A naturally formed pool in the Waitomo Glow-worm Caves which tourists have been using as a wishing well for over 60 years has been cleaned out after scientists found the coins were impacting on the caves ecosystem.
A Groundwater Ecosystems programme by the National Institute of Water and Atmospheric Research(Niwa) found "extremely high levels" of copper and nickel in Aranui Cave, one of the three main tourist caves around Waitomo.
Despite staff removing as many coins as possible over the years to discourage tourists from adding more coins to the water, the levels of metals were found to be "highly toxic to aquatic life".
Niwa freshwater ecologist Mike Scarsbrook said contaminated sediments had been removed and it was hoped that over time the metals levels would drop.
While staff had not encouraged tourists to add money, it took only one tourist to add a coin for others to follow, he said.
"The work we were doing was aimed at wanting to show that what people were doing was having a potentially adverse impact."
Niwa had worked with the Department of Conservation (DOC) and cave management to clean up the pool and remove the sediments.
The biodiversity values of the pools were being examined through the research, Mr Scarsbrook said.
"We know these pools are habitats for various endemic or native species such as snails and shrimps.
"Those animals would be highly sensitive to heavy metals, as most aquatic organisms are.
"Our concerns were if the metals were left in the pool then that pool becomes a habitat that isn't attractive to aquatic life."
The physical features of the cave would not have been affected but the biological features of the cave potentially could have been, he said.
One of the reasons for the research was that very little was known about the biological features of cave systems, he said.
Waitomo caves manager Robert Tahi said about $20 to $30 of coins, weighing in at 1.7kg - some dating back to the 1940s - were now in DOC's possession.
