How clean is the water in the Thames?

The longest river in England, the Thames supplies hundreds of thousands of households with fresh water every day. While most people are content to trust their water company to remove any pollutants before it’s delivered to their homes, a study carried out on the river in 2011 revealed some troubling information.

Led by the University of Bristol, the study comprised an exhaustive examination of water quality in the Thames, drawing on 140 years worth of data. It found that nitrogen levels saw a steady, although fairly restrained rise between 1860 and 1940 – but by around the mid-1970s, levels have spiked upwards to around four times what they were in 1860. They have remained roughly at that level ever since.

Nitrate in water supplies is largely caused by farming methods in which nitrogen is used as fertiliser, and land management around the drainage basin of a river. However, one of the most startling findings of the research is that spikes in nitrate levels do not occur alone. They consist of two parts: an immediate run-off effect, followed by a delayed secondary effect that is just as intense and may occur decades later.

“Generally, Thames water quality has been improving – the fish are returning,” Dr Nicholas Howden of the University of Bristol told BBC News. “But the big challenge that remains is diffuse pollution, and the UK remains the global hotspot for nitrate flux. We have the highest export for nitrate for any country in the world per capita or per land area, depending on how you want to express it. And some of the highest concentrations are coming out through the Thames.”

At low levels, nitrates are not harmful in drinking water. However, in concentrations of 10mg per litre or more, babies and pregnant women are at risk of harmful conditions like methemoglobinemia or blue baby syndrome. According to the most recent data, nitrate levels in the Thames stand at around 8mg per litre – creating a powerful argument for installing drinking water filters in your home if you live in the area.

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