The campaign of Welsh Labour leadership candidate, Vaughan Gething, accepted a donation from a company fined for dumping waste on the Gwent Levels.
Neal Soil Suppliers was fined £50,000 and paid £51,000 in costs in 2013 after illegally dumping waste on the site of Special Scientific Interest.
The founder and one of the directors of the company, David John Neal, has twice admitted to illegal dumping offences.
Mr Neal, who is also the owner of Atlantic Recycling, plead guilty in 2013 and 2017 after an investigation by Natural Resources Wales.
The company had leaked toxic liquid into local water sources on the Gwent Levels and Atlantic Recycling had not cleared it, as they were ordered to do.
The prosecution followed a year-long investigation by Natural Resources Wales, its predecessor the Environment Agency and South Wales Police.
There is an ongoing campaign to protect the Gwent Levels, which face having to make way for an M4 bypass, a project which Gething supports.
As reported by the BBC in 2017, Jon Goldsworthy, of NRW, said: “Despite our best efforts to work with David John Neal and the companies he operates, he continues to show a lack of respect for the rules we enforce and for the health of the environment.”
Mr Gething is yet to publicly comment on the investigation.