Millions litres of orange-coloured, contaminated water flooded into the Fal
The Environment Agency is removing reed beds put in place to help clear up metal contamination in a river from a mine site in Cornwall.
About 50m litres of orange-coloured water flooded into the Fal Estuary from the Wheal Jane mine in 1992.
The agency said the reeds were an experiment to help clear the effects of the orange-coloured water.
It said that the reeds were not sufficient and that the area's active treatment plant would continue working.
Wheal Jane was worked for tin, copper, silver, zinc and arsenic from the 18th Century. It closed in 1991 and began to fill with water.
The failure of a plug caused poisoned water to flow along the Carnon River, Restronguet Creek and into the Carrick Roads.
An active treatment plant was built to deal with the pollution, and other passive schemes were started to help the plant, including the reed beds.
Jeff Boyd, from the Environment Agency, said: "We will continue with that active treatment plant for the indefinite future to make sure that the Fal continues to be protected.
"The passive treatment was an experiment to see if we might do the treatment in a different way with a minimum amount of energy and chemicals and, hence, have a more sustainable way of doing it.
"Unfortunately, it would have required a reed bed area which was so enormous that there just wasn't sufficient room to put enough in."
After the reeds are removed, land in the area next to the Portreath to Devoran cycle trail will be landscaped with fencing and new signs.