Investigating how arsenic gets into groundwater could determine where to site new wells.
Arsenic contaminates millions of people’s drinking water in West Bengal and Bangladesh, but scientists now think they might have figured out how the toxic element gets into the water in the first place. The discovery could help authorities to decide where to site new wells and stop this problem escalating.
For more than twenty years, people in West Bengal and Bangladesh have been drinking water from tube wells that tap into groundwater, rather than using untreated surface water. While this has helped reduce gastrointestinal diseases, it has inadvertently led to the one of the most serious water quality problems in history. More than twenty million people are now drinking water contaminated with arsenic, with tens of thousands already diagnosed with arsenic poisoning.
Scientists think that arsenic in the groundwater comes from sediments washed down from the Himalayas. Oxidised iron in the sediment traps arsenic, also present in an oxidised form known as As(V). Over time, the sediment becomes buried and covered with water. But as bacteria break down organic carbon compounds in the sediment, the iron oxides are reduced to a lower oxidation state, breaking their bond with arsenic and releasing it into the groundwater.
Andy Meharg of Aberdeen University, UK, and colleagues have now analysed levels of arsenic, iron and organic carbon in a range of affected sediment, and say that they can identify the type of river sediments that pose the highest risks.
Since organic carbon ultimately sets the release of arsenic in motion, finding its source is crucial, says team member David Kinniburgh of the British Geological Survey in Wallingford. ’We showed for the first time that there is a definite positive correlation between arsenic and organic carbon, which is not what would necessarily have been expected,’ he says. This implies that arsenic and carbon are buried at the same time in the sediment, the team argues in Environmental Science and Technology.1
Meharg’s team believes that the carbon involved comes from vegetation, such as mangrove swamps. They did not find any evidence, says Kinniburgh, to support alternative theories that organic carbon is derived from buried peat, or from surface pollution, such as latrines.
If they’re right, then the work will have dramatic implications, says Charles Harvey, a geochemist at Massachusetts Institute of Technology, Cambridge, US. Yet he remains sceptical: ’The paper does not show a relation between the sediment characteristics and arsenic in the groundwater,’ he says. ’There may be a strong correlation between solid arsenic and solid organic carbon - but what is the relation to dissolved arsenic?’ he asks.
Harvey points out that rich deposits of solid arsenic do not necessarily go hand in hand with dissolved arsenic. ’Perhaps dissolved arsenic is low in locations where the solid arsenic is high, because the solid arsenic is not dissolving there,’ he suggests. ’We just don’t know.’
Zafar Adeel, an expert on Bangladesh’s water crisis at the UN University in Hamilton, Canada, is also cautious. ’They argue that if arsenic concentrations follow the organic carbon profile then it would be better to have deep aquifers where organic carbon levels are very low,’ he says. ’But I would be concerned if people switched from shallow to deep aquifers without further investigation.’
In a related paper,2 Meharg has found that many Bangladeshis are also consuming arsenic when they eat rice. In southwestern Bangladesh, rice that had been irrigated with contaminated groundwater contains enough arsenic to pose serious health risks, they found in a survey.
The team suggests that in areas with high concentrations of groundwater arsenic, farmers should be encouraged to use surface waters for irrigating rice, or to substitute rice for cereal crops that require less irrigation.
1P N Williams et alEnv. Sci. & Tec.2A A Meharg et alEnv. Sci. & Tech., 2006 (DOI: 10.1021/es060722b)
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