Which gas is a source of natural groundwater pollution

Radon is a gas at the temperatures we live in. 
The gas is slightly radioactive, and it's soluble in water. 
It's a widespread source of natural groundwater pollution,
and is responsible for the elevated level of radiation that's
often measured in the basements of homes.

There is no graph to look at on here, but i shall explain them to you. a line graph is just in its title it’s a graph with a line running through the dots to show an increase or decrease. a bar graph is used to show how much or how little something is so a comparison. a scatter plot is used to graph points to tell the data. and i don’t know what a histogram is.