This is more for fixing the surrounding area than the water, but if some fungi really do eat radiation then there should be a way to filter the water as well.

http://www.scientificamerican.com/article/radiation-helps-fu...

> Based on experiments with three different types of fungi, they believe the melanin-containing breeds absorb the high levels of energy in ionizing radiation and somehow turn it into a biologically useful (and benign) form, akin to a dark and dangerous version of photosynthesis.

http://www.permaculture.co.uk/articles/using-fungi-remediate...

> G. glutinosus has been reported to absorb – via the mycelium – and concentrate radioactive Cesium 137 more than 10,000-fold over ambient background levels. Many other mycorrhizal mushroom species also hyper-accumulate.

The second method still leaves you with radioactive waste (harvested mushrooms are burned, the ash is turned into glass) but it gets the radiation out of the environment.