P L. Smedley, D.G. Kinniburgh/ Applied Geochemistry 17(2002)517-568 Table 4(continued) As concentration average and/ No of Reference r range(mg kg-) analyses lining-contaminated reservoir sediment, Montana 100-800 Moore et al. (1988) Mine tailings. British Colombia 9030396-2000 Soils and tailings- contaminated soil. ul Kavanagh et al. (1997) aminated soil, Montana to 1100 Nagorski and Moore (1999) Industrially polluted inter-tidal sediments, USA 0.38-1260 Davis et al. (1997) Soils below chemicals factory, USA Sewage sludge 9.8(24396 Zhu and Tabatabai (1995) sediments are enriched in As relative to igneous rocks. are also relatively enriched in As, with values up to ca Sands and sandstones tend to have the lowest con- 400 mg kg-I having been reported centrations. reflecting the low As concentrations of thei dominant minerals: quartz and feldspars. Average sand 3. 2. 4. Unconsolidated sediments stone As concentrations are around 4 mg kg-I(Table 4 Concentrations of as in unconsolidated sediments are although Ure and Berrow (1982)gave a lower average not notably different from those in their indurated equiva- value of 1 mg kg- lents. Muds and clays usually have higher concentrations Argillaceous deposits have a broader range and than sands and carbonates. values are typically 3-10 mg igher average As concentrations than sandstones, with kg-, depending on texture and mineralogy (Table 4) a typical average of around 13 mg kg- Table 4: Ure and Elevated concentrations tend to reflect the amounts of Berrow, 1982). The higher values reflect the larger pro- pyrite or Fe oxides present. Increases are also common portion of sulphide minerals, oxides, organic matter and in mineralised areas. Placer deposits in streams can have clays. Black shales have As concentrations at the high very high concentrations as a result of the abundance of end of the range, principally because of their enhanced sulphide minerals. Average As concentrations for pyrite content Data given in Table 4 suggest that mar- stream sediments in England and Wales are in the range ine argillaceous deposits have higher concentrations 5-8 mg kg(AGRG, 1978). Similar concentrations than non-marine deposits. This may also be a reflection have also been found in river sediments where ground of the grain-size distributions, with potential for a water-As concentrations are high: Datta and Sub- higher proportion of fine material in offshore pelagic rumanian (1997) found concentrations in sediments sediments as well as systematic differences in sulphur from the River Ganges averaging 2.0 mg kg-(range and pyrite contents. Marine shales tend to contain 1. 2-2.6 mg kg-). from the Brahmaputra River aver higher S concentrations. Sediment provenance is also a aging 2.8 mg kg-I(range 1. 4-5.9 mg kg-)and from the likely factor. Particularly high As concentrations have Meghna River averaging 3.5 mg kg-I(range 1.3-5.6 mg been determined for shales from mid-ocean settings kg) (Mid-Atlantic Ridge average 174 mg kg-I Table 4) Cook et al. (1995) found concentrations in lake sedi variable but often high. Samples of organic-rich sh"re ments ranging between 0.9 and 44 mg kg-I(median 5.5 Concentrations in coals and bituminous deposits mg kg)but noted that the highest concentrations were (Kupferschiefer) from Germany have As concentrations present up to a few kilometres down-slope of miner of 100-900 mg kg(Riedel and Eikmann, 1986: alised areas. The upper baseline concentration for these Table 4). Some coal samples have been found with sediments is likely to be around 13 mg kg(90th per- extremely high concentrations up to 35,000 mg kg- centile). They also found concentrations in glacial till of (Belkin et al., 2000), although generally low concentra 1.9-170 mg kg(median 9.2 mg kg Table 4) and tions of 2.5-17 mg kg-I were reported by Palmer and noted the highest concentrations down-ice of mine Klizas(1997). Carbonate rocks typically have low con- alised areas (upper baseline, 90th percentile, 22 centrations, reflecting the low concentrations of the kg-). Relative As enrichments have been observed in constituent minerals(ca 3 mg kg; Table 4) reducing sediments in both nearshore and continental- Some of the highest observed As concentrations are shelf deposits(Peterson and Carpenter, 1986; Legeleux found in ironstones and Fe-rich rocks. James(1966) et aL., 1994). Legeleux et al. (1994)noted concentrations collated data for ironstones from various parts of the increasing with depth(up to 30 cm)in continental shelf world and reported As concentrations up to 800 mg sediments as a result of the generation of increasingly g-l in a chamosite- limonite oolite from the former reducing conditions. Concentrations varied between USSR. Boyle and Jonasson(1973) gave concentrations for sites, but generally increased with depth in the range Fe-rich rocks up to 2900 mg kg-l(table 4). Phosphorites 2.3-8.2 mg kg-(Table 4)sediments are enriched in As relative to igneous rocks. Sands and sandstones tend to have the lowest con￾centrations, reflecting the low As concentrations of their dominant minerals: quartz and feldspars. Average sand￾stone As concentrations are around 4 mg kg
