The case was brought by the Environment Agency after it discovered that the water company allowed untreated sewerage to enter Chase Brook which flows through the Chase –a 143 acre nature reserve owned by the National Trust.
On 3 September 2012, untreated sewerage entered the brook following a non-emergency discharge from the Broad Layings Sewerage Pumping Station Emergency Overflow (PSEO) pipe.
The discharge occurred as a result of a blockage at the site on 29 August 2012, and Thames Water failed to act to their alarm system to aremove the blockage.
Despite the fact that Thames Water had fixed the two pumps which were blocked and had organised a tanker to control flows, the raw sewage had already severely impacted aquatic life in over 600 metres of the Chase Brook.
Matthew Rice, environment officer at the Environment Agency, said: “Water quality testing revealed that there were high levels of both ammonia and e-coli in the pond and Brook.
“This combined with low oxygen levels meant that recovery from the incident was a long process.”
Since the incident, Thames Water has sealed up the PSEO outfall to the Chase Brook and created an area into which the contents of the well will overflow, instead of the Chase Brook.
They have also replaced both of the pumps on site with a new type which can deal with blockages.
A Thames Water spokesperson said: “We very much regret this incident and have since carried out a thorough clean-up of the watercourse and funded an ongoing post for a National Trust warden.
“We have also of course reviewed our procedures to reduce the chance of anything like this happening again.”