Radioactivity in drinking water may occur due to radionuclides from natural or human causes. Which of the following is a naturally occurring radionuclide likely to contaminate drinking water? (Both uranium and radon were accepted; uranium is taken here.)

Uranium is a naturally occurring radionuclide that seeps from rocks and minerals into groundwater, contaminating drinking water.

Plutonium is essentially a human-made element from nuclear reactions, not a natural contaminant of water.

Radon is natural too and was also accepted, but it is a gas and uranium is taken here as the answer.

Strontium of concern is largely a product of nuclear fission and fallout rather than natural occurrence.

Natural uranium decays through a chain that also yields radon in the environment.

Prolonged intake of uranium in water can harm the kidneys, so limits are set for drinking water.