Water Quality

Oil from road run-off in CheshamThe water that supplies the River Chess comes mainly from the aquifer and so has the characteristics of water filtered through chalk. The water has a high mineral content, particularly calcium carbonate, which contributes to the unique environment and qualities of chalk streams. However, other sources of water also get into the Chess, including drainage run-off from roads and farmland, which include pollutants like petrol. The outflow from Thames Water's Chesham Sewage Treatment Works also goes into the Chess. Whilst the discharge from the sewage plant has the beneficial effect of helping to maintain flow, these other sources of water can have a detrimental impact on water quality and put pressure upon the river ecosystem.

Four water quality sensors were installed in the river as part of the ChessWatch project initiated in 2019. Read more here.

Pictured left are contaminants on the surface of the Chess resulting from water on roads running off into the channel.


Untreated Sewage

The Problem:

The River Chess Association has been particularly concerned that Thames Water at times discharges a mixture of untreated sewage and storm water runoff into the Chess via the Chesham Sewage Treatment Works (CSTW) outlet. For example, during 2008, they discharged untreated sewage and runoff on 12 separate occasions, in eight separate months, for a total duration of over 40 hours. There were three separate incidents in 2009 totalling 27 hours and two incidents in 2010 totalling 2 h 52 minutes.

Data from 2003 onward demonstrated a marked increase in both the frequency and duration of these releases. These incidents are said to be caused when the capacity of the sewage work's storm surge storage tanks is exceeded following heavy rainfall events. With more housing being built in Chesham, and the possibility of rainfall events becoming more intense as a result of climate change, even more pressure will be placed upon the sewage treatment works. However, we also know that these events occur under high groundwater conditions, in the absence of heavy rainfall. 

The worst incident in recent times took place in 2014 under conditions of high groundwater. Thames Water continually released sewage into the river between February and June of that year. This was an unconsented release and the Environment Agency did investigate, but we were very disappointed that Thames Water were not fined for the 6 months of constant pollution to the Chess.

In 2020, sewage was released into the Chess on at least 36 days. This included releases every day between the 28th February and the 19th March, largely due to high groundwater levels. Again, we are disappointed that the Environment Agency treated these 20 releases as a single pollution event, minimising their severity. 

Why is this allowed?

Discharges are made under the terms of consent conditions granted by the Environment Agency. One argument put forward for allowing this practice is that these events only happen when there is heavy rain. Therefore, any discharge will take place when the river is flowing at higher levels, which should lead to greater dilution. Whilst this may be true of spate rivers or some lowland watercourses, this does not apply to chalk streams where rainfall has a minimal immediate effect on flow rates. From data obtained by the River Chess Association, it is apparent that on occasions the discharges occur at times of very low flow rates. What is more disturbing is that there have been a number of occasions when no rainfall occurred prior to a discharge event; in some cases this can be attributed to high groundwater levels. It has been confirmed by the Environment Agency that sewage discharges resulting from groundwater ingress into the sewers and sewage works, is not licensed under this system and is therefore unconsented. It is now known that groundwater ingress into Chesham's sewers is very significant and Thames Water are working to address this. Read more on this here

What impact is this having on the Chess?

There is not yet a clear picture of the effects of this practice on the Chess, not least because by their own admission, Thames Water do not:

  • Measure the quantity or content of the discharge
  • Warn or inform other river users who rely on the river for their livelihoods or for recreation
  • Understand the health and safety issues arising from these discharges
  • Carry out environmental impact studies on the effect of this practice

From observations made by regular visitors to the Chess, there is a belief that there has been a significant reduction in fly life in recent years, matching the increase in untreated sewage discharges. Such a reduction will have a knock-on effect on the fish population and their predators. In order to monitor this situation, the River Chess Association conducts monthly riverfly monitoring on stretches of the Chess both up- and down-stream of the sewage plant outfall. To find out more, see our riverfly monitoring page.

RCA members with Thames Water staff at Chesham STW

Good News!

Following consistent lobbying by the RCA, Thames Water expanded their storm water overflow storage in Spring 2012, doubling the capacity at the CSTW. This will significantly reduce the chances of future discharges. Thames Water have also set up a system to notify the RCA of any discharges. This has been in place since April 2011 and we were delighted  that there were no discharges for many months, despite some very heavy rainfall events.

....And Bad News

The notification system we had established with Thames Water to tell us of sewage releases from the works was stopped by Thames Water at some point in 2013, meaning that sewage releases may have occurred without our knowledge. Following its re-establishment, we were notified of a sewage release into the river on the 14th February 2014 which lasted for 5 months. We are also disappointed to note that whilst Thames Water are continuing to make improvements to the Chesham Sewage Treatment Works, in late 2016 Thames Water announced that they were planning to reduce their storm water overflow storage. We believe this has led to more pollution incidents.

There are also a number of other problems with the CSTW. Used sanitary towels and nappies are routinely found in the river downstream of CSTW, which is extremely unpleasant for river users, including our volunteers. The works also experienced a number of failures between 2012 and 2014, which led to sewage flooding the site and contaminating neighbouring land, as well as entering the river. Following positive meetings between Thames Water, the Environment Agency, the Chilterns Chalk Streams Project and RCA representatives, Thames Water have been carrying out works at the site to address these issues.

The Future

In late 2020, we met with Thames Water and they showed us the work that they have been doing to identify groundwater ingress into the sewer network in Chesham and how they are going to resolve this problem. 

The capacity of the Chesham Sewage Treatment 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.