What Dirty Business Didn’t Show – The Groundwater Problem Behind Sewer Overflows and ‘Dry Spill Events’
The Hidden Role of Groundwater in Britain’s Sewer Crisis
Summary
The UK’s sewer networks have a hidden problem: groundwater infiltration.
Groundwater reduces sewer capacity. Hidden and overlooked, it’s nationally one of the largest causes of combined sewer overflows and illegal Combined Sewer Overflows (CSO) discharges.
Sewer network design and operations have ignored this issue. Addressing it now is key to stopping raw sewage from getting into our rivers.
The legislation assumes that sewer networks operate to carry sewage with input from rainwater when it rains, and allow overflows when rainfall is too heavy for the system to cope with. It does not recognise the role of groundwater, which is estimated to cause a quarter of spills (and a large portion of illegal ‘dry spill’ events).
Recent analysis by GeoSmart has shown the extent to which groundwater contributions are a cause of sewer overflow problems.
Technological developments are bringing visibility and opportunities to manage the problem. GeoSmart’s live baseflow forecast and sewer infiltration models predict when, where, and how long groundwater infiltration will occur to sewer networks. This enables any issues to be better assessed and managed in future to address what has been plaguing the water industry and leading to worsening pollution in our rivers for many decades.
Introduction
Water companies are facing renewed public and regulatory scrutiny following recent controversy. Channel 4’s Dirty Business and recent prosecutions of Thames Water (with sewer overflows triggering the largest fines ever issued by Ofwat) and the Office for Environmental Protection finding regulatory failures are just a few examples of a growing focus on sewer overflows and pollution events across the UK.
For those who may have assumed the Channel 4 programme is a fictional drama, or perhaps reflects the vivid imagination of film-makers, you will find a relevant reality check in this Environment Agency blog here from 6 years ago that speaks to the real-world story.
This White Paper, published in January 2026, sets out the government’s latest thinking, with one such headline reading:
“Water company storm overflows spilled into England’s rivers, lakes, and seas for 3.61 million hours in 2024”
As a hydrogeologist specialising in groundwater flooding, I’d like to note that groundwater gets no mention in this White Paper.
In this blog, I will bring to light the hidden role of groundwater behind many of these overflows, highlight why this is a problem, and also show how the latest technologies are helping to bring solutions.
Groundwater and sewage
Combined sewers were designed to carry rainwater and sewage in the same pipe to a sewage treatment works, so that when heavy rainfall occurred, the system would divert discharge to rivers at CSOs. This enabled sewage to be treated most of the time, whilst avoiding the additional cost of capacity for peak flows, or needing separate sewers to be built. Peak flows from rainfall may easily exceed 100 times the normal wastewater discharge rates from a property.
The combined sewer design assumed discharge of untreated sewage to rivers during extreme weather conditions, but it did not assume any groundwater infiltration, which causes the overflows to occur much more frequently, when the sun is shining (Dry Spill Days).
With groundwater taking up a substantial part of the sewer capacity, such discharges can quickly become far more frequent than was intended. In a Southern Water study of 19,807 overflows, 25% were primarily caused by groundwater infiltration. It is likely that groundwater also caused a significant additional proportion of the overflows because it increases overflow frequency and flows related to rainfall by reducing capacity in the network.
Legislative and regulatory framework
By law, water companies must ensure that storm overflows only discharge during heavy rainfall when the capacity of the combined sewer has been exceeded.
Rainfall limits can vary, but in essence, overflows to surface watercourses will be legal within the terms of the Act only when heavy rainfall causes sewer capacity to be exceeded. The Act intended to recognise that there will be times when very heavy rainfall creates more runoff than the network and storage systems can accommodate, and in those circumstances, overflows allow the untreated sewage to discharge to rivers.
What the legislators did not take into account was that our sewer networks are mostly not sealed, and in many places lie below the water table. Once groundwater rises above the invert level of sewers, infiltration occurs, and when groundwater gets into the sewers, it takes capacity away from what the sewer is there to do.
Clearly, water companies are responsible for the performance of their own sewer network assets. However, there is a complication that there are many private sewers that flow directly into the main sewers.
Water companies are unable to prevent groundwater from flowing into their networks from these private sewers, but do suffer the reduced capacity and costs associated with such infiltration.
In reality, many sewer networks are often partly full of groundwater, leaving little room for sewage, which consequently may overflow into rivers even when there is no rain. Such ‘Dry Spill’ events are illegal, and it is clear that legislators and risk management authorities did not consider the role of groundwater in sewers when they drew up these regulations. Sewers deteriorate over time, and increased cracks and other defects also lead to groundwater infiltration increasing over time. This, together with increased runoff and increased groundwater levels due to the changing climate, compounds the existing problems.
During extreme weather and dry spill events, untreated sewage combined with groundwater (and often storm runoff) will therefore discharge to streams and rivers with a frequency far in excess of what was assumed in the network design.
