Learning from chemical, taste and odour failures

The table below shows all the chemical failures during 2025, the full list of all chemical parameters sampled in included within Annex B.

Parameter name	Standard	Total number of tests	Tests not meeting the standard	Tests not meeting the standard per company	Percentage compliance (3dp)
Trichloroethene & Tetrachloroethene – sum of 2 substances	10 µg/L	11,156	1	ESP (1)	99.991%
Benzo[a]Pyrene (total)	0.01 µg/L	13,644	1	SVT (1)	99.993%
Polycyclic aromatic hydrocarbons (total by calculation)	0.1 µg/L	13,886	3	IWN(2) SRN(1)	99.978%
Aluminium	200 µg/L Al	52,398	10	BRL (1), NES (4), UUT (5)	99.981%
Bromate	10 µg/L BrO₃	10,026	1	ESP (1)	99.990%
Iron	200 µg/L Fe	53,039	98	SRN(3) SWB(7) IWN(1) WSX(1) TMS(6) SVT(15) AFW(1) NES(9) BRL(2) SST(1) SEW(7) YKS(20) ANH(6) UUT(18) LNW(1)	99.815%
Manganese	50 µg/L Mn	52,918	18	NES(3) WSX(1) SVT(2) UUT(7) SWB(3) LNW(1) BRL(1)	99.966%
Nickel	20 µg/L Ni	14,081	72	WSX(3) BRL(4) YKS(7) NES(2) TMS(20) AWI(2) IWN(3) SES(2) UUT(3) LNW(5) SST(1) ANH(3) ESP(3) SVT(6) MUA(1) AFW(3) SEW(1) SRN(3)	99.489%
Lead	10 µg/L Pb	14,120	60	YKS(6) SST(3) VWP(1) SWB(1) AFW(6) NES(5) SRN(3) SEW(3) IWN(2) AWI(2) UUT(7) WSX(1) ANH(2) ICW(1) SVT(4) TMS(13)	99.575%
Gross alpha	0.1 Bq/L	1,257	15	ESP(4) LNW(1) IWN(10)	98.807%
Odour	Acceptable to consumers and no abnormal change	57,660	43	ANH(4) SVT(5) ESP(1) TMS(1) ICW(1) AFW(1) YKS(3) SWB(4) BRL(2) NES(10) UUT(3) SRN(6) SST(2)	99.925%
Taste	Acceptable to consumers and no abnormal change	57,502	65	UUT(5) ESP(1) BRL(2) SVT(5) NES(6) SWB(8) ANH(13) YKS(5) SRN(6) IWN(6) WSX(1) PRT(1) TMS(6)	99.887%
Copper	2 mg/L Cu	13,785	2	ANH(1) WSX(1)	99.985%
Sodium	200 mg/L Na	14,071	1	NES (1)	99.993%
Nitrite – consumers taps	0.5 mg/L NO₂	24,557	5	TMS(4) NES(1)	99.980%
Nitrate	50 mg/L NO₃	24,518	2	ANH(1) IWN(1)	99.992%

–
Table 9 – Chemical parameter failures

Nitrate

Nitrate in drinking water is most commonly associated with diffuse agricultural pollution, particularly the use of nitrogen fertilisers and organic manures within drinking water catchments. It can persist in groundwater for long periods and may also affect surface water sources following rainfall and runoff. Under the Regulations, nitrate has a prescribed concentration value of 50 mg/L as nitrate at consumers’ taps.

Nitrate exceedances are compliance failures and must be investigated in accordance with regulation 18. Companies are expected to establish the source and extent of elevated nitrate, assess any risk to consumers, review the effectiveness of blending or treatment controls, and take corrective action to restore compliance and prevent recurrence.

In 2025, two nitrate exceedances were reported from 24,518 tests. These were reported by Anglian Water and Independent Water Networks. Regulation 27 risk assessments should identify sources vulnerable to nitrate pollution and set out appropriate control measures, including catchment management, source blending, treatment, operational monitoring and contingency arrangements where required. The Inspectorate expects companies to manage nitrate risks proactively, recognising that catchment pressures and groundwater lag times can make deterioration slow to reverse. Monitoring data should be used to identify upward trends before standards are exceeded, and companies should maintain clear evidence that control measures are effective and resilient under seasonal and operational variation.

