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Over time this gets deposited underground.
For the study, the researchers collected samples of groundwater from a depth of 820 feet under the Earth’s surface.
The water at this level is known as "fossil", mainly because it is almost 12,000 years old.
This fossil water was then tested and the researchers found traces of modern-day rainwater mixed with the ancient sample.
The scientists mainly detected tritium — a radioactive isotope of hydrogen — which is generally found in human contaminated waters.
It was once thought that the fossil water was impervious to the pollutants on the surface, but the study proved that groundwater pollution is a reality.
While groundwater pollution and contamination was discovered perchance, the researchers were originally focused on ascertaining how much of Earth’s fossil water was over 12,000 years old.
The researchers noted that a significant amount of groundwater reserves today had deposits of fossil water, which was at least 12,000 years old.
Being from such an old period, the scientists were certain that this groundwater would not possess any human contaminants.