1 (Table 4) although Ure and Berrow (1982) gave a lower average value of 1mg kg
1 . Argillaceous deposits have a broader range and higher average As concentrations than sandstones, with a typical average of around 13 mg kg
1 (Table 4; Ure and Berrow, 1982). The higher values reflect the larger pro￾portion of sulphide minerals, oxides, organic matter and clays. Black shales have As concentrations at the high end of the range, principally because of their enhanced pyrite content. Data given in Table 4 suggest that mar￾ine argillaceous deposits have higher concentrations than non-marine deposits. This may also be a reflection of the grain-size distributions, with potential for a higher proportion of fine material in offshore pelagic sediments as well as systematic differences in sulphur and pyrite contents. Marine shales tend to contain higher S concentrations. Sediment provenance is also a likely factor. Particularly high As concentrations have been determined for shales from mid-ocean settings (Mid-Atlantic Ridge average 174 mg kg
1 ; Table 4). Concentrations in coals and bituminous deposits are variable but often high. Samples of organic-rich shale (Kupferschiefer) from Germany have As concentrations of 100–900 mg kg
1 (Riedel and Eikmann, 1986; Table 4). Some coal samples have been found with extremely high concentrations up to 35,000 mg kg
1 (Belkin et al., 2000), although generally low concentra￾tions of 2.5–17 mg kg
1 were reported by Palmer and Klizas (1997). Carbonate rocks typically have low con￾centrations, reflecting the low concentrations of the constituent minerals (ca. 3 mg kg
1 ; Table 4). Some of the highest observed As concentrations are found in ironstones and Fe-rich rocks. James (1966) collated data for ironstones from various parts of the world and reported As concentrations up to 800 mg kg
1 in a chamosite-limonite oolite from the former USSR. Boyle and Jonasson (1973) gave concentrations for Fe-rich rocks up to 2900 mg kg
1 (Table 4). Phosphorites are also relatively enriched in As, with values up to ca. 400 mg kg
1 having been reported. 3.2.4. Unconsolidated sediments Concentrations of As in unconsolidated sediments are not notably different from those in their indurated equiva￾lents. Muds and clays usually have higher concentrations than sands and carbonates. Values are typically 3–10 mg kg
1 , depending on texture and mineralogy (Table 4). Elevated concentrations tend to reflect the amounts of pyrite or Fe oxides present. Increases are also common in mineralised areas. Placer deposits in streams can have very high concentrations as a result of the abundance of sulphide minerals. Average As concentrations for stream sediments in England and Wales are in the range 5–8 mg kg
1 (AGRG, 1978). Similar concentrations have also been found in river sediments where ground￾water-As concentrations are high: Datta and Sub￾ramanian (1997) found concentrations in sediments from the River Ganges averaging 2.0 mg kg
1 (range 1.2–2.6 mg kg
1 ), from the Brahmaputra River aver￾aging 2.8 mg kg
1 (range 1.4–5.9 mg kg
1 ) and from the Meghna River averaging 3.5 mg kg
1 (range 1.3–5.6 mg kg
1 ). Cook et al. (1995) found concentrations in lake sedi￾ments ranging between 0.9 and 44 mg kg
1 (median 5.5 mg kg
1 ) but noted that the highest concentrations were present up to a few kilometres down-slope of miner￾alised areas. The upper baseline concentration for these sediments is likely to be around 13 mg kg
1 (90th per￾centile). They also found concentrations in glacial till of 1.9–170 mg kg
1 (median 9.2 mg kg
1 ; Table 4) and noted the highest concentrations down-ice of miner￾alised areas (upper baseline, 90th percentile, 22 mg kg
1 ). Relative As enrichments have been observed in reducing sediments in both nearshore and continental￾shelf deposits (Peterson and Carpenter, 1986; Legeleux et al., 1994). Legeleux et al. (1994) noted concentrations increasing with depth (up to 30 cm) in continental shelf sediments as a result of the generation of increasingly reducing conditions. Concentrations varied between sites, but generally increased with depth in the range 2.3–8.2 mg kg
1 (Table 4). Table 4 (continued) Rock/sediment type As concentration average and/ or range (mg kg
1 ) No of analyses Reference Mining-contaminated reservoir sediment, Montana 100–800 Moore et al. (1988) Mine tailings, British Colombia 903 (396–2000) Azcue et al. (1995) Soils and tailings-contaminated soil, UK 120–52,600 86 Kavanagh et al. (1997) Tailings-contaminated soil, Montana up to 1100 Nagorski and Moore (1999) Industrially polluted inter-tidal sediments, USA 0.38–1260 Davis et al. (1997) Soils below chemicals factory, USA 1.3–4770 Hale et al. (1997) Sewage sludge 9.8 (2.4–39.6) Zhu and Tabatabai (1995) 532 P.L. Smedley, D.G. Kinniburgh / Applied Geochemistry 17 (2002) 517–568