Irrigation alters arsenic levels in Bangladesh wells

Scientists from the US Massachusetts Institute of Technology have discovered that irrigation pumping over the last few decades has dramatically altered the chemistry of groundwater flowing through a Bangladesh aquifer. The chemical changes can influence the levels of arsenic in water, according to the study published in the latest issue of Science.

In his study of 17 Bangladesh wells, MIT Professor Charles Harvey and colleagues found that arsenic can be mobilised by the breakdown of dissolved organic carbon, mediated by microbes. Organic carbon from untreated waste can be drawn into the aquifer through the irrigation pumping, thus increasing the levels of arsenic in the water. However, Harvey also stresses that irrigation can lower arsenic concentrations when nitrates, oxygen or other oxidants are introduced. “Curtailing irrigation pumping is not a solution,” he warns.

In a separate paper to be published in Water Resources Research, Harvey’s group says that replacing 31% of the country’s most tainted wells with deeper wells will eliminate about 70% of arsenic-associated illnesses, assuming that arsenic levels remain low in the deep wells.

Regions of Bangladesh are prone to arsenic contamination of groundwater from surrounding soil. Thousands of people are affected by chronic poisoning, characterised by skin changes such as blackened knotty palms and eventually, cancer. According to Harvey, long-term exposure to present arsenic levels will result in around 1.2 million cases of hyperpigmentation, 600,000 of keratosis, 125,000 cases of skin cancer and 3,000 deaths a year from internal cancers.

In 1998 the World Bank gave Bangladesh US$2.4 million to develop a method of controlling the metal, but the problem persists. Wells drilled down to 300 feet still contain 50 parts per billion of arsenic, above the maximum WHO limit of 10ppb. The arsenic is believed to come from the Himalayan Mountains bordering the country, where the metal is transported downstream in sediment.