Critical Research Journal | Graham Wilson

Two separate requests were made to Thames Water (TW) for details of the water supply to Lower Heyford. I made the first, which was a request for statutory information. The other was made by another resident, and I don’t know the details of how it was made and what it asked for, they were separated by about a month, in which time TW will have become more aware of a wider public concern.

Thames Water first gave the following account of the local water supply:

“The water supply zone name is Deddington. The source type is borehole water that goes through Heyford Hill. The treatment works are valve-fed off Duns Tew, as far as we know. Unfortunately, we don’t have any information regarding the source protection zone.”

On the basis of this, I wrote a post explaining the geological principles at play (How PFAS chemicals get into our water supply (DRAFT) – Critical Research Journal | Graham Wilson). However, Thames Water has now given this more detailed and different account:

“The drinking water for 0200 Deddington is supplied by Cleeve, Farmoor and Swinford water treatment works (WTWs). Farmoor and Swinford are surface water sites and abstract river water from the upper River Thames. Both abstract river water upstream of RAF Upper Heyford and the Gallos Brook, Leys Farm Ditch, River Ray, Oxfordshire Canal and River Cherwell. As a result, any PFAS contamination from RAF Upper Heyford will have no impact on the water abstracted for drinking water treatment at these sites. Cleeve WTW is a groundwater site located in Berkshire in which water is abstracted from an underground aquifer. The aquifer in which Cleeve WTW abstracts water is located ~45km to the south of the Heyford Park area as the crow flies and is not hydrogeological linked to any groundwater sources that may be present in and around RAF Upper Heyford. However, the River Thames does flow through the Cleeve WTW groundwater catchment, and a number of the boreholes that supply Cleeve WTW are surface water influenced- the area in which the boreholes abstract from the aquifer is considered hydraulically connected to the river, and water abstracted for drinking water treatment at Cleeve is therefore a mix of River Thames water and water from the underground aquifer. As the section of the River Thames that flows through the Cleeve WTW groundwater catchment is downstream of RAF Upper Heyford, it is theoretically possible that any contamination to watercourses from the RAF Upper Heyford site could reach Cleeve WTW due to the surface water influence with the River Thames.

That said, the distance the contamination must travel from watercourses in and around Heyford Park to reach the River Thames near Cleeve WTW is in the region of 60km. This is a significant distance and allows for mixing and dilution before any interaction with the Cleeve WTW boreholes. PFAS sampling is undertaken in the raw and treated water at Cleeve WTW, and the results are well within the wholesomeness limit. As a result, it is highly unlikely that contamination observed in and around RAF Upper Heyford is having any impact on the drinking water quality at Cleeve WTW.”

These two answers do not say the same thing. The first answer describes the Deddington zone as “borehole water” going through Heyford Hill, with treatment works “valve-fed off Duns Tew”. The later answer says the Deddington supply comes from Cleeve, Farmoor, and Swinford water treatment works. It says Farmoor and Swinford draw surface water from the upper Thames, while Cleeve draws groundwater in Berkshire, with some boreholes influenced by the River Thames.

The later account may be more accurate, but it does not explain the earlier one. It may be that the Deddington zone receives blended water from several sources, or that the network changes with demand, repair work, or seasonal supply needs. It may also be that the first answer was a partial or poor account. What matters is that residents cannot judge the risk until Thames Water sets out precisely how much water comes from each source, when that changes, and whether any local borehole or Duns Tew-linked supply still plays a role.

The most credible part of Thames Water’s later answer concerns Farmoor and Swinford. These sites are associated with water abstracted from the upper River Thames, in the Oxford area. If the relevant abstraction point is upstream of the Cherwell confluence, then it is also upstream of the route by which contamination from RAF Upper Heyford would be most likely to enter the wider Thames system.

The likely route for this would run from the former airbase through local ditches and streams, into Gallos Brook, then into the River Cherwell system, and only later into the Thames. If Farmoor and Swinford draw water above that point, then PFAS from RAF Upper Heyford should not affect water abstracted at those works.

This is an important point. It means that the local contamination near Heyford Park does not create the same risk for every named water source. Geography and direction of flow both matter. A site upstream of the likely pollution pathway should not be exposed to that pathway. On this part of the answer, Thames Water’s claim seems plausible.

The Cleeve part is more complex. Thames Water says Cleeve is a groundwater site in Berkshire, around 45 km south of Heyford Park. It also says the Cleeve aquifer is not hydrogeologically linked to groundwater around RAF Upper Heyford. That may well be right. If Cleeve drew only from a separate chalk or other aquifer with no groundwater link to Upper Heyford, the Upper Heyford groundwater plume would not be the direct issue.

Yet Thames Water then adds a crucial qualification. It says that the River Thames flows through the Cleeve groundwater catchment, and that a number of the boreholes supplying Cleeve are “surface water influenced”. In plain terms, this means the boreholes are not isolated from the river. Thames Water says the area from which the boreholes draw water is hydraulically connected to the Thames. The water abstracted at Cleeve is therefore a mix of underground water and river-influenced water.

