How does groundwater pollution occur?
Groundwater is simply water — mainly from rain and snow, but also from some human activities — that has soaked into the soil. That’s the end of its journey from our perspective, but the water keeps going long after it’s gone underground. It percolates downward, with dirt and rock particles filtering out dangerous bacteria as it sinks. When it finally reaches an impermeable layer of bedrock deep below the surface, it stops and begins to saturate the surrounding soil. Over many millennia, this pool of purified groundwater can grow into vast subterranean aquifers.