Where Does Clean Water Come From?

A few years ago, I was talking to a friend who had recently visited India. “I’ve seen the pollution,” he told me. “I get why people need clean water. But how does the water underground stay clean? Why is it different?”

 The answer to this question is kind of miraculous.

 Earth has two main kinds of fresh water: surface water and groundwater. Because of everything that happens at the surface (industry, farming, animal life, etc.), surface water tends to be polluted by viruses, bacteria, and chemical pollutants, and has to be treated before it can be safely used. This is a major problem for remote rural communities, who live too far from water treatment plants to access the safe water they provide. Instead, people end up walking miles to collect polluted surface water, which then makes them sick.

Simplified diagram of a deep underground aquifer.

 But groundwater is different. Groundwater in deep aquifers has percolated into cavities underground, where it is protected by layers of sand and clay. The sand acts as a filtration layer, attracting contaminants and pulling them out of the water. Clay acts as a confining layer, like a lid on a pot, keeping chemical compounds from seeping into the aquifer. Working together, these rocks insulate the water from surface pathogens and keep it clean. Then, the only thing people need to access that clean water is a well!