If there's enough radioactive material and it is mobile enough (due to ground water or wind driven mixing) to stay near the surface it could sterilize any organic material that comes in faster than it can accumulate.

"An area of oak-pine wood was selected East of Upton, and a tower was constructed that could raise and lower a canister from underground that contained radioactive source material, allowing for controlled dosage levels that emanated in a radius from the tower. The canister contained Cesium-137, which would emit ionizing gamma radiation without making the surrounding area radioactive itself."

They said specifically that it were a cesium-137 source. I think the commentator is fair in pointing out that this finding is ultimately quite odd.