The private land is a former gravel works and protected for the importance of its neutral marsh as well as its scrub, grassland and pool habitats, which support a diverse range of breeding birds, according to government body Natural England.
Air and water quality are now being monitored, the Environment Agency said, and the public are urged to stay away.
Ian Jones, area director for the West Midlands at the agency, said: “This is a sickening case of large-scale illegal waste dumping, and we share the public’s disgust.
“We have launched an investigation with Natural England to find the criminals and ensure they pay the price for their despicable actions.”