Nitrate: there’s something in the water

Time lag: concentrations of nitrate in groundwater are continuing to rise in some areas.

The United Kingdom’s four nations have the highest river nitrate concentrations in Europe. The European Environment Agency is demanding change, writes James Fitzgerald.

The UK’s chemical-intensive farming methods have caught us between a rock and a hard place, according to scientists. The main concern is nitrate, and the levels leaching from the soil into the ground water, which is then used for public supply.

“The principal source of nitrate in ground water is coming from agriculture — at least 70 per cent,” says Professor Robert Ward, director of science at the British Geological Survey.

Large amounts of nitrates have been applied as fertiliser since the 1950s, until a peak in the 1980s, when controls were put in place. “That nitrate has been moving slowly through the subsurface, through the soils, the rocks overlying our aquifers. Because of the downward slow movement the nitrate is stored in the unsaturated zone. We have a peak of nitrate moving through, particularly in chalk, and that peak has still to reach the ground water, so concentrations are continuing to rise in some areas,” says Prof Ward.

The subterranean landscape of the UK contains chalk aquifers, red sandstone and limestone. All these rocks are permeable — the water seeps through until it finds the water table. Once it reaches the zone of saturation it will move laterally. That water will remerge at the surface — at low lying springs. Water companies also take advantage of permeable rocks by drilling wells to pump it for the public water supply and farming irrigation.

“Four water companies have reported increasing concentrations in nitrate, despite the controls that have been applied on the surface,” says Prof Ward. This would suggest we are witnessing the effects of a lag time from when those high levels of nitrate entered the ground.

“It’s not just about nitrate. If you imagine nitrogen as a pizza, it’s just one part of the pizza. So, it’s about nitrogen and not just nitrate,” says Penny Johnes, professor of biogeochemistry at Bristol University.

Every biological organism requires nitrogen to exist, says Prof Johnes — along with carbon and phosphorous, and some other minor elements. “Nitrogen is playing a part in driving eco-system responses, but it’s not the only driver of those.”

Policy development so far has sought a “silver bullet” approach, whereby “if we just control this thing, everything will be hunky dory, but it won’t. These biota have been around for billennia, they have evolved over time; they need these nutrient forms, and have evolved every possible means of obtaining them.”

Nitrate levels vary throughout the UK. In some areas it may not be the dominant form of nutrient that is driving ecological degradation.

“We have nitrite, which is small in terms of quantity, but is quite toxic to some organisms. Nitrite is one oxygen molecule less than nitrate and can be found in sewerage or slow-moving water, where the oxygen molecule can be knocked off,” says Prof Johnes.

Then there is ammonia (NH3), which becomes available through ammonium compounds. “There is a range of legislation to protect fresh water fish from ammonia. These are considered inorganic nutrients,” she says.

The other half of the story is organic compounds, which includes dissolved organic nitrogen and particulate nitrogen. This makes it highly bio-available and very reactive, so when you have a lot of organic waste, from sewerage works or livestock production, it drives the system into over production, due to the abundance of available chemical elements.

The Danes have gone from about 7.5 milligrams per litre of nitrogen to about 3.5 in river water. ‘The country has approached the problem by forcing the application of fertiliser to crops to below the optimum.’

The health effects of nitrate in food has recently garnered some headlines in the press. It is present in the water supply, but also in smoked meats. “Nitrite can be part of the trigger for stomach cancer… with blue baby syndrome, what happens is the nitrite gets into the blood, and it locks onto the haemoglobin molecule,” says Prof Johnes.

If this chemical is so diverse and its levels are increasing, then is there a legislative framework in place that covers the full nitrogen cycle?

“We have a number of directives and each one is trying to bite off a chunk of the problem,” says Prof Johnes.

The first is the Nitrates Directive that is concerned with controlling the nitrogen in the water supply, but which has no ambition to tackle nitrate and its role in eco-systems.

“We have the European Water Framework Directive where we have no nitrogen standards in the UK for surface fresh waters and a limited range for ground water-fed wetlands in relation to nitrate,” says Prof Johnes. “But the evidence is out there that most of the water bodies we are looking at are driven by multiple stressors; nitrogen is one of those and phosphorous is another, and we have thresholds for the phosphorous but we don’t have them for the nitrogen.”

She suggests we are out of line with thinking in continental Europe. The Dutch have a limit of 1.5 milligrams per litre of nitrogen going into their shallow lakes; other countries have a limit of 4 milligrams per litre. In the UK, limits have been proposed of 2 milligrams per litre of nitrate in the winter months, so there’s a whole range of different standards.

So, each piece of legislation directed at the nutrients is trying to tackle a different part of the problem.

Germany has designated the whole country as a Nitrate Vulnerability Zone (NVZ) and compensates farmers for implementing ground water protection measures.

Policies developed in a piecemeal way can create a phenomenon known as “pollutant swapping”. For example, attempts to reduce airborne ammonia from farming inject slurry into the ground, instead of letting it evaporate. While this may alleviate one environmental demand it contributes to another — as the nitrogen-rich slurry would then find its way more easily into the water table.

