Isle of Anglesey County Council has appealed for the safe return of a stolen pump which maintains the level of settlement ponds at the Parys Mountain mining site which prevents them drying out and releasing arsenic and lead-contaminated ochre dust into the air.
The pump was installed with funding from the Welsh Assembly Government at the complex historic site which has seen many hundreds of years of metal mining.
The Henwaith Settlement Ponds were determined as contaminated last year.
"This pump is such a specialised piece of equipment that it really isn't much use to anyone else as it will not run off normal mains electricity," said principal environmental health officer Huw Thomas.
"The theft leaves the project in jeopardy and increases the potential for people to come into contact with contaminants in the affected ponds. We have limited financial resource available to us, and the theft places an additional unnecessary burden upon the local authority, particularly in this financial climate."
He said there was no immediate risk to residents but there could be long-term health risks from breathing in the dust.
The ponds were determined because of the way the ponds dried out in summer months and the wind whipped up the ochre dust.
Discussions with the Environment Agency, Countryside Council for Wales, Gwynedd Archaeological Trust, Amlwch Industrial Heritage Trust, Anglesey Mining and local residents resulted in the decision to install the pump.
In March, the Heritage Lottery Fund made a £497,000 grant to improve visitor facilities at Parys Mountain.
