Sustainable development is a tricky thing. At least 140 million people in Asia are drinking arsenic-contaminated water, and the ever-expanding use of groundwater wells—a growing population needs water to drink, and farmers need it to grow crops to feed them—has been making the situation worse. Pumping out this water has changed the courses of underground streams, so previously clean water now flows through arsenic-laden sediments, and wells that used to be pure in villages once healthy suddenly pump out death. The naturally occurring arsenic kills human cells, leading first to skin scarring and then, as it slowly builds up in the body, to brain damage, heart disease and cancer.

Arsenic-laced groundwater has been found in at least 30 countries, from Argentina to China, Cambodia and Vietnam, as well as parts of Canada and the U.S., but the problem is particularly bad in India and Bangladesh as irrigation expands. Scientists have recently been trying to map the underground landscape in an attempt to pinpoint safer places to sink the wells.  But so far the subterranean flow changes and rates of chemical reactions have been outpacing the predictive ability of the maps. Alexander van Geen, a geochemist at Columbia Univeristy's Lamont-Doherty Earth Observatory, and other scientists discuss the problem and related issues of groundwater management in the video below.