The system used to supply drinking water to the public removes large quantities of nitrogen, a potential pollutant, from aquatic ecosystems such as rivers, lakes and groundwater, new research shows.

However, leaking drinking water from pipes may also be an important source of nitrogen within the environment, according to the study which is published in Environmental Science and Technology.

For the first time, scientists from Lancaster University and the British Geological Survey have quantified the impact of England’s public water supply system on how and where nitrate, a form of nitrogen that is a potentially harmful pollutant, moves around the environment.

The amount of nitrogen in our soils, rivers, lakes and groundwater has increased dramatically over the last century, largely as a result of humans using inorganic nitrogen fertiliser to increase crop production in agriculture.

Nitrogen that is lost from agricultural soils and from other sources can create major problems if it reaches aquatic environments, including contributing to eutrophication which may be associated with harmful algal blooms.

Some of the nitrate reaching watercourses is removed naturally through the process of denitrification, which produces gases that enter the atmosphere. But now scientists have shown for the first time that the system we rely on to supply clean drinking water also removes large quantities of nitrate, potentially reducing the downstream movement of this pollutant towards the oceans.

“Water companies spend a significant amount of money in order to abstract water from the environment and to remove nitrate from this water before it enters the distribution network and arrives at our taps,” said Dr Ben Surridge, from the Lancaster Environment Centre and co-author of the study.

“Our research shows that the processes involved in supplying drinking water to the public can change the nitrogen cycle within aquatic ecosystems in ways that have not been recognised until now. For example, we calculate that the amount of nitrate removed through abstracting water may equal up to 40% of the nitrogen that has been estimated to be lost from aquatic environments through denitrification.

“This may be seen a good thing, because nitrogen is being abstracted from rivers, lakes and groundwater and may be prevented from entering the oceans.”

However, some nitrate remains in drinking water, even after treatment. This water may then be pumped long distances to supply customers. Moving water through distribution networks also has its own risks.

“Leakage of water from distribution networks returns nitrogen to the environment. We estimate that the equivalent of about 15% of the nitrate that is removed from rivers, lakes and groundwater through abstraction is returned to the environment through leakage,” said lead author of the paper Matthew Ascott, a PhD student at the Lancaster Environment Centre and the British Geological Survey.

“The loss of nitrate through leakage is higher in more heavily populated areas of the country. In some areas in and around London for instance, we’ve estimated that up to 20% of the total amount of nitrogen entering the environment could come from nitrate within leaking drinking water.

“Historically, we’ve assumed that agriculture, domestic and industrial wastewater are the key sources of nitrogen pollution and, nationally, that is still true. However, our research demonstrates that, particularly in urban areas, a significant proportion could be coming from leaking drinking water,” said Mr Ascott.

With increasing pressure on water resources due to climate change and population growth, we may be transferring more water around the country and over greater distances in order to meet demand in the future.

“Our research suggests that we may also be transferring pollutants and therefore risks and costs around the country too. In the future, water transfers could increase the risk of eutrophication in some areas compared to others, and mean that some areas have to shoulder more of the costs of nitrogen removal from the environment,” said Dr Surridge.

Dr Surridge believes the research highlights the importance of understanding how human action alters the cycle of elements such as nitrogen.

“We need to be more aware of the full range of human activities that influence element cycles,” he said. “Our research here has shown that human management of water resources can significantly alter the nitrogen cycle in ways that need to be properly accounted for, if we are to understand and manage this important nutrient.”

The study is co-authored by Professor Daren Gooddy from the British Geological Survey. Professor Gooddy is also an Honorary Professor at the Lancaster Environment Centre.