Difference between revisions of "Geogenic contamination - Arsenic"
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== Occurrence == | == Occurrence == | ||
| − | Arsenic in drinking water is a global threat to health, potentially affecting about 140 million people in at least 70 countries worldwide (Ravenscroft et al., 2009). It is considered by some researchers to have more serious health effects than any other environmental contaminant (Smith and Steinmaus, 2009). | + | [[File:Figure 2.2.jpg|thumb|400px|]]Arsenic in drinking water is a global threat to health, potentially affecting about 140 million people in at least 70 countries worldwide (Ravenscroft et al., 2009). It is considered by some researchers to have more serious health effects than any other environmental contaminant (Smith and Steinmaus, 2009). |
Drinking water can contain arsenic at concentrations of up to several mg/L, most commonly as the reduced species, As(III) (arsenite), or the oxidised species, As(V) (arsenate). As(III) is uncharged (H3AsO3) under ambient pH conditions, and as such, is more mobile than As(V) (H2AsO4- or HAsO42-). Under most geochemical conditions, arsenic in aquifers remains tightly bound to the sediments, and concentrations of dissolved arsenic in the groundwater remain low. However, two geochemical conditions have been identified under which elevated concentrations of arsenic can be found in the groundwater even when solid-phase concentrations in the sediments are unremarkable: reducing conditions in young alluvial aquifers and arid, high pH oxidising conditions in rocks and/or sediments with relatively low permeability (Smedley and Kinniburgh, 2002). Figure 2.2 shows the modelled probability of the occurrence of geogenic arsenic contamination in groundwater for both of these conditions combined. Geothermal contributions and sulphide oxidation in mine tailings can also lead to elevated arsenic concentrations in groundwater, but these conditions tend to be more localised and have not been included in the model. | Drinking water can contain arsenic at concentrations of up to several mg/L, most commonly as the reduced species, As(III) (arsenite), or the oxidised species, As(V) (arsenate). As(III) is uncharged (H3AsO3) under ambient pH conditions, and as such, is more mobile than As(V) (H2AsO4- or HAsO42-). Under most geochemical conditions, arsenic in aquifers remains tightly bound to the sediments, and concentrations of dissolved arsenic in the groundwater remain low. However, two geochemical conditions have been identified under which elevated concentrations of arsenic can be found in the groundwater even when solid-phase concentrations in the sediments are unremarkable: reducing conditions in young alluvial aquifers and arid, high pH oxidising conditions in rocks and/or sediments with relatively low permeability (Smedley and Kinniburgh, 2002). Figure 2.2 shows the modelled probability of the occurrence of geogenic arsenic contamination in groundwater for both of these conditions combined. Geothermal contributions and sulphide oxidation in mine tailings can also lead to elevated arsenic concentrations in groundwater, but these conditions tend to be more localised and have not been included in the model. | ||
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== Health effects == | == Health effects == | ||
| − | Arsenic causes a wide range of adverse health effects. The acute toxicity of As(III) is somewhat greater than that of As(V), but because As(V) is reduced to As(III) in the body, the two species can be considered equally toxic. | + | [[File:Figure 2.3.jpg|thumb|400px|]]Arsenic causes a wide range of adverse health effects. The acute toxicity of As(III) is somewhat greater than that of As(V), but because As(V) is reduced to As(III) in the body, the two species can be considered equally toxic. |
The first obvious symptoms are often skin lesions (keratosis, melanosis; Fig. 2.3), but other effects can include weakness, diarrhoea, bronchitis, vascular disease and diabetes mellitus (UNICEF/Chinese Ministry of Health, 2004). | The first obvious symptoms are often skin lesions (keratosis, melanosis; Fig. 2.3), but other effects can include weakness, diarrhoea, bronchitis, vascular disease and diabetes mellitus (UNICEF/Chinese Ministry of Health, 2004). | ||
The main health concerns, however, are cancers of the skin or internal organs. In particular, arsenic-related lung and bladder cancers can cause heavy mortality (Argos et al., 2010; Sohel et al., 2009). More recently, arsenic exposure has been linked to cardiac effects such as myocardial infarction (heart attacks) (Yuan et al., 2007). Cardiac risks are elevated within short exposure periods, while cancers can take decades to develop, even long after arsenic exposure has stopped (Fig. 2.4). In addition, arsenic is linked with a range of impacts on children, including reduced birth weight, infant mortality and impaired cognitive development (Smith and Steinmaus, 2009). | The main health concerns, however, are cancers of the skin or internal organs. In particular, arsenic-related lung and bladder cancers can cause heavy mortality (Argos et al., 2010; Sohel et al., 2009). More recently, arsenic exposure has been linked to cardiac effects such as myocardial infarction (heart attacks) (Yuan et al., 2007). Cardiac risks are elevated within short exposure periods, while cancers can take decades to develop, even long after arsenic exposure has stopped (Fig. 2.4). In addition, arsenic is linked with a range of impacts on children, including reduced birth weight, infant mortality and impaired cognitive development (Smith and Steinmaus, 2009). | ||