“I think if we move towards a system that is cross-compliant and wholistic — a policy that controls the whole system — then we can look for the win-wins. If we think we can do this for carbon, then we can manage nitrogen at the same time and, by the way, also reduce fertiliser costs for the farmer,” says Prof Johnes.

So, there could be an economic benefit, a carbon benefit and a nitrogen benefit.

This approach would go beyond just reducing fertiliser use on the farm, by asking farmers to treat manures and slurries as a resource — as a source of nitrogen, phosphorous and carbon.

It would “not just be a waste product to dress on the surface of the land, because it’s expensive to store it. You want the farmer to develop a farm-scale nitrogen and phosphorous budget — and maybe carbon. And from there look to drive up nutrient use efficiency — so that what we bring in the farm gate in nutrients is balanced by what we take out in our crops or what we store within the soil system,” says Prof Johnes.

Does that mean the problem is tied to eastern England, if measurable nitrogen levels are high there? Prof Johnes says it is countrywide. “Wherever we farm intensively the problem exists; it just exists in a slightly different chemical form, so one of the problems is that we are not monitoring the right things and it is very infrequent.”

Most monitoring is done on a monthly basis, but if there is a weather event, such as a flood, then huge amounts of material can pass into the system over a few hours and get missed by the data collectors. “If you look at the uncertainties associated with sampling at that frequency they are huge. You could be estimating your concentration at 75 per cent below what it actually is, or 500 per cent higher than the mean concentration is.”

She says there is a scientific argument that we should revisit what we do as a nation, and measure at fewer sites — given the budgetary restraints — but measure for the right things at a higher frequency.

“High risk and high concentration don’t necessarily mean the same thing,” says Professor David Reay, chair in carbon management at Edinburgh University. “Your risk might be greatest at a public water supply but you don’t just want to monitor there if a pollutant is migrating towards that, moving through the subsurface. You want to monitor it in a location that allows you to take action before it reaches the receptor, whether that be a water supply or a wetland.”

Annual average river nitrate concentration averaged by National River Basin Districts (2009). Source: European Environment Agency

Advances in robotics now allow for fertilisers to be deployed very precisely to crops.

With the European Environment Agency criticising all of the UK’s four nations over river nitrate concentrations — which are the highest in Europe — what is happening on the continent that isn’t here? In Denmark, reductions have been “absolutely staggering”, according to Professor Helen Jarvie of the Centre for Ecology and Hydrology. The Danes have gone from about 7.5 milligrams per litre of nitrogen to about 3.5 in river water. “The country has approached the problem by forcing the application of fertiliser to crops to below the optimum,” she says.

Germany has designated the whole country as a Nitrate Vulnerability Zone (NVZ) and compensates farmers for implementing ground water protection measures.

“In relation to NVZs, it’s never made any sense that we have cherry picked bits of the UK, as nitrate leaches in every environment, and so there is no reason to suggest there are certain areas which are vulnerable to nitrate movement,” says Prof Johnes. “So, the Germans have been quite pragmatic, and it shows what can be achieved if this was rolled out across the UK.”

Denmark has set benchmarks to help achieve a nutrient balance per farm — whereby what is brought in for food and feed is offset by an export to other systems, so that there is very little left to leach into the ground of evaporate into the air.

DEFRA in the UK has been looking at trading partnerships between farms where, for example, a poultry operation can share material with a large grain producer to replenish carbon levels in the soil. Such collaborations could address manures in appropriate ways to meet needs on a countrywide basis. The Sustainable Intensification Platform, one of DEFRA’s flagship programmes, is gathering data on possible partnership offsets across the country.

These advances are timely, given that the government’s 25-Year Environment Plan and the new rules for water came into effect on April 2.

“We think there should be scope for water companies and farmers to work together more than they are but also in different ways,” says Will Andrews Tipper, head of natural environment at Green Alliance, a think-tank.

“I think there is a limiting factor on the timescales of projects. Water company investments are calculated over decades, but that is not very applicable to catchment management methods, which work with natural processes, to deliver reductions in nutrient application — which have to be justified over a five-year cycle.

“But if you can find ways to take a longer term view, and find ways to displace what can be millions of pounds worth of water company investment into nitrate removal, you could have a lot more money available to support farmers in maintaining thriving agricultural businesses which aren’t just being managed for eco-system services but that go further than we might otherwise do in reducing emissions of nitrate to water.”

It seems any system will need to go beyond being purely compliance-driven if it is to meet legal requirements and address crucial ecological needs.

The prospect of a fertiliser tax is viewed as a blunt approach to reducing nitrogen levels and would “need to be introduced alongside fairly effective measures to support lower input farming models,” according to Mr Tipper.

Some water companies have also invested in anaerobic digestion, which can turn waste into a higher value product and potentially recover energy.

With legislative and environment bodies now more focused on the issue, they will be hoping for a groundswell of support from farmers and industry on wholistic and expedient solutions to nitrate pollution.

 


Headline Image Credit: Ihor Matsiievskyi/Shutterstock.com

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