The Thames Swim Against Sewage

On 1st – 4th September, eight world-class swimmers will take on the 208km Thames Swim Against Sewage – a non-stop relay from the river’s source in Lechlade to the heart of Westminster. It’s more than a test of endurance. It’s a call for urgent action, bringing together athletes, citizen scientists, activists and communities in the fight to end sewage pollution.

During the swim, we’ll be collecting and sharing data from the river using a Seneye water quality sensor.

The live tracker tells you ammonia levels as an indicator of water quality. Ammonia is one indicator of pollution as it spikes when sewage or agricultural pollution enters the water. The readings come direct from the Thames, with updated data coming through every 15 minutes. It also gives you the location of the swimmers, so you can track their progress too.

When sewage spills into rivers, it contains multiple pollutants. One of these indicators is free ammonia (NH₃). In simple terms, free ammonia is a chemical that can be toxic to fish, aquatic life, and can pose a risk to humans using the water for recreation. It’s present in higher concentrations after sewage discharge, agricultural runoff, and some industrial pollution.

Here’s how to interpret readings from the tracker:

*Levels relate to toxicity for marine organisms.

Academic studies confirm that free ammonia blocks biological processes needed to naturally clean water, is toxic to aquatic life, and disrupts wider river ecosystems. You can’t see or smell ammonia in water, but even tiny amounts can kill wildlife and make rivers unsafe for swimming.

Measuring ammonia is a way to monitor UK rivers, because it spikes directly after sewage discharge – making it a marker in both research and environmental protection.

We’ll be working to add additional markers as the swim progresses, but ammonia will remain our main indicator of sewage events.

Here’s how the sensor works…

On our journey down the Thames, we’re joining the tireless work of those monitoring and safeguarding their local stretches. We’re highlighting the real gaps in current legislation and the urgent need for reform – because our water system is broken, and the time for action is now. In short, we’re taking your calls to #EndSewagePollution all the way to Westminster.