River Chess Association

Above: CCTV footage reveals groundwater gushing into a sewer which has been damaged by piling

The RCA and Chilterns Chalk Streams Project met with Thames Water representatives this week to discuss its investigations on groundwater infiltration into Chesham’s sewers. Groundwater infiltration is a problem because it adds to the amount of water heading to the Chesham Sewage Treatment Works for processing. The sewage works becomes overwhelmed at times of high groundwater or heavy rainfall, resulting in the pollution of the river with untreated sewage mixed with groundwater/rainwater. 

CCTV and ‘look and lift’ surveys covering nearly 4,000 metres identified that over a quarter of the sewers looked at had some groundwater infiltration. In areas where the river is culverted, river water is also getting into the sewers. Up to 15 million litres of groundwater and river water per day are entering the town’s sewers. This a greater volume than the amount abstracted from the upper Chess catchment in recent years. If this problem can be addressed, it is hoped that this will benefit the flows of the river, as well as reducing the frequency of pollution events from the sewage treatment works. 

Why is this happening? Many of the issues result from crumbling infrastructure and problems caused by new developments damaging existing sewers, e.g. by driving piling into the sewers. The most severe infiltration points are being targeted for repair as soon as possible. Groundwater monitors were installed in November to help with further investigation and where problems persist, Thames Water aim to seal these areas to prevent infiltration. 

Thames Water also provided an update on the upgrade to Chesham Sewage Treatment Works. The capacity of the works will be increased by 30%, which should reduce the frequency of pollution events. And by 2024, phosphorus levels in treated effluent will be reduced from 2mg per litre to 0.25 mg per litre. Phosphorus pollution is known to cause the process of eutrophication in rivers, a highly problematic issue that causes excessive growth of algae, which smothers and blocks out light for other aquatic plants and animals. 

The upgrade to the sewage treatment works is a very positive step, but the RCA would wish to see a 100% increase in storm storage capacity to eradicate the majority of sewage pollution events caused by heavy rainfall. We would also wish to see tertiary water quality ‘polishing’ through reed beds and water meadows. 

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