A leaking Wellington landfill is causing snails to grow abnormally and wiping out invertebrate populations from a nearby stream, a report has found.
Sampling carried out by scientists at Aquanet for the Greater Wellington Regional Council found high levels of iron, manganese and ammonia in the Ōwhiro Stream and its tributaries, usually after heavy rainfall.
They found abnormally formed snails and the near-disappearance of other macroinvertebrate species - creatures without a backbone large enough to be seen without a microscope.
The testing was done after complaints were made about the stream's water being discoloured and foaming in November last year.
The report recommended taking urgent action to divert Ōwhiro Stream tributaries away from the landfill, and capturing and treating any liquid coming from the landfill.
It also raised doubts about plans to treat the wetland, saying the proposal was not an efficient option for cleaning up ammoniacal nitrogen.
There had been spikes in the amount of contaminants in the stream in 2012, 2013 and last year following heavy downpours, the report said.
Sampling is usually done every three months, but the report recommended doing it within two to four weeks following heavy rain.
Report no surprise - freshwater group
The group working to protect the stream, Friends of Ōwhiro Stream, said Aquanet's results were no surprise.
Group co-ordinator Martin Payne said they had been calling for more protection for the stream for years.
"We were certainly making a lot of noise in 2013. It's really only now that we're getting some traction in terms of things happening.
"We're pleased that the publicity is being created but we certainly would have preferred action to have been taken before this."
Diverting the tributaries away from the landfill was something the group had pushed for in 2013 when an extension of the landfill was proposed, Mr Payne said
"We spent a lot of time at the resource consent ... arguing for those but for some reason those conditions didn't require them to build those until the very end of the consent which is a number of years off yet.
"There's some gains happening but these landfills, which we've got three of in the catchment, continue to be a bit of a concern and obviously have these impacts."
"People have been talking to me, particularly about the pictures of the snails, it's just a surprise to them that we could be having those kinds of effects."