Datchet tunnels targeted in mussel-removal operation
Thousands of mussels that have collected over 50 years inside underground water tunnels connecting two reservoirs are being cleared out.
Thames Water is removing hundreds of tonnes of zebra mussels from the 8ft (2.5m) wide tunnels, 130ft (40m) below ground in Datchet, Berkshire.
They link the River Thames to the Queen Mother and Wraysbury reservoirs, in Berkshire and Surrey respectively.
The molluscs are considered a pest and can block water pipes if left.
The invasive shellfish, which is native to Russia and Ukraine, is similar in shape to marine mussels but smaller in size and covered in distinctive brown and yellow zig-zag stripes.
Mussels sense poison
The species, Dreissena polymorpha, is thought to have arrived in the UK from southern Russia in the early 1800s attached to the hulls of cargo ships.
They are very difficult to get rid of using poisons since they can sense them in the water and simply close their shells for up to three weeks until it has dissipated.
Most water companies therefore have to remove the molluscs by hand.
Thames Water has said it is currently testing a new environmentally-friendly toxin to get rid of them, which could be adopted if successful.
Since the network of Thames Water tunnels in Datchet was built 50 years ago, zebra mussels have lodged themselves inside and multiplied.
The tunnels have now been drained and engineers are being lowered into the tunnels by crane through specially constructed shafts.
They are also strengthening the walls with concrete in work that is due to finish in March 2014.