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Perfluoroalkyl and polyfluoroalkyl substances (PFAS)

Perfluoroalkyl and polyfluoroalkyl substances (PFAS)

PFAS are a group of man made compounds, the basis of which are chains of carbon and fluorine atoms. They contain at least one fully fluorinated methyl or methylene group. The carbon-fluorine bond is very strong and so these compounds do not degrade easily in the environment. They are resistant to grease, oil, water, and heat and so they have found a large range of uses, for example, in stain and water-resistant fabrics and carpets, as well as in paints and firefighting foams, cookware, and food packaging.

The production of these substances, their use in products and their use and disposal means that they are now being found widely in the environment throughout the world. Ultimately, because of the persistence of the substance they are eventually found in water, be that groundwater or surface water.

In August 2024 the Inspectorate updated its guidance to the industry regarding requirements to submit sample results and summary risk assessment information for PFAS, this remains a major focus. Companies have continued with their targeted sampling strategy to focus on those sites which are likely to be at risk from PFAS contamination. In 2024 companies collectively carried out over 770,000 analyses for individual PFAS, and in total, over 1.7 million analyses have been performed since 2012, providing a significant dataset to understand the challenges in drinking water.

PFAS tiers are now applied to any PFAS chemicals of interest detected in the final or raw water. Sites are classified as:

  • Tier 1 where PFAS concentrations are <0.01 µg/L;
  • Tier 2 where PFAS concentrations are <0.1 µg/L; and
  • Tier 3 where PFAS concentrations are ≥0.1 µg/L.

PFAS tiers are to be applied to all PFAS chemicals of interest identified in the parameter list (Annex C of the Information Direction) detected in final water. The latest guidance on the actions companies should be taking for the three tier levels is available in our most recent guidance – DWI_PFAS-Guidance_Mar_2025.pdf

Monitoring by water companies has highlighted a further PFAS compound of potential concern; 6:2 fluorotelomer sulfonamide alkylbetaine (6:2 FTAB). This compound has been added to the parameter list and companies are expected to initiate monitoring and reporting for this parameter as soon as practical.

Companies are also expected to consider the effect of combined concentrations of the PFAS chemicals of interest identified in the parameter list. As such, the requirement to implement a prioritised mitigation methodology to progressively reduce PFAS concentrations in drinking water has been further extended to include combined PFAS on a ‘sum of’ basis. Companies have now implemented reporting for the ‘sum of PFAS’ based on the 48 named PFAS compounds in the parameter list. Where sites now fall into tier 2 or 3 for the sum of parameter these sites should be rolled into the existing site prioritisation approach based on their relative classification and an appropriate risk reduction strategy should aim to progressively reduce PFAS concentrations in drinking water.

The analysis targets the source water because this provides information on the hazard and informs the necessary mitigation required, be that blending, removal of the source water or treatment. During 2024 there were no samples reported in tier 3 for any treated water being supplied to consumers. In total 351 samples taken from raw water abstractions had results within tier 3. This helps to demonstrate that the current industry strategy of mitigating the risks from PFAS is working effectively to protect consumers. 96% of the samples taken were less than the detection limit for their analytical methodology.

TierRaw water testsTreated water testsTotal
Tier 1287,840413,084700,924
Tier 228,88817,22346,111
Tier 33510292
Total317,079430,307747,386
Table 32 – Number of tests in 2024 within tiers including results less than the limit of detection

The most prevalent species in raw water during 2024 is PFOS 21.1% of all positive sample results, the below table sets out all the other parameters which contributed greater than 1% of detections by number of samples rather than by result for all tiers.

