Toxic arsenic removed from fields by monsoon floodwaters
22 December 2009
As a result of irrigation with arsenic-rich groundwater, this toxic substance can accumulate in paddy soils and – at high concentrations – may ultimately find its way into rice plants. In a study published in Nature Geoscience, researchers from Eawag and the ETH Zurich collaborating with scientists from Bangladesh have shown that a certain amount of arsenic is released from soils into floodwaters during the monsoon season.
In a flooded paddy, a boy
from Bashailbhog village (Bangladesh)
helps researchers to collect porewater
samples from various sediment depths.
Worldwide, millions of people drink water with arsenic concentrations far above the WHO guideline value of 10 µg/L. The situation is particularly dire in Bangladesh. Here, between 1980 and 1990, tube wells were sunk (with support from the WHO and Unicef) to provide supplies of clean groundwater from a depth of 5–50 m. The aim was to reduce the incidence of diarrhoeal diseases and cholera outbreaks caused by the consumption of polluted surface water. In many places, however, water from these depths is contaminated – not with dangerous pathogens, but with arsenic. Concentrations of 0.5 to 2500 µg/L have been measured by Eawag. According to Ruben Kretzschmar, a professor in the Department of Environmental Sciences at the ETH Zurich and co-author of the study, “Contamination levels are typically around 400 micrograms.”
In humans, excessive arsenic intake leads to insidious poisoning: initial changes in skin pigmentation may subsequently develop into skin cancer, followed by organ failure and neurological disorders. After 15–30 years, the risk of cancer is high.
Over 1000 tonnes of arsenic per year
As paddy fields in Bangladesh are irrigated with arsenic-rich water during the dry season, an estimated 1360 tonnes of arsenic is added to arable soils per year. Experts are concerned about the possible long-term accumulation of arsenic in paddy soils. In fields that are flooded, arsenic concentrations in soils decrease over the monsoon season. To date, the reasons for this decline have not been clear. It has been suggested that arsenic could migrate to deeper soil layers, be released into the atmosphere as a result of microbial activity, or diffuse into monsoon floodwaters and be carried away as the waters recede.
Across the fields by boat
With analyses of soil porewater and overlying floodwater, it has now been possible to demonstrate the pathway for arsenic removal. For this study, Linda Roberts – a doctoral student at Eawag and the ETH Zurich and lead author of the publication – used a boat to collect samples from two flooded rice paddies in Bangladesh during the monsoon season in 2006 and 2007. The analyses showed that arsenic is predominantly mobilized into the soil solution in the uppermost 10 cm of soil, where the greatest accumulation occurs during irrigation; it then enters the overlying floodwater by diffusion. The patterns of arsenic distribution in water on the fields and in canals leading to rivers indicate that arsenic is transported to the rivers by the receding floodwaters. Thus, according to the researchers’ calculations, 51–250 mg/m2 arsenic is washed out of paddy soils into floodwater and ultimately into the ocean. Each year, via this process, 13–62% of the arsenic added to soils through irrigation is released again.
A
villager in his boat crossing paddy fields in the Munshiganj
district of
Bangladesh. During the monsoon season, many
paddies are submerged to a depth of
several metres for
up to 4 months.
Greater risk of reduced yields in non-flooded areas
In an earlier project, the researchers studied how arsenic is distributed across fields as a result of irrigation. They demonstrated that concentrations are particularly high near the irrigation inlet. However, concentrations also increase gradually in more distant parts of the fields, so that there is a risk of arsenic accumulating in rice plants and grains. As well as entering the food chain, arsenic could then produce toxic effects in the plants, leading to reduced yields. Roberts comments: “We conclude from our findings that arsenic probably accumulates more rapidly in areas that are not regularly flooded, and that there is a greater risk of future yield reductions in these areas.” Non-flooded fields should therefore be sparingly irrigated, so that the amount of arsenic added to soils is kept to a minimum.
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A global problem Contamination of drinking water with arsenic is a global problem. Regions affected include West Bengal, Vietnam, Thailand, Taiwan, Inner Mongolia, several South American countries, the US, Canada and parts of Europe. Elevated concentrations of arsenic in groundwater are also found in certain areas of Switzerland. High levels of arsenic in drinking water are only partly due to human influences. Although, in the past, arsenic entered soils and groundwater via animal feed additives and pesticides, for example, contamination of drinking water is primarily due to natural arsenic deposits. These occur in rocks, black shales, volcanic sediments and soils close to geothermal springs, but especially in river and lake sediments. Arsenic is released into these sediments as a result of the weathering of natural arsenic-bearing minerals such as arsenopyrite. Dissolved arsenic is strongly adsorbed to iron oxide or iron hydroxide particles transported by surface waters. Since the last ice age 15,000 years ago, when sea levels were about 100 m lower than today, rivers such as the Ganges have carried enormous amounts of sediment from the Himalayas into the delta. As well as the sediment load, substantial quantities of arsenic have been deposited in river beds and particularly in the slow-flowing river mouth areas. If the sediments are adequately supplied with oxygen, arsenic remains bound to them and does not contaminate groundwater. Generally, however, organic matter is also deposited in these sediments, providing a food source for sediment-dwelling microbes. To degrade organic matter, microorganisms first use the unbound oxygen available in the sediments. When this has been consumed, they resort to the iron oxides or iron hydroxides contained in the sediments; these are reduced to divalent iron, which is readily soluble. As a result, arsenic bound to these compounds is also released. |
- Interview with Linda Roberts on the BBC „Science in Action“
programme,
18 December 2009
[mp3-File, 2MB] - Roberts LC et al.: Arsenic release from paddy soils during monsoon flooding, Nature Geoscience Published online: 13 December 2009, doi:10.1038/ngeo723
- Article from Eawag-News (2006): Arsenic in paddy fields – a hazard? [pdf]
The above is a slightly expanded version of an article originally written in German by Simone Ulmer for the online newsletter ETH Life. www.ethlife.ch (Photos: Linda Roberts, Eawag)
Amendment 20 January 2010
Eawag-Research shows response
The publication in Nature Geoscience shows already some response in Bangladesh. According to two articles in the Daily Star, Bangladesh’s most important newspaper, the government is already engaging in the problems.
Daily Star, 15 January 2010 [pdf]
Daily Star, 16 January 2010 [pdf]