P L. Smedley, D G. Kinniburgh/Applied Geochemistry 17(2002 )517-568 3. 2.5. Soils the mineral type(Krause and Ettel, 1989). There is some Baseline concentrations of As in soils are generally of onfusion in the analysis of these solubility relationships the order of 5-10 mg kg. boyle and Jonasson(1973) between congruent dissolution, incongruent dissolution quoted an average baseline concentration in world soils and sorption/desorption reactions. Secondary arsenolite of 7.2 mg kg-I(Table 4)and Shacklette et al.(1974) (As O3)is also relatively soluble. Arsenic bound to Fe quoted an average of 7. 4 mg kg(901 samples) for oxides is relatively immobile, particularly under oxidising American soils. Ure and Berrow (1982)gave a higher conditions average value of 11.3 mg kg. Peats and bog soils can have higher concentrations (average 13 mg kg 3.3. The atmosphere Table 4), principally because of increased prevalence of sulphide mineral phases under the reduced conditions The concentrations of As in the atmosphere are Acid sulphate soils which are generated by the oxidation usually low but as noted above, are increased by inputs of pyrite in sulphide-rich terrains such as pyritic shales, from smelting and other industrial operations, fossil mineral veins and dewatered mangrove swamps can also fuel combustion and volcanic activity. Concentrations be relatively enriched in As. Dudas(1984) found As amounting to around 10--10-3 ug m-3 have been concentrations up to 45 mg kg-I in the b horizons of recorded in unpolluted areas, increasing to 0.003-0.18 acid sulphate soils derived from the weathering of pyr ug m-3 in urban areas and greater than 1 ug m close ite-rich shales in Canada. Concentrations in the over- o industrial plants (WHO, 2001). Much of the lying leached (eluvial, E) horizons were low (1.5-8.0 mg atmospheric As is particulate. Total As deposition rates kg-')as a result of volatilisation or leaching of As to have been calculated in the range <1-1000 ug m-2a lower levels. Gustafsson and Tin(1994) found similarly depending on the relative proportions of wet and dry evated concentrations(up to 41 mg kg-)in acid sul- deposition and proximity to contamination sources phate soils from the Mekong delta of vietnam. (Schroeder et al., 1987). Values in the range 38-266 ug Although the dominant source of As in soils is geo- m-2 a-l(29-55% as dry deposition)were estimated for gical, and hence dependent to some extent on the the mid-Atlantic coast(Scudlark and Church, 1988) concentration in the parent rock material, additional Airborne As is transferred to water bodies by wet or dry inputs may be derived locally from industrial sources deposition and may therefore increase the aqueous con such as smelting and fossil-fuel combustion products entration slightly. However, there is little evidence to and agricultural sources such as pesticides and phos- suggest that atmospheric As poses a real health threat phate fertilisers. Ure and Berrow (1982)quoted con- for drinking-water sources. Atmospheric As arising centrations in the range 366-732 mg kg in orchard from coal burning has been invoked as a major cause of soils as a result of the historical application of arsenical lung cancer in parts of China( Guizhou Province), but pesticides to fruit crops. threat is from direct inhalation of domestic coal- smoke and especially from consumption of foods dried 3. 2.6. Contaminated surficial deposits over domestic coal fires, rather than from drinking water Arsenic concentrations much higher than baseline affected by atmospheric inputs( Finkelman et al., 1999) values have been found in sediments and soils con- laminated by the products of mining activity, including mine tailings and effluent Concentrations in tailings piles 4. Mineral-water interactions and tailings-contaminated soils can reach up to several thousand mg kg(Table 4). The high concentration 4. Controls on arsenic mobilisation reflect not only increased abundance of primary As-rich sulphide minerals, but also secondary Fe arsenates and As with most trace metals. the concentration of as in Fe oxides formed as reaction products of the original ore natural waters is probably normally controlled by some minerals. The primary sulphide minerals are susceptible form of solid-solution interaction. This is most clearly to oxidation in the tailings pile and the secondary the case for soil solutions. interstitial waters and minerals have varying solubility in oxidising conditions groundwaters where the solid/solution ratio is large but in groundwaters and surface waters. Scorodite(FeA- it is also often true in open bodies of water(oceans, sO4. 2H20)is a common sulphide oxidation product and lakes and reservoirs) where the concentration of solid its solubility is considered to control As concentrations particles is small but still significant. In these open bodies in such oxidising sulphide environments. Scorodite is the particles can be of mineral and biological origin. It is metastable under most groundwater conditions and likely that in most soils and aquifers, mineral-As inter tends to dissolve incongruently, forming Fe oxides and actions are likely to dominate over organic matter-As releasing As into solution(Robins, 1987; Krause and interactions, although organic matter may interact to Ettel, 1989). In practice, a wide range of Fe-As mineral some extent through its reactions with the surfaces of olubility relationships are found which in part relate to minerals. Knowing the types of interaction involved is3.2.5. Soils Baseline concentrations of As in soils are generally of the order of 5–10 mg kg
