Officers are investigating whether zinc has found its way into a small stream that runs into lake at Orrell Water Park.
Dog walkers say they have been warned to be on the look out for signs of poisoning after pets have suddenly taken ill and died.
The brook runs alongside the new housing estate in Billinge, which was previously the site of Billinge Hospital.
An Environment Agency spokesperson said: “Yesterday evening (8 February), a member of the public notified us of potential pollution in the watercourse leading into Orrell Water Park, Wigan.
“We have responded quickly and are now on site investigating the situation. Our priority is to protect people and the environment from any potential damage.
“It’s important people remember to report any environmental incident, such as pollution, to the Environment Agency’s incident line, on 0800 80 70 60, 24/7, so we can investigate.”
Coun Michael Winstanley, representing Orrell ward said: "Hopefully the Environment Agency can get to the bottom of the issue, as there is a big concern for residents in the area about the negative effects this pollution is causing.
"Once the cause is known, the necessary action can be taken to stop anything like this from happening again."