Sewage pollution hits chalk streams

Friday 28 February 2014

A rash of sewage pollution incidents has hit rivers and local communities across the Chilterns in recent weeks as sewer networks struggle to cope with record levels of rainfall.

Although the floods that have affected large areas of the country since December are beginning to subside as river levels drop, in the Chilterns flows are continuing to strengthen as a result of rapidly rising groundwater.  High groundwater is good news in some respects for the area’s chalk streams as they are flowing strongly, but with levels now approaching those seen last in 2001, the sewer network in many places is failing to cope, leading to widespread pollution of waterways by sewage.

Incidents have been reported along a number of the Chilterns internationally rare chalk streams recently including: sewage surcharging in Dudswell village near Tring into the River Bulbourne; sewage pollution in the Misbourne from blocked sewers at Chalfont St. Peter; combined sewage overflows into the Chess at Chesham; and surcharging sewers in the Hughenden and Hambleden valleys.

This is not the first time this has happened however.  It is a consequence of an ageing sewerage system that is leaky and insufficient in capacity to deal with prolonged periods of heavy rainfall and high groundwater. sewage surcharge Hambleden

“Pollution of the area’s chalk streams is of great concern to the Chilterns Conservation Board,” says Allen Beechey, Officer for the Chilterns Chalk Streams Project, “At a time when our rivers should be looking healthy and flowing crystal clear, they are instead murky, smelly and, in some cases, wreathed in sewage debris. This pollution is potentially very damaging to freshwater environments as it can kill sensitive species that live in these streams. Beyond the risk to wildlife sewage pollution also presents a significant health hazard to people.”

Most modern sewerage systems are in two parts, foul sewers and surface water sewers.  These are designed to keep sewage separate from surface water runoff. In older systems however, both are often combined.  This means that in times of prolonged, heavy rainfall combined sewers can quickly become overwhelmed. In addition, where sewers are in contact with saturated ground, they are particularly vulnerable to groundwater infiltration which further adds to the problem.   The end result is that diluted sewage escapes to the surface through manhole covers.  This often means that untreated sewage can end up flowing down roads and pavements and invariably ends up in a nearby stream or river.

Thames Water Utilities, the company responsible for sewage treatment in the Chilterns and the Thames Valley has a sewer network renewal programme and had done a great deal to improve its network in recent years.  However, further significant investment is required to find long term solutions for problem areas like the Hughenden Valley and to build in resilience to the network to enable it to cope with the increasing likelihood of exceptional weather events  as a result of climate change and to accommodate future development.  Without it, the Chilterns precious chalk streams will increasingly become the dumping ground for human waste whenever there is heavy rain.


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