The only real way to balance geothermal in-game would be to add heat energy as an underground resource, limiting what maps this could be built on to those intended to have geothermal sources.

A bit of pollution would not be realistic. Geothermal is clean energy. The only reason it's not more commonly used to solve all the world's problems is it only works in areas where there is extremely high levels of heat close to the earth's surface, which tend to be near volcanos. Since energy is "lossy" when transmitted over long distance we can't just paint volcanos with geothermal plants and ship it around the world. If we could, Iceland would be rich.

A bit of pollution would be great so it can not be used as a place everywhere free energy thing. If this makes sense scientificially, of course. I dont know much about geothermal power.