolo- by geothermal, mining and industrial activity. Evapora- gical productivity during the summer months are a tive concentration can also increase concentrations sub- ely cause of the higher As stantially. Most high-As groundwater provinces are the lake waters result of natural occurrences of As. Cases of mining- induced As pollution are numerous in the literature but 2. 2. 4. Seawater and estuaries tend to be localised. Cases of industrially-induced As Average As concentrations in open seawater usually pollution (including that from agriculture) may be show little variation and are typically around 1.5 ug l-l severe locally (Table 1) but occurrences are relatively (Table 1). Concentrations in estuarine water are more rare. Arsenic occurrences in groundwater are discussed variable as a result of varying river inputs and salinity or more fully in Section 5 than 4 Hg I- under natural conditions. Peterson and 2.2.6. Mine drainage Carpenter (1983)found concentrations between 1. 2-2.5 Under the extremely acid conditions of some acid ug I- in waters from Saanich Inlet, British Columbia. mine drainage, whicl have negative pH values Values less than 2 ug I-I were found in Oslofjord, Nor- (Nordstrom et al., 2000), high concentrations of a wid way(Abdullah et al., 1995: Table 1) Concentrations are range of solutes are found, including Fe and As. The commonly higher when riverine inputs are affected by highest reported As concentration of 850,000 ug 1-I is industrial or mining effluent(e. g. Tamar, Schelde, Loire from an acid seep in the richmond mine at Iron Estuaries; Table D)or by geothermal water. Unlike some Mountain, California(Nordstrom and Alpers, 1999). In other trace elements such as B, saline intrusion of sea- a compilation of some 180 samples of mine drainage water into an aquifer is unlikely to lead to a significant from the USA, Plumlee et al. (1999)reported concentra increase of As in the afected groundwater tions ranging from detection limits(<l ug I-I or more Arsenate shares many chemical characteristics with to 340,000 ug l-, again the highest values being from phosphate and hence in oxic marine and estuarine waters. the richmond mine. Gelova(1977) also reported an A depletions in phosphate in biologically productive surface concentration of 400,000 ug I- from the Ural Moun- waters are mirrored by depletions in arsenate. Arsenate tains. Dissolved As in acid mine waters is rapidly concentration minima often coincide with photosynthetic removed as the Fe is oxidised and precipitated and the axima evidenced by high concentrations of chlor As scavenged through adsorption. At Iron Mountail ophyll a( Cullen and reimer, 1989). Several studies have an efficient neutralization plant removes the As and noted variations in the behaviour of As during estuarine metals for safe disposal mixing. Some have reported conservative behaviour. In the unpolluted Krka Estuary of Yugoslavia, Seyler and 2. 2.7. Sediment porewaters Martin(1991)observed a linear increase in total As with Some high concentrations of As have been found in increasing salinity ranging from 0.13 Hg I- in fresh porewaters extracted from unconsolidated sediments waters to 1.8 ug I- offshore (i.e. seawater value). How- and often form sharp contrasts to the concentrations ever, other studies have observed non-conservative observed in overlying surface waters (e.g. Belzile and behaviour (departures from simple mixing) in estuaries Tessier, 1990). Widerlund and Ingri (1995) found con due to processes such as diffusion from sediment pore centrations in the range 1.3-166 ug I-I in porewaters waters, coprecipitation with Fe oxides or anthropogenic from the Kalix River estuary of northern Sweden. Yan inputs(e.g Andreae et al., 1983: Andreae and Andreae, et al. (2000) found As concentrations in the range 3. 2-99 1989). The flocculation of Fe oxides at the freshwater- ug I-i in porewaters from clay sediments in Saskatch- saline interface is an important consequence of increases ewan, Canada(Table 1). Increased concentrations have in pH and salinity. This can lead to major decreases in been found in porewaters affected by geothermal inputs. the As fux to the oceans( Cullen and Reimer, 1989) Aggett and Kriegman(1988) found As concentrations u to 6430 ug I- in anoxic porewaters from New Zealand 2.2.5. Groundwater Even higher concentrations can be found in porewaters Background concentrations of As in groundwater are from sediments affected by mining contamination(tail in most countries less than 10 ug l-(e.g. Edmunds et ings, mineral-rich deposits). McCreadie et al.(2000) al., 1989 for the UK: Welch et al, 2000 for the USA) reported As concentrations up to 100,000 ug I-I inNriagu (1995) found that concentrations increased with depth (up to 10 m) in lake waters from Ontario, prob￾ably because of an increasing ratio of As(III) to As(V) with depth and an influx of mining-contaminated sedi￾ment porewaters at the sediment-water interface. The concentrations were higher in summer when the pro￾portion of As(III) was observed to be higher. Depleted O2 levels in the bottom lake waters as a result of biolo￾gical productivity during the summer months are a likely cause of the higher As concentrations in the deeper lake waters. 2.2.4. Seawater and estuaries Average As concentrations in open seawater usually show little variation and are typically around 1.5 mg l