Raw water data – number of tests per tier greater than the limit of detection (LOD)

ParameterTier 1 >LODTier 2 >LODTier 3 >LODTotal >LOD
PFOS1,499541742,114
PFHxS1,254428251,707
PFOA1,6039371,703
PFHxA1,405197151,617
PFBS1,4764921,527
PFBA1,22346 1,269
PFPA821200151,036
PFHpA8831014907
Sum_of_PFAS39213236560
PFecHS216122 338
PFPS264323299
FBSA206127225
6:2 FTAB189293221
6:2 FTSA; 6:2 FTS978413194
PFDA1931 194
FHxSA1344910193
PFHpS12313 136
8:2 FTSA1159 124
HFPO-TA115  115
PFNA115  115
PFUnDA111  111
EtFOSAA103  103
MeFOSAA92  92
PFNS92  92
FOSA829 91
HFPO-DA (Gen X)82  82
PFDS81  81
PFDoDA79  79
PFHxDA78  78
PFODA78  78
PFTeA78  78
11Cl-PF3OUdS77  77
NFDHA75  75
EtFOSA74  74
PFTrDA74  74
4:2 FTSA72  72
MeFOSE72  72
PFEESA72  72
3:3 FTCA71  71
DONA71  71
PFDoS71  71
5:3 FTCA2 6870
PFMOPrA70  70
PFMOBA69  69
PFUnDS69  69
7:3 FTCA68  68
MeFOSA68  68
EtFOSE63  63
9Cl-F3ONS34  34
Total14,4512,05629216,799
Table 33 – Raw water PFAS data

Treated water data – number of tests per tier greater than the limit of detection (LOD)

ParameterTier 1 >LODTier 2 >LODTotal
PFBA2,939753,014
PFHxA1,9433612,304
PFOS1,4932141,707
PFBS1,489141,503
PFPA1,0583841,442
PHFHxS1,1351671,302
PFOA1,068571,125
PFHpA698 698
6:2 FTSA; 6:2 FTS84367451
Sum_of_PFAS15451205
6:2 FTAB988106
PFPS106 106
PFDA86 86
FBSA70 70
FHxSA402262
PFHpS53 53
PFecHS41849
8:2 FTSA43144
PFNA36 36
NFDHA31 31
FOSA28 28
PFUnDA27 27
PFNS25 25
9Cl-F3ONS22 22
EtFOSAA14 14
MeFOSAA14 14
PFEESA14 14
PFDoDA11 11
PFDS8 8
PFTeA8 8
11Cl-PF3OUdS7 7
HFPO-TA5 5
MeFOSE2 2
EtFOSA1 1
PFHxDA1 1
PFMOBA1 1
PFODA1 1
PFUnDS1 1
Total12,8551,72914,854
Table 34 – Treated water PFAS data – zero tier 3 detections in 2024

Affected sites

Where PFAS is identified, action is taken by the company in line with the risk prioritisation strategy. For instance, in 2022, there were two tier 3 results from the outlet point of water treatment works, which were subsequently blended within the distribution network to reduce levels to well below the tier 1 threshold prior to supply to consumers. In 2024, no samples were within tier 3 for treated water. Whilst no tier 3 detections occurred within 2024, similar action will be taken in the future to ensure the continuing protection of public health and consumer confidence should there be any further tier 3 detections, including the ‘sum of PFAS’ for the 48 named parameters.

For the PFAS sampling the company undertakes risk based sampling which is specifically focused on the assets most at risk of failing, therefore the number of samples in tier 2 might look high but these are taken from a small number of sites. Therefore out of 1067 treatment works in supply across England and Wales in 2024 tier 2 samples were detected at only 46 locations – 4.3%.

Company nameCount of Sample Result
in tier 2		Actual number of
affected sites
Affinity Water Ltd2667
Anglian Water Services Ltd109716
Bristol Water Plc242
Isles of Scilly41
Northumbrian Essex and Suffolk water62
SES Water31
Severn Trent Water Ltd31
South Staffordshire Water Plc436
Southern Water Services Ltd2402
Thames Water Utilities Ltd84
United Utilities Water Ltd21
Wessex Water Services Ltd333
Table 35 – number of sites affected by PFAS Tier 2 results
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