| − | Because of the ongoing uncertainty about low-level effects and the difficulties involved in measuring arsenic concentrations below 10 µg/L or in reducing arsenic concentrations to this level, the WHO has set 10 µg/L as a provisional guideline value (WHO, 2011). Many countries have a less strict drinking-water standard of 50 µg/L. The disease burden at these levels can be considerable; Argos et al. (2010) found all-cause mortality levels to be 34% higher in a population exposed to 0.01–50 µg/L arsenic, compared to a control group with <10 µg/L exposure. Those exposed to 150 µg/L or more showed 68% higher all-cause mortality. | + | [[File:Figure 2.4.jpg|thumb|400px|]]Because of the ongoing uncertainty about low-level effects and the difficulties involved in measuring arsenic concentrations below 10 µg/L or in reducing arsenic concentrations to this level, the WHO has set 10 µg/L as a provisional guideline value (WHO, 2011). Many countries have a less strict drinking-water standard of 50 µg/L. The disease burden at these levels can be considerable; Argos et al. (2010) found all-cause mortality levels to be 34% higher in a population exposed to 0.01–50 µg/L arsenic, compared to a control group with <10 µg/L exposure. Those exposed to 150 µg/L or more showed 68% higher all-cause mortality. |
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There is no effective medical treatment for chronic arsenicosis, except for switching to an arsenic-free drinking-water source. However, palliative care such as application of ointments to cracked skin lesions can ease suffering. Chelation therapy is effective for short-term, acute poisoning, but not for long-term exposure. | There is no effective medical treatment for chronic arsenicosis, except for switching to an arsenic-free drinking-water source. However, palliative care such as application of ointments to cracked skin lesions can ease suffering. Chelation therapy is effective for short-term, acute poisoning, but not for long-term exposure. | ||
Revision as of 18:43, 21 November 2024
Occurrence
Arsenic in drinking water is a global threat to health, potentially affecting about 140 million people in at least 70 countries worldwide (Ravenscroft et al., 2009). It is considered by some researchers to have more serious health effects than any other environmental contaminant (Smith and Steinmaus, 2009).
Drinking water can contain arsenic at concentrations of up to several mg/L, most commonly as the reduced species, As(III) (arsenite), or the oxidised species, As(V) (arsenate). As(III) is uncharged (H3AsO3) under ambient pH conditions, and as such, is more mobile than As(V) (H2AsO4- or HAsO42-). Under most geochemical conditions, arsenic in aquifers remains tightly bound to the sediments, and concentrations of dissolved arsenic in the groundwater remain low. However, two geochemical conditions have been identified under which elevated concentrations of arsenic can be found in the groundwater even when solid-phase concentrations in the sediments are unremarkable: reducing conditions in young alluvial aquifers and arid, high pH oxidising conditions in rocks and/or sediments with relatively low permeability (Smedley and Kinniburgh, 2002). Figure 2.2 shows the modelled probability of the occurrence of geogenic arsenic contamination in groundwater for both of these conditions combined. Geothermal contributions and sulphide oxidation in mine tailings can also lead to elevated arsenic concentrations in groundwater, but these conditions tend to be more localised and have not been included in the model.
Health effects
Arsenic causes a wide range of adverse health effects. The acute toxicity of As(III) is somewhat greater than that of As(V), but because As(V) is reduced to As(III) in the body, the two species can be considered equally toxic.
The first obvious symptoms are often skin lesions (keratosis, melanosis; Fig. 2.3), but other effects can include weakness, diarrhoea, bronchitis, vascular disease and diabetes mellitus (UNICEF/Chinese Ministry of Health, 2004).
The main health concerns, however, are cancers of the skin or internal organs. In particular, arsenic-related lung and bladder cancers can cause heavy mortality (Argos et al., 2010; Sohel et al., 2009). More recently, arsenic exposure has been linked to cardiac effects such as myocardial infarction (heart attacks) (Yuan et al., 2007). Cardiac risks are elevated within short exposure periods, while cancers can take decades to develop, even long after arsenic exposure has stopped (Fig. 2.4). In addition, arsenic is linked with a range of impacts on children, including reduced birth weight, infant mortality and impaired cognitive development (Smith and Steinmaus, 2009).
Because of the ongoing uncertainty about low-level effects and the difficulties involved in measuring arsenic concentrations below 10 µg/L or in reducing arsenic concentrations to this level, the WHO has set 10 µg/L as a provisional guideline value (WHO, 2011). Many countries have a less strict drinking-water standard of 50 µg/L. The disease burden at these levels can be considerable; Argos et al. (2010) found all-cause mortality levels to be 34% higher in a population exposed to 0.01–50 µg/L arsenic, compared to a control group with <10 µg/L exposure. Those exposed to 150 µg/L or more showed 68% higher all-cause mortality.
There is no effective medical treatment for chronic arsenicosis, except for switching to an arsenic-free drinking-water source. However, palliative care such as application of ointments to cracked skin lesions can ease suffering. Chelation therapy is effective for short-term, acute poisoning, but not for long-term exposure.
