Coal Mining - Environmental Geology?

The reduced environment would probably mean anaerobic- no oxygen. All available oxygen is used up decomposing organic material underwater. The Radon is a naturally occurring radioactive gas, a product of the radioactive decay of Uranium through a whole chain of different elements including Thorium, protactinium, and radium. Radon just happens to be a gas so it can move through liquids and through cracks in rock.