Efforts to prevent contaminated waters leaching out of a disused, flooded mine shaft appear to have paid off.
It was feared that rising water at the abandoned mine at Wheal Jane, near Baldhu – a hamlet near Truro – could spill out into the Carnon River.
This could have leaked into the Fal Estuary, a designated marine Special Area of Conservation, and home of the UK's only wild oyster breeding ground, it was feared.
However, the Environment Agency said yesterday an additional high volume pump, which was deployed by Cornwall Fire and Rescue Service, had helped stem rising water levels.
The pump was put in place on Wednesday evening and remained there overnight and yesterday.
A spokesman for the Environment Agency said that along with the seven pumps that were already being used, the additional equipment meant it was now possible to treat water leaving the mine.
The threat was first reported by site operator, Veolia Ltd, which notified the authorities after registering rising water levels in the main mine shaft.
In normal circumstances, mine water leaving the site is pumped and treated to control pollution.
The Environment Agency said it had been working with the site operator and the coal authority, which manages the treatment of water from the mine, to monitor rising water levels at the site.
The agency warned that if the mine water levels in the shaft continued to rise faster than the pumps could extract the water then there was a possibility that some of the water could leave the mine and enter the Carnon River untreated.
A spokesman for the Environment Agency said the situation at Wheal Jane was under constant monitoring.