Cothill Fen: Keeping it clean! 2018

How volunteers are protecting Cothill Fen from the growing tide of water pollution

Cothill Fen, just south of Oxford, is a special place. It is internationally important, a rare alkaline fen, home to many sensitive freshwater plants and animals. But surrounded by land used intensively by humans, it’s under threat.

Habitats like Cothill Fen develop on permanently water-logged soils with peat forming at the surface. In healthy fen systems water levels stay pretty constant with water just at, or just below the surface. At Cothill, the fen is fed by calcareous waters both from the surface and aquifer. Crucially, the water needs to be very low in nutrients.

Sadly, a water quality survey using data collected by local people, revealed that nutrient pollution is impacting on the site, something that previous studies had failed to fully recognise. A spring is bringing in nitrates that could really change the nature of the fen, and lead to the dying out of plants and animals that make it special.

Armed with this new information, techniques to manage nutrients and ameliorate the effects of pollution are being applied and tested on the site.

Volunteers have been digging a channel, shored up with logs, to intercept the nitrate-laden water and send it to a ‘clean-up zone’. Without this work, the polluted water would continue seeping out over a large part of the fen, stimulating vigorous growth of reed, water cress and duckweed, all at the expense of the more sensitive, specialist fen species.

In the channel and ‘clean-up zone’, rotting reed and rush stems are being used to to clean the nitrates out of the water – rotting reed and rush allow anaerobic denitrifying bacteria to turn nitrate to harmless nitrogen gas. Once the nitrate is reduced, the cleaner water can seep back into the fen.

So far, the results are very promising. Quick test kits are showing very high levels of nitrate in the water fresh from the spring, whilst water just below where cut reed and rush has been in the channel for a while revealed no detectable nitrate!

So the water in the channel is getting cleaned up. The challenge is to stop water leaking out from the spring straight into the fen and force it all to go through the clean up zone.

11th October 2018

Find out more

• Read more about the Oxfordshire Fens Project
• Find out more about Flagship Ponds