The Environmental Audit Committee (EAC) of MPs has today warned the UK is facing a pollution 'timebomb' as the excessive use of nitrates in fertilisers works its way through groundwater tables, posing a potential threat to water sources and biodiversity.

Nitrates are a critical component of modern fertilisers, but environmental campaigners have long warned that their overuse leads to oxygen depletion in water, fuels ocean algal blooms, and can also contribute to air pollution from nitrogen oxides and ammonia.

Today's report from the EAC reiterates these concerns and warns that high levels of nitrate pollution in some groundwater sources might not peak for another 60 years. 

"Though progress has been made in reducing nitrates in surface waters, levels are high in some areas, especially in parts of England, and we still lag behind a number of our European neighbours," the report states. "The Committee is particularly disturbed to hear of the high levels of nitrate pollution in some of our groundwater sources, which supply nearly a third of our drinking water, which might not peak for another 60 years. Water companies are having to invest substantial sums of money in nitrate removal and water blending plants, the costs of which are being passed on to customers through water bills."

The report, entitled UK Progress on Reducing Nitrate Pollution, also argues that the management of nitrate pollution provides a prime example of wider concerns about how a post-Brexit 'governance gap' could undermine environmental progress.

Current regulation of water and air quality is based on EU legislation and the report reiterates fears that while such regulations will be transposed into UK law there is a risk of a 'governance gap', whereby 'zombie EU legislation' is adopted in the UK but is divorced from EU institutions that monitor, update, administer and ensure compliance.

The report acknowledges there is an opportunity to address these fears through the government's planned reform of agricultural subsidies and the promised introduction of a new post-Brexit green watchdog.

But the EAC said MPs remain particularly concerned about the danger that existing standards, including the target of water bodies reaching a good status by 2027, are weakened.

There are also fears the Environment Agency, which was recently subjected to staff being transferred to Defra to aid Brexit planning, lacks the resources to ensure compliance with existing nitrate rules.

The report recommends the government should conduct a thorough assessment of future nitrate risks and how pressures, such as population growth and climate change, might impact upon air, water and soil quality.

It also calls for robust interim targets and milestones should be included in the upcoming Environment Bill, underpinned by legally binding water quality targets, and reiterates previous calls for a new Environmental Enforcement and Audit Office (EEAO) to ensure targets are met.

The issue of post-Brexit environmental governance and whether or not to set legally binding targets in the planned Environment Bill is still the subject of intense debate in government.

The planned Brexit Withdrawal Agreement would commit the UK to broadly maintaining EU environmental goals and principles, while retaining a degree of flexibility on how they are delivered. It also commits the UK to ensuring robust enforcement mechanisms are put in place.

However, the Treasury is known to have opposed Defra proposals for a more robust watchdog and ambitious targets, citing fears they could hamper economic growth - a forecast that is fiercely contested by others in government and green groups.

Chair of the Environmental Audit Committee, Mary Creagh MP, said the UK was facing a nitrate 'time bomb' and it was critical that any post-Brexit governance regime has the reach and authority to address the threat.  

"If we are to deal effectively with the challenges nitrates pose to the environment, it is vital that, if Brexit happens, we do not end up with zombie legislation where EU laws apply but there is no oversight or governance," she said. "Targets on water quality must be enforced and any new UK legislation should not undermine the important principle that those causing the pollution are the ones who pay."

She also argued reforms to agricultural subsidies should encourage farmers to tackle nitrate pollution. "If we leave the European Union, there could be a new joined-up approach that can bring about standards that are even higher than those currently in place," she said. "Only by supporting farmers to invest in infrastructure and processes to reduce artificial fertiliser application will we see better, more sustainable, environmental outcomes. We will be scrutinising the Environment Bill closely."

A Defra spokeswoman said the government would respond to the EAC in due course, but stressed that steps were being taken to tackle nitrate pollution. "We have dedicated £400m to improve water quality - we are helping farmers improve their farming practices to reduce pollution, the Environment Agency assesses all our waters for levels of nitrate pollution and we set limits for fertiliser use in areas affected by nitrate pollution," she said.

The EAC report comes on the same day as a major new survey of UK farmers carried out for McDonalds revealed significant interest across the sector in undertaking environmental improvements, with. Ninety-eight per cent said they already had measures in place to bolster their environmental performance and 62 per cent claimed they are planning further green investments on their farms in 2019.

Separately, concerns over post-Brexit environmental governance were further fuelled by an explosive intervention from the outgoing chair of Natural England, Andrew Sells, who told MPs the agency was chronically under-resourced and no longer fully independent from Defra.

"The difficulty is that all of our money effectively comes from Defra," he said. "There is an inherent contradiction there. They want us to deliver their priorities. We say we have 500 statutory duties and responsibilities. We want to deliver those."

He added that steep and sustained cuts had significantly impacted the organisation. "The staff are very hardworking, frankly overworked, underpaid and many of them are stressed and have been through endless reorganisations and uncertainty," he said. 

A Defra spokeswoman said Natural England continued to have a crucial role to play in addressing the UK's environmental challenges.

"The work of Natural England and its staff to protect our invaluable natural spaces, wildlife and environment is vital and its independence as an advisor is essential to this," she said. "With the government's 25 Year Plan for the Environment, Natural England will continue to have a central role in protecting and enhancing our environment for future generations."