Brenda Myers is among the volunteers who have helped with the clean-up operation at Bumble Hole Nature Reserve in Netherton, where birds have died and been rescued.
The spillage was first reported on Tuesday with the substance believed to have leaked through a culvert at Bumble Hole Pool.
Local residents have since been working hard to help the authorities look after the wildlife and clean up the surrounding area.
The Environment Agency, which is leading the operation, is trying to establish the cause of the leak but has not yet confirmed the source.
Ms Myers, who has been a volunteer at the nature reserve for the past three years, said: "Apparently it is an illegal dumping of the pollutant.
"It has gone down the drain, and because of the culvert it has entered through, into the Bumble Hole Pool, that is like a storm drain.
"It has been washed from the drain it was dumped, along with all the rain water, into the Bumble Hole Pool. Some irresponsible person has decided to dump their diesel, which I presume it was, down the drain rather than getting rid of it in a responsible manner and pay for it.
"My message would be 'please don't do it because you don't know what the knock on effects are'.
"I don't think these people have really got a brain in their head. They just don't care.
"As long as there is someone else to pick up the bill, they will just do whatever they want to do."
The RSPCA said eight Canada geese, one swan and ducks have been affected.
Ms Myers said she found one coot dead, having been killed by another animal as it was unable to escape due to being covered in diesel.
She said upon arriving at the site, she found "horrendous wildlife everywhere, Canadian geese, mallards, coots, swans, all covered in what I presume diesel".
However agencies and the community acted fast in dealing with the pollution.
Make-shift barriers and absorbent pads were placed into the water to contain the spillage.
She added: "The community spirit has been outstanding, second to none.
"People have come along with bags of towels, vinyl gloves, curtains, sheets, anything and everything just to try and assist with the capture of wildlife and to help them.
"Some people have given up hours of their time because they cannot believe what has happened."
Council workers and contractors were on-site yesterday, alongside volunteers who included John Smith from Scared Animal Wildlife Rescue.
He praised the community response on Facebook, writing: "Can I just use this post to say a big thank you to all the friends of Bumble Hole! I have been on many large oil spills this year and this is the only place that the local community has got together in mass to help the wildlife, you are all wonderful! It truly makes me proud to be a local lad!