Lead

Lead is a toxic metal that can dissolve into the drinking water supply when water comes into contact with lead pipes. The source of lead in drinking water is mainly communication pipes owned by water companies and supply pipes and fittings owned by consumers within their property which are made from lead. There were 60 compliance sample failures for lead during 2025 out of 14,122 samples analysed for lead. The highest concentration of lead found was 562 µg/L in Blackheath zone during April where the supply pipe is over 100m in length and the owner is reluctant to change it. The company changed their communication pipe.

Consumers are, to a large extent, protected from exposure to lead by the practice of phosphate dosing at water treatment works to reduce plumbosolvency. To further reduce exposure to consumers from lead, national policy and company strategy is required to mitigate the risk. Lead contributed to 57 events reported to the Inspectorate in 2025. The combined ERI score for these events was 0.043.

The Inspectorate collects data on lead via the compliance process which sets out how many compliance samples must be taken each year from water quality zones. This number varies by population but as a maximum this is eight samples per year per zone as stipulated within the Regulations. Therefore, the size of the data set held for lead is small compared to the amount of data held on other parameters. Companies, however, do collect far more data than the compliance samples we require them to take. The Inspectorate has been in discussions with the industry lead working group to see whether all lead sample data can be collated on behalf of the industry to help the Inspectorate and form an overall picture of the lead risk within the country, with the assistance on the lead working group this will form part of the data return from 2027 onwards. Holding this data set would enable the Inspectorate to work better with Defra and central government to be able to advocate for changes to the governmental policy for lead management within water.

Lead is a problem which to a large extent is confined to within the boundaries of homes and therefore is under the control of the homeowners to rectify. It is however a relatively invisible problem and one which is tough to compel homeowners to spend the often large amount of money required to remove lead from their properties. Large-scale company-led removal schemes have been successful in discrete areas, but scaling up to national levels is not currently feasible.

Of note is a lead event from Advanced Water Infrastructure Networks who are a new appointment and variant company, NAVs predominantly supply new build housing estates given that lead has been banned since the 1970s. However, this failure of 11.7 µg/L taken from a show home was believed to be due to the low turnover and the properties fittings. The company received two recommendations related to their investigation and follow up sampling which was not timely.

The issue is also compounded by the fact that some plumbers still use lead solder despite it being banned for use on cold water systems. Using lead solder on hot water systems is still possible and lead solder can still be purchased from plumbing and DIY shops should the homeowner be willing to carry out their own modifications which, unlike the gas industry, is still legal to do so. There is an approved plumber scheme in place, but there is no legal requirement to use this service. The World Health Organization (WHO) has declared there is no safe level of lead in drinking water. Many lead pipes still exist within company distribution networks, particularly on the final sections of pipe supplying consumer properties. The Inspectorate set out its expectations in respect of lead in its long-term planning guidance. Water companies have collectively declared an ambition to be lead pipe free by 2050.

Iron, manganese and aluminium

Iron, manganese and aluminium are commonly detected in drinking water and are associated with source water quality, treatment performance, asset condition and distribution system operation. Under the Regulations, these parameters are subject to indicator standards. Although they are not normally health-based limits at the concentrations typically encountered, they are important indicators of treatment control, network stability and consumer acceptability.

Elevated iron and manganese concentrations are most often associated with groundwater sources or the mobilisation of deposits within the distribution system, particularly following changes in flow, pressure or system operation. Aluminium exceedances are more commonly linked to treatment performance, especially where aluminium-based coagulants are used and where coagulation, filtration, pH control or residual management is not sufficiently effective.

Exceedances of these standards are compliance failures and must be investigated in accordance with regulation 18. They may indicate weaknesses in treatment control, asset maintenance, network operation or operational response, including inadequate flushing, insufficient optimisation of treatment processes, or poor understanding of deposits within the distribution system. Failures may also be associated with turbidity, discolouration and consumer rejection of water.

