The pollution we see affecting the river Misbourne and the Amersham/Chalfonts area is not unique within the 402 square miles of river Colne catchment, but it is certainly one of the worst affected at present.

The Problem

So, what is going on – what is the problem? The simple answer to that is that the very high levels of groundwater we are experiencing, after all this last winter’s rain is finding its way into the sewage system and overloading it.

Each of the sewage treatment works has pre-agreed (with the Environment Agency) flow rates they have to be able to fully treat, before discharge to the various rivers or groundwater. When those flow rates are exceeded the water company is allowed to discharge dilute but untreated sewage to the rivers or groundwater – it is referred to as ‘spilling’ or ‘discharging under storm conditions’ and this essentially ensures the sewage does not back into people’s homes or businesses.

What should not happen however is that such discharges continue for periods beyond the storm. Groundwater ingress into sewers, at times other than the storm itself that caused discharges, should be deemed illegal. There are a number of ways that ingress can occur – too complex to enter into here but with the right investment could be significantly reduced if not ceased. These storm discharges, whether legal or not, are happening across the whole catchment.

In the current case of the Gerrards Cross, Chalfont St Peter and River Misbourne areas the situation is complex. Gerrards Cross Sewage Treatment Works (STW) has been performing poorly for years and appears to operate under storm conditions all too readily – we argue that many of those discharges do not comply with the discharge permits. At Amersham there are large ‘balancing tanks’ that receive overflow from a trunk sewer when the levels in it are very high. That trunk sewer actually takes sewage to Maple Cross STW in Harefield. When the tanks overflow however that dilute sewage then enters the river Misbourne too, so now we have two points that sewage is entering that river. A third has been created because Thames Water are having to try and minimise flooding and sewage at a number of locations including Chalfont St Peter itself and the HS2 Southern Portal site downstream. That is being achieved in part by pumping to the Misbourne but much of the sewage is being taken to Maple Lodge which, you will recall us saying earlier, is already ‘spilling’ so all that achieves is making the situation worse for the river Colne (which is way upstream in the catchment) into which Maple Lodge discharges.

What is a Category 1 pollution incident?

The Environment Agency (EA) are investigating the incident in the Misbourne Valley as a potential Category 1 incident because of groundwater contamination. What is a Cat 1 incident? The EA use this set of benchmarks to explain:

Category 1 incidents have a serious, extensive or persistent impact on the environment, people or property. Category 2 incidents have a lesser, yet significant, impact. Category 3 incidents have a minor or minimal impact on the environment, people or property with only a limited or localised effect on water quality.

Definite indicators to achieve Cat 1 status are for eg groundwater pollution or a fish mortality in rivers/lakes.

As environmentalists we believe that metric is seriously flawed but that can be the subject of another news piece.

How is the whole catchment affected?

We have referred to the river ‘catchment’ – by that we mean the main river Colne from its source near Water End at North Mymms to its confluence with the Thames at Staines. It also includes all the tributaries (including some of the Chiltern chalk streams) and the distributaries as the Colne flows downstream – The Frays, Colne Brook, Wraysbury River to name a few. It is a very complex and huge catchment area but you will understand that pollution in any one area has the potential to spread and affect large areas.

The effect of local river pollution on our work

The problems we are facing currently are so acute that much of the ‘normal’ ColneCAN activity cannot be completed. The removal of Invasive species, riverfly monitoring, flow gauging and litter picking cannot proceed. That means our interaction with volunteers, school groups etc is all on hold.

If all this fills you with despair – welcome to our world!

Will everything be OK when the storms pass?

Let us take a further reality check, however. You might assume that when the storms have passed, groundwater has reduced, and we do not have a single ‘spill’ occurring in the catchment that all will be well again.

Actually, maybe not. There will still be numerous routes by which pollution is entering our rivers but as we are focused on Sewage treatment works here let’s continue in that vein. We mentioned earlier about STW’s having to ‘full treat’ the sewage it receives before that treated effluent is discharged to the river. That is true but what does ‘treated’ mean?

STW’s are mandated to treat human waste using a biological process and they do a pretty good job of that. The setting of parameters for the rates of Ammonia, Phosphates etc etc included in that discharge will (or should) monitor performance.

What it does not do however is monitor for a host of other micropollutants that STW’s are not mandated to remove, and which were never designed to do so.

So, all the chemicals, pharmaceuticals, pet medicines, cosmetics and the myriad of other substances we send down our drains and toilets will pass though the STW and into our rivers. Any removal that occurs is by good fortune rather than by design.

ColneCAN are commencing a broad-based study to determine the extent of such pollution. We are not alone – there is a growing recognition Nationally that we potentially have a ticking time bomb on our hands. Determining the nature of the pollutants present is relatively easy albeit very time consuming and expensive. Determining exactly what impacts these pollutants will have on our natural environment and indeed on human health is a much more complex piece of work that has many years to run yet.

Many thanks to Tony Booker, ColneCAN co host, for this detailed explanation.

More information

Storm discharge and event duration monitoring (EDM)

the Current State of Our Rivers and how we can move forward in a positive way 25/02/23

The River Colne - A Toxic Soup of Chemicals ? 20/04/23

Major sewage pollution in rare chalk stream stops extraction of drinking water i newspaper 13/02/24

Chiltern chalk stream polluted by sewage after heavy rain BBC news 12/02/24

Design by LTD Design Consultants and build by Garganey Consulting. From an original concept by the Herts and Middlesex Wildlife Trust.