Arsenic poisoning is no fun, as you can imagine. A 2007 study found that over 137 million people in more than 70 countries are probably affected by arsenic poisoning from drinking water.
Symptoms of arsenic poisoning begin with headaches, confusion, severe diarrhea, and drowsiness. As the poisoning develops, convulsions and changes in fingernail pigmentation called leukonychia striata may occur. When the poisoning becomes acute, symptoms may include diarrhea, vomiting, blood in the urine, cramping muscles, hair loss, stomach pain, and more convulsions. The organs of the body that are usually affected by arsenic poisoning are the lungs, skin, kidneys, and liver. The final result of arsenic poisoning is coma to death. (source)
Their findings suggest that 19.6 million people in China could be exposed to unsafe levels in their drinking water, including some living in areas areas not previously thought to be at risk. [...]
[Dr Annette Johnson, from the Swiss Federal Institute of Aquatic Science and Technology (EAWAG)] explained: "What it is very important to do is to go to the areas that are hotspots and screen those first. The chances are you will find more contaminated wells than wells that are not contaminated.
"And in the other areas, you still have to make sure you do screening for arsenic, but it is probably not such a high priority."
Of course, China isn't the only place where this could work. Many other countries could no doubt benefit from this way of predicting where arsenic is most likely to be found in drinking water.