A massive increase in housebuilding across Harrogate and Knaresborough is worsening pollution in the River Nidd, according to the Environment Agency.
Yorkshire Water is allowed to release sewage into the Nidd when the sewerage system is at risk of being overwhelmed through what are called storm overflows.
It has led to human waste being released over thousands of hours, and sampling by the Nidd Action Group has reported that the bacteria E. coli is at “concerningly high” levels.
Mr Duncan’s message to councillors was stark as he warned the problem could get worse without a recognition of the impact that development is having, and improvements to the town’s creaking Victorian sewerage system.
He said the Environment Agency was trying to tackle historic pollution problems, such as peat bog erosion and metal mining, which wash into the river at Nidderdale and travel downstream.
But he said its attempts are being made more difficult due to the thousands of new homes that have been built in the outskirts of Harrogate over the last decade — and thousands are more planned.
“If you are building housing estates on the urban fringe, on greenfield sites that historically have sewers just for servicing a pub and a few farms… and you’re putting hundreds of houses into these pipes then you only need a very small amount of rain [for waste] to spill into rivers.
“That’s untreated sewage. You’ve sieved out contraception and sanitary products, nothing more.”
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During the 2010s the now defunct Harrogate Borough Council did not have a local plan for several years, which gave the authority little control over where developers chose to build.
Harrogate now has a local plan but Mr Duncan said North Yorkshire Council must give more consideration into what impact new housing is having on the sewerage system, which he said is struggling to cope.
He added that the situation is leading to more storm overflows and more sewage being pumped into the Nidd.
A working group of councillors was set up last year to tackle pollution in the river, following an incident last summer where several children ended up in hospital after swimming there.
A campaign is also underway to clean up the river so it can be designated with bathing water status. Harrogate and Knaresborough Conservative MP Andrew Jones submitted an application to government last month.
North Yorkshire Council is also in the early stages of developing a new county-wide local plan that will set out where housebuilding can take place over the next 15 years.
Paul Haslam, Conservative councillor for Bilton and Nidd Gorge, said he hoped the council can view the sewage network as a “critical part” of infrastructure, like roads.
In the meantime, Mr Duncan urged councillors to factor in the sewerage system when granting planning permission for new developments.
“If you’re going to grant it, please understand the knock-on effects. Houses might be new, but the sewage pipes might run a very long way through central Harrogate to a very old system that’s at capacity.”