As part of the ongoing review into the use of the Duxford Aquifer in the drinking water supply to some homes in South Cambridge, we have modelled a series of theoretical scenarios to test if Duxford Aquifer water had been supplied direct and unblended to customer taps in the first half of 2021.

Water from this source is usually blended with other water sources before arriving at the tap owing to the presence of PFOS. Levels of PFOS are overseen by our regulator the Drinking Water Inspectorate. Although there is no limit for PFOS specified in the Regulations, a guidance note was provided by our regulator on levels of PFOS detected and actions to be taken.

The investigation has identified three situations where this could have happened for a short period.

These situations were due to brief periods of disruption to the supply of other water sources used for blending. Due to the design of the water network, these gaps in blending supply would need to exceed 11.5 hours before unblended water was present at customer taps. There is no certain way of knowing if this situation ever actually occurred, due to the many variables affecting the raw water quality and travel time of water. We do know that if it did, it would likely be for a short time period, based on the known disruption times of the other sources used for blending.

However, that it theoretically could have happened further vindicates our decision to remove the aquifer from the supply in June of 2021.

The Health Security Agency has considered these modelled situations and, in a report, outlined three theoretical levels of PFOS tap water may have contained.

At the highest level considered in this modelled scenario, it concluded that ‘…the occasional and short duration of exposure does not necessarily indicate a risk to public health…’ but ‘it further reduces the margin of safety generally considered desirable.’

The Duxford Aquifer currently remains out of supply and has done since disconnection in June 2021. It will remain out of supply until the proven treatment solution we have installed has been fully tested and commissioned.

We are working closely with our regulators on this, and initial testing data shows that PFOS levels are below the limit of detection in the treated water, even if unblended with other sources.

Background facts:

• Duxford Aquifer has been used in the water supply for many years but is currently not in use
• PFOS is known to be present in the water due to historic testing
• In January 2021, new lower limits were introduced by the Drinking Water Inspectorate
• Water testing for PFOS led us to respond in line with Drinking Water Inspectorate guidance
• These meant that measurable levels in some tap water required us to make further investigations and report our findings
• Ongoing testing led us disconnect the aquifer from the supply in June 2021

Posted: 26 November 2022