Geogenic contamination - Arsenic
NOTE: Modified article from the Geogenic Contamination Handbook
Under most geochemical conditions, arsenic in aquifers remains tightly bound to the sediments, and dissolved concentrations in the groundwater remain low. However, elevated concentrations of arsenic in groundwater can be found even when solid-phase concentrations in the sediments are unremarkable. (It is the 55th most abundant element in the Earth’s crust with an average content of 1.8 mg/kg). Such sediments are typically young and contain organic carbon that can be consumed by microbes. Microbes use oxygen or other oxidants present and create chemically reducing conditions in which reduced species, such as arsenic (arsenite, As(OH)3), iron (Fe(II)) and manganese (Mn(II)) predominate. Groundwater in low-permeable host rocks/sediments with chemically oxidising conditions, high pH and generally arid conditions may also be contaminated, though to a lesser degree (Smedley and Kinniburgh, 2002). Figure 2.2 shows the modelled global probability of the occurrence of geogenic arsenic contamination in groundwater for both of these conditions combined. Geothermal contributions and sulphide oxidation from sulphide minerals in host rocks 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.
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.
For references, please visit the page References - Geogenic Contamination Handbook.