How An Overspill Contaminated A Rare Chalk Stream

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The most inescapable story of the last few years for many water consultants wanting to get the best water supply deal for their customers has been the increasingly controversial policies surrounding storm overflow discharge.


These policies, now the subject of a lengthy Environment Agency investigation, have led to many water sources becoming polluted, and one of the most recent and controversial of these is a rare chalk stream used to supply the local area.


According to an article by the i Newspaper, a chalk stream that flows through several Chiltern villages in Buckinghamshire has become so polluted by untreated sewage discharged by wastewater provider Thames Water that potable water cannot be extracted from a nearby borehole.


The complex situation has both immediate and long-term consequences for people who live on the banks of the River Misbourne, and to understand the implications for people in the area it is important to explain the context, and potential implications on water pricing.


What Is A Rare Chalk Stream?


The River Misbourne is a chalk stream, one of the rarest and most vulnerable habitats in the world. According to The i, 85 per cent of the world’s chalk streams are located in England.


Chalk streams are rivers and bodies of water that have a bedrock of calcium carbonate, which water can easily flow through, creating a supply of clean, clear water rich in minerals generated by a natural filter.


This water is then extracted by drinking water companies, in this case, Affinity Water, supplying the local area with hard water.


The clear streams are also teeming with fish and invertebrates due to the filtered, calcium-rich water.


However, pollution and dredging efforts have made chalk streams more vulnerable, affecting these consistent, clean conditions and replacing the vibrant plant life that called the river home with sewage fungus, something that also affects the breeding patterns of fish.


This is something that came to a head in the first two months of 2024.


The Thames Water Incident


The frequency of sewage discharges in the area near Gerrards Cross, Amersham and Chalfont St Giles has increased, and alongside the contamination of the Misbourne, several towns and villages have suffered from flooding, the consequence that storm discharges are intended to avoid.


Initially reported in January and already subject to criticism and debate, the situation has escalated into what the Environment Agency has classified as a Category 1 incident, their highest level that is used to describe an event with a potential “major” environmental impact.


The flooding was caused by rising groundwater levels entering the Thames Water sewer network, backing up into manholes and flooding places such as Chalfont St Peter.


The result is that certain parts of villages have been cordoned off whilst Thames Water pumps up the contaminated water using their fleet of tankers, described by residents as dilute but unpleasant.


It has resulted in businesses closing and residents in the area unable to flush toilets. That this took place during half term when many children were on holiday from school, compounds the issue.


Volunteers from the Misbourne River Action group were warned not to go into the river to test the water quality, and one representative, Bob Older, claimed in an interview with The i that the contaminated water from the village was pumped directly into the Misbourne.


However, another action group, the Chiltern Chalk Streams Project, noted in an interview with the BBC in January that many of the storm discharges took place when there had been little to no rainfall for several days.


The point of a storm overflow is that by mixing the sewage with overflowing rainwater, the polluting impact can be reduced, something the EA claims minimises the environmental impact of any given discharge event.


Without the excess water, more concentrated sewage enters the environment, something which can have grave, long-term consequences and is the reason for the sweeping investigations into wastewater treatment works and their lack of compliance under the terms of their environmental permits.


Poisoning The Well


One of the biggest effects, and the cause of the EA’s escalation of the incident, was the temporary suspension of a borehole in the area.


Affinity Water, a company that only supplies drinking water to the area, relies heavily on the Misbourne due to its natural aquifer properties. According to The i, over 60 per cent of the drinking water they provide is sourced by these aquifers.


However, in what the company describes as a “precautionary measure”, their borehole based near Chalfont St Giles was halted, although they stress that this would not affect the water quality for their customers.


They did not comment on the report by the EA that significant sewage fungus was found in the river, with the potential to cause short and long-term harm.


Instead, they noted that they do not manage wastewater but focus on drinking water, noting that they are continuing to monitor the situation.


Compounding Crises


This incident will do little to ease the crisis Thames Water has faced over the past year, with the

company struggling to manage its debt obligations totalling £14bn, forced to provide reductions to water bills owing to poor performance as announced in September 2023, and has had several near-misses with loan obligations.


Whilst shareholders have provided £750m in funding alongside a £500m loan, Thames Water declared to MPs that it would not have the finances to pay off a loan valued at £190m due at the start of the 2024/25 financial year.


These financial struggles may explain the poor performance, noted by Ofwat as amongst the worst in the country, and emergency plans have been drafted since December 2022 in the event that Thames Water collapses entirely.


Should this be the case, a temporary nationalisation of Thames Water could be enacted, bringing the company into a special administration regime, a process that effectively nationalises it temporarily to keep the vital public service running.


Local MP Gareth Williams called for compensation and swift action and pledged to “keep the pressure on”, according to a follow-up report by the BBC.