Arsenic and Old Clays

All across Southern and Southeastern Asia, in countries such as India, Thailand, Nepal, and Vietnam, contamination of drinking water by arsenic is an enormous public health concern. An estimated 100 million people consume the carcinogenic chemical through naturally contaminated groundwater on a regular basis throughout the region. For years now, the general consensus has been that to avoid arsenic, you simply needed to dig your wells deeper. A recent study published in the Proceedings of the National Academy of Sciences tells a very different story, one which could help to explain how this threat has survived and how it continues to grow.

Researchers out of Stanford examined the deep wells of the Mekong Delta in Southern Vietnam, using data the Vietnamese government had collected in a nationwide survey through its Department of Water Resources Management. According to the data, deep wells were far more contaminated than shallow ones, with the study’s focus area possessing seven times more deep wells containing 10μg of arsenic per Liter of water than shallow wells. On top of that, contrary to popular wisdom, mean contamination levels for deep wells were much higher than in those close to the surface (four times as high to be precise).

The question now was why? The levels were far too high and had increased too quickly to be a result of the water’s natural travel downwards towards uncontaminated reservoirs. At the same time, it was found to be impossible that shallow well drilling in nearby cities had contaminated deeper aquifers, thereby contaminating the entire region’s deep water resources.

Researchers found their answer in the clay which encapsulates those deep aquifers. The clay, and the aquifers themselves, date from the Pliocene-Miocene age, when conditions were optimal for deposition of arsenic in soil along with those compounds that encourage arsenic’s dissolution in water. As these wells were pumped, they caused compaction of the surrounding clay as a result of subsidence, the process by which land drops, here caused by the removal of subterranean water. This vacuum-like effect caused dissolved arsenic, which had been stored in the highly porous clay, to be squeezed out into the surrounding aquifers, thereby contaminating water in the entire region.

What this means for the Mekong Delta is that these deep wells are no longer guaranteed sources of safe drinking water. Water that may have tested clean when the well was originally dug may now be contaminated with dangerous levels of arsenic. This is also true throughout South and Southeast Asia, where these deep aquifers have been assumed to be safe sources of potable water. This new finding underscores the importance of developing new means for providing clean water in these countries. Simply digging deeper is no longer a viable response to questions of how to access safe water.

Original article: http://www.pnas.org/content/110/34/13751.abstract

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