Deep groundwater in coastal deltas resilient to contamination

Groundwater pumped from the depths of the coastal Bengal Basin supporting more than 80 million people is largely secure from contamination, according to new research by UCL and the British Geological Survey. The study shows that groundwater pumped from depths below 150m in the coastal regions of the Bengal Basin is thousands of years old, and generally secure from contamination by salinity and arsenic found in shallow groundwater. Understanding how deeper groundwater is replenished and how vulnerable it is to contamination from saline and arsenic groundwater from shallow depths is critical to safeguarding public drinking water supplies. "Our results reveal no modern components in deep groundwater with ages ranging from 1,000 to 10,000 years. Traces of modern groundwater are, however, evident in a few pumping wells where they are associated with short-circuiting of vertical leakage within inadequately sealed boreholes," said lead author, Dr Dan Lapworth (UCL Geography and British Geological Survey). The discovery of arsenic contamination of shallow groundwater in Bangladesh in the early 1990s led to arsenic-mitigation strategies that included very rapid development of deep groundwater. There have, however, been serious concerns surrounding the security of deep groundwater to the arrival of both arsenic-rich and saline shallow groundwater.