THOUSANDS of people in Bangladesh and neighbouring West Bengal are chronically exposed to arsenic-contaminated groundwater, which causes skin lesions and increases the risk of certain cancers. A team of researchers from the Lawrence Livermore National Laboratory (LLNL), Barnard College, Columbia University, the University of Dhaka, the Desert Research Institute and the University of Tennessee has found that the arsenic in the groundwater in the region is part of a natural process that predates any recent human activity, such as intensive pumping.
Bacterial respiration of organic carbon releases naturally occurring arsenic from sediment into groundwater, but the source of this organic carbon remains unclear. The work was published in the recent edition of Proceedings of the National Academy of Sciences (PNAS). Scientists of Barnard College, led by Brian Mailloux, isolated microbial deoxyribonucleic acid (DNA) from several depth intervals in arsenic-contaminated aquifers in Bangladesh and analysed the DNA’s radiocarbon signature, which reflects whether the organic carbon used by the microbes derives primarily from younger, surface-derived sources that are transported by groundwater into the aquifers or older, sediment-derived sources.
Using “bomb pulse” radiocarbon analysis, the DNA of the groundwater bacteria was dated at the LLNL. It was found that the DNA samples were consistently younger than the sediment, suggesting that the microbes favour using surface-derived carbon. Thousands of litres of groundwater from the sampling site was passed through filters to collect bacteria for DNA dating.
The surface-derived carbon has flowed into the aquifer over hundreds to thousands of years—a rate that is approximately 100 times slower than groundwater flow. The results suggest that recent human activities, such as intensive groundwater pumping, have not yet significantly affected the release of arsenic into the groundwater at this site.