https://phys.org/news/2018-11-villagers-geology-safer-bangladesh.html

"Groundwater is popular because it is generally free of bacterial pathogens, unlike surface water," explained Alexander van Geen of Columbia University, Lamont-Doherty Earth Observatory. Groundwater must travel through rocks and sediments, which filters out most harmful bacteria. The same process adds minerals to groundwater—including a lot of arsenic in some shallow wells of Bangladesh.

In 2000, van Geen and his team surveyed 6,000 wells, and then recruited a health study cohort of 12,000 people. Then in 2013, they conducted larger survey of 50,000 wells serving 350,000 people. They found that government wells that were more than 150 meters deep were typically low in arsenic. However, they also found that the more than 900 deep were distributed in a way that suggested they had been taken by elite and politically connected households, and were not accessible to the public. This interpretation has since been confirmed by development economist Mushfiq Mobarak at Yale University, van Geen explained.