In recent years, the Environment Agency and Ofwat have become more aware of this problem and have acted to require water companies to develop infiltration reduction plans.
In January 2026, the Environment Agency issued a regulatory position statement (RPS) on discharges from groundwater surcharged sewers where such overflows could cause significant pollution.
The RPS requires water companies to take forward additional infiltration reduction plans for those networks that suffer significant groundwater inflows.
The RPS recognises that groundwater-inundated sewers can lead to unpermitted discharges to the environment when flows exceed system capacities. It states that such discharges can be allowed as long as the water company is implementing infiltration reduction plans (IRPs) that resolve or minimise infiltration and prevent any further discharges from April 2030 (beginning of the AMP9 5-year funding period). There is, therefore, a high degree of regulatory urgency (as well as other fundamental duties and responsibilities of water companies) on water companies tackling this issue effectively over the next 4 years.
Water companies are progressing their analyses, including infiltration and inflow studies, and need to have detailed plans setting out the required measures with their sequence and timing. These plans must explain how the measures will ensure satisfactory progress toward eliminating unpermitted discharges resulting from groundwater infiltration. They must present the plan within the IRP and, from November 2027, within the DWMP.
Groundwater infiltration to sewers is also widespread in private sewers, which feed the water company network. This reduces capacity in the main sewers, but water companies do not have legal powers to reduce such infiltration or prevent the flow from entering their system. Achieving the 2030 goals will be very challenging and will entail solutions that are not yet in contemplation.
Even if more robust solutions are identified, the impact of the changes required will be significant. Some may lead to unexpected consequences, and there is a need to consider the impact of infiltration reduction on the wider environment. For example, previous analysis by GeoSmart has demonstrated that about 25% of the total catchment flow in a typical Chalk valley may take place by groundwater flow within the sewer network due to capture of the natural catchment flows. This exacerbates low flow and quality problems in many Chalk streams where the water was naturally providing baseflow in the stream and keeping these habitats healthy for the fish and other aquatic life. The groundwater flow within the sewer networks discharges at the sewage works, where it is treated and discharged to the river at the bottom of the catchment. Reducing this groundwater inflow will lead to the following impacts:
- Reduced CSO discharges
- Less dilute sewage and reduced volumes at the sewage works, with implications for the treatment system
- Increased groundwater flood risk and fluvial flood risk locally in some places
- Increased river flows, particularly increased baseflow, which are generally beneficial for the water environment
- Reduced costs for wastewater operations, which are currently managing and treating a very significant groundwater flow over many networks nationwide
Solutions
GeoSmart is a specialist in groundwater and catchment flooding, and we have many years of experience managing flood and climate risk to infrastructure, and providing data and analytics support to the water industry. Our realtime groundwater forecast has been providing early warning of upcoming flooding and sewer and drain infiltration events for the last 10 years for the Flood Forecasting Centre, water companies, local authorities and others.
More recently, we have completed machine learning analysis of the role of groundwater in Chalk stream baseflow nationwide and developed a model that enables us to quantify the contribution to sewer flows and predict timing, location and severity of sewer infiltration. The analysis demonstrates that following initial setup, our model can deliver a live realtime sewer infiltration forecast (provided by API or via text alerts) that provides up to 30 days’ advance notice of future sewer infiltration and overflow events.
We welcome enquiries from water companies and drainage authorities who would like to subscribe to a daily forecast of baseflow via API, as well as (following initial calibration) prediction of timing, location and severity of groundwater inflows to sewer and drainage networks.
Conclusions
Groundwater is a systemic threat central to (albeit with an unplanned role) sewer networks. It is often overlooked hitherto in the legislation and arrangements related to both private and water company sewer networks.
With our changing climate and deteriorating sewer networks, groundwater infiltration challenges are increasing, and the cost to customers and the environment is worsening rapidly.
Groundwater is not a secondary factor and can no longer be overlooked without jeopardising targets that are vital for an industry under unprecedented pressure.
Groundwater is a primary cause of CSO problems that impact our rivers and coasts through ongoing and worsening pollution, and it is a systemic issue that can influence treatment loads, performance metrics and investment planning for water companies and drainage authorities.
Understanding infiltration is becoming increasingly important in the context of climate variability, AMP investment cycles and ageing sewer assets.
The latest modelling and real-time forecast services from GeoSmart bring visibility to the role of groundwater in the sewer network, enabling infiltration reduction programmes to be significantly improved and accelerated to meet the requirements of the recent Environment Agency RPS and the wider pressures on the water industry.
Please call 07976 800730 or email MarkFermor@GeoSmartInfo.co.uk if you would like to arrange a free initial chat about the challenges you are facing and how to access our real-time forecast service, or if you are interested in further information on the contribution of groundwater to the national wastewater and drainage network.
