It wasn't really "without check" though, was it? Courts could still rule that an agency exceeded its authority, it's just that Chevron meant the courts had to give deference to the agency's interpretation if Congress left the law ambiguous – and if the agency's interpretation was "reasonable."
So for example, if a law grants an agency power to regulate pollution emitted into the air, the agency already couldn't simply decide on its own that it was also able to regulate toxins dumped into rivers. But it could decide, if the law was vague on this point, whether "emitted into the air" included car exhaust vs. only stationary factories, for example.
The principle was that if Congress left a definition or meaning ambiguous in the law, it's implied that defining its precise meaning is part of the regulatory work they wanted the agency to do. Now, instead of that principle, the meaning of every ambiguity is open for litigation to select a different interpretation if the court finds it preferable to the agency's.
It pretty much was "without check" unless the action was facially against the text of the governing statute for the agency.
Your example about pollution in the air and in the water is actually close to a good one: The EPA decided recently that they had power over CO2 emissions because of some very mild toxicity, even though that's not the reason why anyone wants to control CO2 emissions. They could easily have argued in your scenario that toxin dumped in the water evaporates and ends up in the air, so it's in their purview, and it might not be wrong under Chevron.
So for example, if a law grants an agency power to regulate pollution emitted into the air, the agency already couldn't simply decide on its own that it was also able to regulate toxins dumped into rivers. But it could decide, if the law was vague on this point, whether "emitted into the air" included car exhaust vs. only stationary factories, for example.
The principle was that if Congress left a definition or meaning ambiguous in the law, it's implied that defining its precise meaning is part of the regulatory work they wanted the agency to do. Now, instead of that principle, the meaning of every ambiguity is open for litigation to select a different interpretation if the court finds it preferable to the agency's.