Answer: The source of natural groundwater pollution is Radon gas.
Explanation: Groundwater pollution is caused when the pollutants are released to the ground which then enters the groundwater.
Natural groundwater pollutants are arsenic and radon.
Arsenic is present in the rocks and is in solid form.
Radon is a radioactive product formed by the decay of naturally occurring Uranium element in the Earth's crust. This is present in the gaseous form.
Hence, Radon gas is a source of natural groundwater pollution.