In a paper published in Nature Communications, researchers from the British Geological Survey (BGS) and Lancaster University show that vast quantities of nitrate are stored in the rocky layer between our soils and the water table. Using modelling techniques they estimate that rocks contain up to 180 million tonnes of nitrate, twice the amount believed to be stored in soil.

This nitrate, from nitrate fertiliser used to boost food production, is leached quickly through soil but can move much more slowly through rocks. Depending on the thickness and geology of the rock layer, it can take around 100 years or more to reach the water table. This creates a hidden time bomb of nitrate pollution with potential to cause future damage to our drinking water and human health, and to the ecosystems of our rivers, lakes and coastal zones.

Matthew Ascott, lead author on the study, is a hydrogeologist at the BGS and is doing a PhD at Lancaster University. He said: “With big investments being made to reduce water pollution through changes in farming, it is vital that we understand what pollution is already in the environment.

“Water and the pollutant travels through the rocks below our feet very slowly. This and a history of intensive agriculture means that a large store of nitrate pollution has built up over time. When this pollution is released it will continue to impact water quality for decades, in some cases, even where controls on fertiliser use have been put in place.”

Professor Andrew Binley, from Lancaster Environment Centre, is a groundwater specialist with a background in computer modelling of water in the ‘unsaturated zone’ - the sub surface above the water table. He is currently working on how land use changes in some regions of China are affecting the movement of nitrate into groundwater. The researchers approached him for help with improving their modelling approach and linking their study to his work in China.

“There has been a recent trend of moving some hydrogeological modelling to much larger continental and global scales. There is now a vast amount of information available at these large scales, which presents exciting opportunities for looking at subsurface processes. But when you move to a global scale you have to simplify everything down so everything tends to look a little bit cruder. You can take pre-calculated maps of the depth to the water table, estimates of leaching rates from fertiliser application and, from typical travel times through rocks, get an idea of how long the nitrate remains above the water table.

“There has already been quite a lot of work done on the nitrate store in rocks in particular regions, such as the UK and the USA, what is different about this work is that it is the first time it has quantified this problem on a global scale,” said Professor Binley.

The model showed that most nitrate is stored in North America, China and Europe where huge quantities of fertiliser have been applied for decades. In developing countries, while the problem many not currently be so severe, there is evidence of a worsening situation in some regions that requires early intervention to avoid the same problems and environmental damage experienced by highly developed countries.

The researchers hope the results will be useful for policy makers.

“What the work allows us to do is to look at areas where we might be able to see future impact, as that water moves through to the water table - it might lead to some countries taking initiatives to look at this in detail, particularly in regions where groundwater is widely used for drinking water,” said Professor Binley.

Another of the authors, Professor Daren Gooddy, is Team Leader of Groundwater Processes at BGS and an Honorary Professor at the Lancaster Environment Centre. View the paper “Global Patterns of Nitrate Storage in the Vadose Zone" here.