1 (Table 1). Concentrations in estuarine water are more variable as a result of varying river inputs and salinity or redox gradients but are also usually low, at typically less than 4 mg l
1 under natural conditions. Peterson and Carpenter (1983) found concentrations between 1.2–2.5 mg l
1 in waters from Saanich Inlet, British Columbia. Values less than 2 mg l
1 were found in Oslofjord, Nor￾way (Abdullah et al., 1995; Table 1). Concentrations are commonly higher when riverine inputs are affected by industrial or mining effluent (e.g. Tamar, Schelde, Loire Estuaries; Table 1) or by geothermal water. Unlike some other trace elements such as B, saline intrusion of sea￾water into an aquifer is unlikely to lead to a significant increase of As in the affected groundwater. Arsenate shares many chemical characteristics with phosphate and hence in oxic marine and estuarine waters, depletions in phosphate in biologically productive surface waters are mirrored by depletions in arsenate. Arsenate concentration minima often coincide with photosynthetic maxima evidenced by high concentrations of chlor￾ophyll a (Cullen and Reimer, 1989). Several studies have noted variations in the behaviour of As during estuarine mixing. Some have reported conservative behaviour. In the unpolluted Krka Estuary of Yugoslavia, Seyler and Martin (1991) observed a linear increase in total As with increasing salinity ranging from 0.13 mg l
1 in fresh waters to 1.8 mg l
1 offshore (i.e. seawater value). How￾ever, other studies have observed non-conservative behaviour (departures from simple mixing) in estuaries due to processes such as diffusion from sediment pore￾waters, coprecipitation with Fe oxides or anthropogenic inputs (e.g. Andreae et al., 1983; Andreae and Andreae, 1989). The flocculation of Fe oxides at the freshwater￾saline interface is an important consequence of increases in pH and salinity. This can lead to major decreases in the As flux to the oceans (Cullen and Reimer, 1989). 2.2.5. Groundwater Background concentrations of As in groundwater are in most countries less than 10 mg l
1 (e.g. Edmunds et al., 1989 for the UK; Welch et al., 2000 for the USA) and sometimes substantially lower. However, values quoted in the literature show a very large range from <0.5 to 5000 mg l
1 (i.e. four orders of magnitude). This range occurs under natural conditions. High concentra￾tions of As are found in groundwater in a variety of environments. These include both oxidising (under condi￾tions of high pH) and reducing aquifers and areas affected by geothermal, mining and industrial activity. Evapora￾tive concentration can also increase concentrations sub￾stantially. Most high-As groundwater provinces are the result of natural occurrences of As. Cases of mining￾induced As pollution are numerous in the literature but tend to be localised. Cases of industrially-induced As pollution (including that from agriculture) may be severe locally (Table 1) but occurrences are relatively rare. Arsenic occurrences in groundwater are discussed more fully in Section 5. 2.2.6. Mine drainage Under the extremely acid conditions of some acid mine drainage, which can have negative pH values (Nordstrom et al., 2000), high concentrations of a wide range of solutes are found, including Fe and As. The highest reported As concentration of 850,000 mg l
1 is from an acid seep in the Richmond mine at Iron Mountain, California (Nordstrom and Alpers, 1999). In a compilation of some 180 samples of mine drainage from the USA, Plumlee et al. (1999) reported concentra￾tions ranging from detection limits (<1 mg l
1 or more) to 340,000 mg l
1 , again the highest values being from the Richmond mine. Gelova (1977) also reported an As concentration of 400,000 mg l
1 from the Ural Moun￾tains. Dissolved As in acid mine waters is rapidly removed as the Fe is oxidised and precipitated and the As scavenged through adsorption. At Iron Mountain, an efficient neutralization plant removes the As and metals for safe disposal. 2.2.7. Sediment porewaters Some high concentrations of As have been found in porewaters extracted from unconsolidated sediments and often form sharp contrasts to the concentrations observed in overlying surface waters (e.g. Belzile and Tessier, 1990). Widerlund and Ingri (1995) found con￾centrations in the range 1.3–166 mg l
1 in porewaters from the Kalix River estuary of northern Sweden. Yan et al. (2000) found As concentrations in the range 3.2–99 mg l
1 in porewaters from clay sediments in Saskatch￾ewan, Canada (Table 1). Increased concentrations have been found in porewaters affected by geothermal inputs. Aggett and Kriegman (1988) found As concentrations up to 6430 mg l
1 in anoxic porewaters from New Zealand. Even higher concentrations can be found in porewaters from sediments affected by mining contamination (tail￾ings, mineral-rich deposits). McCreadie et al. (2000) reported As concentrations up to 100,000 mg l
1 in P.L. Smedley, D.G. Kinniburgh / Applied Geochemistry 17 (2002) 517–568 525