Rodent poison deployed to deal with rats can be bad for the local water supply.
Although the poison is organic and breaks down a over time, the build up of poisoning in the water will only increase the toxicity in the water. Eventually as the poison is washed away the water will get toxic for local mammals.
Avoid using poison in controlling rats. It's both inhumane and will kill non target species.
There was a piece on the bbc's Farming Today about the use of poison on slugs and how it affects the water supply.
This is no different to rat poison.