Arsenic in Groundwater Worldwide

The average crustal abundance of arsenic is 1.5 mg/kg. The element is strongly chalocophile. Approximately 60% of natural arsenic minerals are arsenates, 20% sulphides and sulfosalts, and the remaining 20% are arsenides, arsenites, oxides, alloys and polymorphs of elemental arsenic. Arsenic concentrations of more than 10⁵ mg/kg have been reported in sulphide minerals and up to 7.6x10⁴ mg/kg in iron oxides (Smedley and Kinniburgh, 2002). However, concentrations are typically much lower. Arsenic is incorporated into primary rock-forming minerals only to a limited extent, for example, by the substitution of As³⁺ for Fe³⁺ or Al³⁺. Therefore, arsenic concentrations in silicate minerals are typically ~1 mg/kg or less (Smedley and Kinniburgh, 2002). Many igneous and metamorphic rocks have average arsenic concentrations of 1-10 mg/kg. Similar concentrations are found in carbonate minerals and carbonate rocks (Plant et al, 2004).

Arsenic concentrations in sedimentary rocks can be more variable. The highest arsenic concentrations (20-200 mg/kg) are typically found in organic-rich and sulphide-rich shales, sedimentary ironstones, phosphatic rocks, and some coals (Smedley and Kinniburgh, 2002). In sedimentary rocks arsenic is concentrated in clays and other fine-grained sediments. The average concentration of arsenic in shale is an order of magnitude greater than in sandstones, limestones and carbonate rocks. Arsenic is strongly sorbed by oxides of iron, aluminium and manganese as well as some clays, leading to its enrichment in ferromanganese nodules and manganiferous deposits (Plant et al, 2004).

Alluvial sands, glacial till and lake sediments typically contain <1-15 mg/kg arsenic (Plant et al, 2004).

The arsenic concentrations in soils show a similar range to that found in sediments except where contaminated by industry or agricultural activity (Plant et al, 2004). By and large, the arsenic release potential seems to be correlated with three soils or hydrogeological situations (this does not mean, however, that all soils of these types necessarily carry measurable levels of arsenic) (Alaerts and Khouri, 2004):

• Peaty or peaty-clayey soils with high humic organic content and with a high water table, which contain arsenopyrite crystals. When these soils, often associated with wetlands and marshes, are drained to bring them into production, oxygen penetrates the peat and oxidises the arsenopyrite crystals, thus releasing a sulphate-rich acid as well as dissolved arsenic into ground and drainage water. Arsenic concentrations are usually below 100 µg/l. Such conditions are found for example in England, Germany, Netherlands and many tropical peaty lowlands.
• Young volcanic deposits or thermal water sources. These often contain elevated dissolved arsenic concentrations that can well exceed 1 mg/l. This water usually enters surface water streams as experienced for example in Argentina, Bolivia, Chile, Greece and Taiwan.
• Loamy and clayey deposits (especially in deltaic areas) that may contain arsenic in dissolved state and/or adsorbed onto clay particles. The release of the arsenic is thought to be primarily governed not by changes in the redox potential, but by the physical-chemical circumstances that control desorption equilibria. Arsenic concentrations can vary from high to low. Such conditions are found for example in Bangladesh and China (Inner Mongolia and Xinjiang).
• Anthropogenic arsenic sources. Mine tailings, and soil under factories processing arsenic-based pesticides or under fields where these pesticides are applied can contain arsenic. This contamination commonly is of very localised nature. Tailing drainage may seep into rivers and aquifers. Concentrations can vary from low to medium. Pollution from mines has reported for example in Ghana and Thailand.

The concentration of arsenic in most groundwaters is <10 μg/l and often below the detection limit of routine analytical methods. The physicochemical conditions favouring arsenic mobilisation in aquifers are variable, complex and poorly understood, although some of the key factors leading to high groundwater arsenic concentrations are know. Mobilisation can occur under strongly reducing conditions where arsenic, mainly as As(III), is released by desorption from, and/or dissolution of, iron oxides. Immobilisation under reducing conditions is also possible. Some sulphate-reducing micro organisms can respire As(V) leading to the formation of an As₂S₃ precipitate. Some immobilisation of arsenic may also occur if iron sulphides are formed (Plant et al, 2004).

Reducing conditions favourable for arsenic mobilisation have been reported most frequently from young (Quaternary) alluvial, deltaic sediments where the interplay of tectonic, isostatic and eustatic factors have resulted in complex patterns of sedimentation and rapid burial of large amounts of sediment together with fresh organic matter during delta progradation. Thick sequences of young sediments are quit often the sites of high groundwater arsenic concentration (Plant et al, 2004). Recent groundwater extraction, either for public supply or for irrigation, has induced increased groundwater flow. This could induce further transport of arsenic (Harvey et al, 2002).

High concentrations of naturally occurring arsenic are also found in oxidising conditions where groundwater pH values are high (ca. >8) (Smedley and Kinniburgh, 2002). In such environments, inorganic As(V) predominates and arsenic concentrations are positively correlated with those of other anion-forming species such as HCO₃¯, F¯, H₃BO₃, and H₂VO₄¯. The high-arsenic groundwater provinces are usually in arid or semi-arid regions where groundwater salinity is high. Evaporation has been suggested to be an important additional cause of arsenic accumulation in some arid areas (Welch and Lico, 1998).

High concentrations of arsenic have also been found in groundwater from areas of bedrock and placer mineralization which are often the sites of mining activities. Arsenic concentrations of up to 5000 μg/l have been found in groundwater associated with the former tin-mining activity in the Ron Phibun area of Peninsular Thailand, the source most likely being oxidised arsenopyrite (FeAsS) (Plant et al, 2004).

International standards for drinking water

The standards for maximum concentrations of arsenic in drinking water have been declining since the high toxicity of arsenic has become apparent. The 1903 report of the Royal Commission on Arsenic Poisoning in the UK set a standard of 150 μg/l. In 1942, the US Public Health Service set a drinking water standard of 50 μg/l for interstate water carriers. The WHO guideline value for arsenic in drinking water was reduced from 50 μg/l to a provisional value of 10 μg/l in 1993. Most western countries adopted this limitin their current drinking water standards (Yamamura, 2003).

On the other hand, many affected countries still operate 50μg/l standard due to lack of adequate testing facilities.