Massive groundwater withdrawals promote arsenic transport into an uncontaminated aquifer
The 6.5 million inhabitants of Vietnam's capital, Hanoi, are facing a growing threat: as more and more water is pumped from a deep - previously uncontaminated - aquifer, arsenic-laden water from shallow sediments is advancing towards the city's wells. These findings, confirming what had been suspected for some time, are reported in a study - with Eawag participation - published today in Nature.
A study carried out near Hanoi by researchers from the US, Switzerland and Vietnam indicates that excessive pumping of groundwater poses a long-term threat to millions of people. According to lead author, Professor Alexander van Geen of Columbia University, the study published in Nature today demonstrates for the first time that a clean aquifer can be contaminated by accelerated flows of groundwater containing high levels of arsenic. Although the movement of arsenic contamination is slow, the process appears to be unstoppable.
Hanoi is expanding at a pace typical of many metropolitan areas worldwide, and pumping for municipal water supplies doubled between 2000 and 2010, to around 900 million litres a day. In the city, water is treated and filtering ensures that any arsenic is largely removed. However, this only solves the problem if water users are connected to the municipal supply, which is not the case in many suburban areas. Here - for example, near the Red River - water for household use is pumped from numerous private wells and does not undergo treatment. In the past, high water levels in the safe aquifer have ensured that water pumped from these wells is free of arsenic. But as a result of Hanoi's growing groundwater withdrawals, flow conditions have changed: contaminated water from arsenic-rich sediments and from the river is now increasingly intruding into the previously uncontaminated aquifer.
Co-author Michael Berg, a geochemist at Eawag, comments: "It's a huge, unintended experiment. We are altering systems all over the world." He believes that processes similar to those observed around Hanoi - involving not only arsenic but also other contaminants - may be under way in other areas, such as the megacities of Dhaka (Bangladesh) and Beijing (China). But fundamental changes in groundwater conditions could also occur in regions where large amounts of water are pumped for irrigation, e.g. in parts of sub-Saharan Africa or arid areas of North America.
At some of the sites investigated near Hanoi, arsenic concentrations in groundwater are already 10-50 times higher than the guideline level of 10 µg/L recommended by the World Health Organization. Other sites have yet to be affected by geogenic arsenic contamination: according to Professor Rolf Kipfer of Eawag, who measured groundwater ages using helium and hydrogen isotope dating techniques, the movement of dangerous arsenic levels is much slower than the migration of groundwater itself. While, over the last 40-60 years, water from the contaminated aquifer has migrated more than 2 kilometres towards the city centre, arsenic contamination has only advanced around 120 metres. This means that, where water quality is still acceptable today, sufficient time remains for water managers to address the problem - by reducing withdrawals or introducing water treatment. However, as Berg points out, "Arsenic contamination is constantly moving, and as it continues to advance, more people are exposed."