1 . Boyle and Jonasson (1973) quoted an average baseline concentration in world soils of 7.2 mg kg
1 (Table 4) and Shacklette et al. (1974) quoted an average of 7.4 mg kg
1 (901samples) for American soils. Ure and Berrow (1982) gave a higher average value of 11.3 mg kg
1 . Peats and bog soils can have higher concentrations (average 13 mg kg
1 ; Table 4), principally because of increased prevalence of sulphide mineral phases under the reduced conditions. Acid sulphate soils which are generated by the oxidation of pyrite in sulphide-rich terrains such as pyritic shales, mineral veins and dewatered mangrove swamps can also be relatively enriched in As. Dudas (1984) found As concentrations up to 45 mg kg
1 in the B horizons of acid sulphate soils derived from the weathering of pyr￾ite-rich shales in Canada. Concentrations in the over￾lying leached (eluvial, E) horizons were low (1.5–8.0 mg kg
1 ) as a result of volatilisation or leaching of As to lower levels. Gustafsson and Tin (1994) found similarly elevated concentrations (up to 41mg kg
1 ) in acid sul￾phate soils from the Mekong delta of Vietnam. Although the dominant source of As in soils is geo￾logical, and hence dependent to some extent on the concentration in the parent rock material, additional inputs may be derived locally from industrial sources such as smelting and fossil-fuel combustion products and agricultural sources such as pesticides and phos￾phate fertilisers. Ure and Berrow (1982) quoted con￾centrations in the range 366–732 mg kg
1 in orchard soils as a result of the historical application of arsenical pesticides to fruit crops. 3.2.6. Contaminated surficial deposits Arsenic concentrations much higher than baseline values have been found in sediments and soils con￾taminated by the products of mining activity, including mine tailings and effluent. Concentrations in tailings piles and tailings-contaminated soils can reach up to several thousand mg kg
1 (Table 4). The high concentrations reflect not only increased abundance of primary As-rich sulphide minerals, but also secondary Fe arsenates and Fe oxides formed as reaction products of the original ore minerals. The primary sulphide minerals are susceptible to oxidation in the tailings pile and the secondary minerals have varying solubility in oxidising conditions in groundwaters and surface waters. Scorodite (FeA￾sO4 .2H2O) is a common sulphide oxidation product and its solubility is considered to control As concentrations in such oxidising sulphide environments. Scorodite is metastable under most groundwater conditions and tends to dissolve incongruently, forming Fe oxides and releasing As into solution (Robins, 1987; Krause and Ettel, 1989). In practice, a wide range of Fe–As mineral solubility relationships are found which in part relate to the mineral type (Krause and Ettel, 1989). There is some confusion in the analysis of these solubility relationships between congruent dissolution, incongruent dissolution and sorption/desorption reactions. Secondary arsenolite (As2O3) is also relatively soluble. Arsenic bound to Fe oxides is relatively immobile, particularly under oxidising conditions. 3.3. The atmosphere The concentrations of As in the atmosphere are usually low but as noted above, are increased by inputs from smelting and other industrial operations, fossil￾fuel combustion and volcanic activity. Concentrations amounting to around 10
5 –10
3 mg m
3 have been recorded in unpolluted areas, increasing to 0.003–0.18 mg m
3 in urban areas and greater than 1 mg m
3 close to industrial plants (WHO, 2001). Much of the atmospheric As is particulate. Total As deposition rates have been calculated in the range <1–1000 mg m
2 a
1 depending on the relative proportions of wet and dry deposition and proximity to contamination sources (Schroeder et al., 1987). Values in the range 38–266 mg m
2 a
1 (29–55% as dry deposition) were estimated for the mid-Atlantic coast (Scudlark and Church, 1988). Airborne As is transferred to water bodies by wet or dry deposition and may therefore increase the aqueous con￾centration slightly. However, there is little evidence to suggest that atmospheric As poses a real health threat for drinking-water sources. Atmospheric As arising from coal burning has been invoked as a major cause of lung cancer in parts of China (Guizhou Province), but the threat is from direct inhalation of domestic coal-fire smoke and especially from consumption of foods dried over domestic coal fires, rather than from drinking water affected by atmospheric inputs (Finkelman et al., 1999). 4. Mineral–water interactions 4.1. Controls on arsenic mobilisation As with most trace metals, the concentration of As in natural waters is probably normally controlled by some form of solid-solution interaction. This is most clearly the case for soil solutions, interstitial waters and groundwaters where the solid/solution ratio is large but it is also often true in open bodies of water (oceans, lakes and reservoirs) where the concentration of solid particles is small but still significant. In these open bodies, the particles can be of mineral and biological origin. It is likely that in most soils and aquifers, mineral–As inter￾actions are likely to dominate over organic matter–As interactions, although organic matter may interact to some extent through its reactions with the surfaces of minerals. Knowing the types of interaction involved is P.L. Smedley, D.G. Kinniburgh / Applied Geochemistry 17 (2002) 517–568 533