This matters because the River Thames at Cleeve lies downstream of Oxfordshire. Thames Water accepts that, in theory, contamination from watercourses around RAF Upper Heyford could reach the Thames near Cleeve and then influence the boreholes.

That does not prove that unsafe drinking water has reached anyone’s tap. It does, however, remove a simple reassurance. Thames Water cannot say that Cleeve is irrelevant merely because it is a groundwater site. By its own account, Cleeve is a groundwater site partly linked to a downstream river.

Thames Water then relies on distance, mixing, and dilution. It says contamination would need to travel around 60 km from watercourses near Heyford Park to the Thames near Cleeve. This distance would allow the contaminant to mix with other water and become diluted before it could interact with the Cleeve boreholes.

That argument has some force, but it does not settle the issue. Dilution can reduce concentration. It can make a pollutant harder to detect and less likely to breach a regulatory value. But PFAS are not like many short-lived pollutants. PFAS are persistent, mobile in water to varying degrees, and often resistant to breakdown. The Drinking Water Inspectorate treats PFAS as a matter that requires targeted monitoring, risk assessment, and source control. The fact that a pollutant travels a long way does not by itself prove that it has ceased to matter.

The phrase “well within the wholesomeness limit” also needs to be unpacked. The Drinking Water Inspectorate has adopted a tiered approach to PFAS. For PFAS detected in raw or final water, Tier 1 applies below 0.01 micrograms per litre, Tier 2 below 0.1 micrograms per litre, and Tier 3 at or above 0.1 micrograms per litre. Current guidance also refers to a guideline value of 0.1 micrograms per litre for the sum of 48 named PFAS. These figures matter because “within the limit” may still mean that PFAS have been detected. A result can be lawful, or within current guidance, while still being a matter that should be disclosed and tracked.

A resident therefore needs more than a broad assurance. We need the actual results. That means the named PFAS compounds tested, the dates of sampling, the raw water results, the treated water results, the limits of detection, and the total for the 48 PFAS group.

We also need to know the DWI tier assigned to each relevant site. If Cleeve, Farmoor, and Swinford all sit in Tier 1, that would give a stronger form of reassurance. If one or more sources sit in Tier 2, that would not mean the water is unsafe, but it would mean that further risk reduction and closer scrutiny may be needed.

The local setting increases the need for clarity. Cherwell District Council now accepts that PFAS and land contamination at Heyford Park have become matters of public concern. The known history of the former airbase, including fire-fighting foam use, makes PFAS a foreseeable issue rather than a vague fear. Former airfields and fire-training sites are well known PFAS risk settings. In that context, the residents are entitled to expect a clear source-to-tap account, not a set of partial answers that shift from Duns Tew and boreholes to Cleeve, Farmoor, and Swinford.

The central question is not whether Thames Water has lied. The central question is whether Thames Water has given a complete and verifiable account. At present, it has not. Its later answer is more detailed and contains some plausible hydrological reasoning, but it still leaves too much undisclosed.

A quick summary, so far – TW’s claim about Farmoor and Swinford is plausible if those sites abstract from the Thames upstream of the Cherwell and upstream of the likely Upper Heyford pollution route. Its claim about Cleeve is more guarded, because Thames Water accepts a possible downstream surface-water pathway through the River Thames. Its reassurance about Cleeve rests on distance, dilution, treatment, and monitoring. Those factors may well reduce risk. They do not remove the need to see the data.

The inconsistency between the two Thames Water replies also remains unresolved. If Deddington is supplied by Cleeve, Farmoor, and Swinford, what did the earlier reference to Heyford Hill and Duns Tew mean? If the earlier answer was out of date, Thames Water should say so. If it was incomplete, Thames Water should correct it. If the Deddington zone can receive water from different sources at different times, Thames Water should provide the operating pattern and the proportion supplied by each source over the last two years.

The most reasonable next step is to ask Thames Water for six specific items:

(1) The current source-to-zone schematic for Water Supply Zone 0200 Deddington.

(2) The percentage contribution from Cleeve, Farmoor, Swinford, Duns Tew, Heyford Hill, and any other source over the past 2-3 years.

(3) A statement explaining the earlier reference to borehole water, Heyford Hill, and Duns Tew.

(4) The raw and treated PFAS results for each relevant works, with dates, compounds, detection limits, and totals for the 48 PFAS group.

(5) The DWI PFAS tier for each works and any linked supply source.

(6) The risk assessment that considers RAF Upper Heyford, Gallos Brook, Leys Farm Ditch, the Cherwell, the Thames, and the surface-water-influenced boreholes at Cleeve.

Until those items are provided, Thames Water’s statement should be treated as a plausible assurance, but not a settled answer. It explains why some sources may be low risk. It does not yet prove that the whole Deddington supply is free from concern linked to RAF Upper Heyford. For a pollutant class as persistent as PFAS, and for a former military airbase with known local watercourse concern, public trust depends on disclosure, not simply “trust us” reassurance.