Companies are expected to maintain effective process control, use monitoring data to identify deteriorating trends and manage treatment and network risks proactively. Regulation 27 risk assessments should identify where iron, manganese or aluminium may present a risk to wholesomeness, consumer acceptability or treatment performance, and should set out appropriate control measures. Where failures are persistent, repeated or linked to wider asset or operational deficiencies, companies must take corrective action to restore compliance and prevent recurrence.

In samples taken from water supply zones, iron, manganese and aluminium accounted for 29% of all chemical failures in 2025. These parameters are significant because they can cause visible discolouration and affect the acceptability of drinking water, leading consumers to reject the supply even where the primary concern is aesthetic rather than toxicological. Each failure is assessed by the Inspectorate to determine whether the cause lies in source water quality, treatment performance, asset condition or distribution system operation. Where appropriate, the Inspectorate makes recommendations or takes enforcement action to secure improvement and protect consumers.

Trichloroethene and tetrachloroethene

Trichloroethene (TCE) and tetrachloroethene (PCE) are chlorinated hydrocarbons included in the Regulations under the combined parameter for the sum of trichloroethene and tetrachloroethene. Their presence in drinking water is most commonly associated with groundwater sources affected by historic industrial contamination, including metal degreasing and dry-cleaning activities, because these compounds can persist in aquifers for long periods, they represent a long-term source water risk which must be identified and controlled through company risk assessments.

The parametric value applies to the combined concentration of TCE and PCE in water supplied for human consumption. Any exceedance is a compliance failure and must be investigated in accordance with regulation 18. Companies are expected to establish the cause, assess any risk to human health, determine the extent of the issue, and take appropriate action to restore compliance and prevent recurrence.

In 2025, the reported TCE and PCE failure did not reflect deterioration in water quality. The investigation established that the failure arose from a transcription error by the company when transferring laboratory data, which resulted in an erroneous compliance failure being recorded.

Although the supplied water did not exceed the regulatory standard in this instance, the error demonstrates the importance of robust data governance. Companies must maintain effective validation, verification and reporting controls to ensure that compliance data submitted under the Regulations is accurate, reliable and traceable. Errors in regulatory reporting can misrepresent the quality of water supplied, lead to unnecessary investigation, and undermine confidence in published compliance information.

Bromate

A bromate exceedance of 200 µg/L was reported by ESP Water Limited and subsequently confirmed to be incorrect. The investigation showed that the true laboratory result was 0.2 µg/L, which is well below the legal limit of 10 µg/L. The error occurred during the manual data entry, where an incorrect multiplier was applied to the analytical result. This led to an inaccurate value being reported. No additional sampling was required, as the original laboratory data was valid.

To prevent a similar error in the future, the incorrect multiplier was removed from the company’s reporting spreadsheet. Previous compliance returns were reviewed, and no other errors of this type were identified. The company also improved its reporting arrangements to reduce reliance on manual processes, including additional automated checks to ensure that any genuine exceedances are identified promptly before data are submitted.

Data integrity and reporting errors

Although the supplied water did not exceed the bromate standard, the incident demonstrates the importance of robust data governance in regulatory reporting. Companies must ensure that laboratory results are transferred, calculated, validated and submitted accurately. Manual data handling, unsupported calculation steps, or weak validation controls can lead to erroneous compliance reporting, unnecessary investigation and inaccurate public information.

In response, the company removed the incorrect multiplier from its reporting system and reviewed previous compliance returns. That review did not identify further errors of this type. Additional automated validation checks were also introduced to reduce reliance on manual processes and to improve the accuracy of future compliance reporting.

The TCE/PCE and bromate as did the heptachlor epoxide cases arose from deficiencies in data handling rather than from deterioration in water quality. In addition laboratory errors account for the PAH failures for example. Such errors are not acceptable. Companies are required to maintain effective data management, validation and verification arrangements so that compliance data submitted under the Regulations is accurate, reliable and traceable.

The Inspectorate expects companies to minimise manual handling of compliance data wherever practicable, maintain clear audit trails, apply automated validation checks, and investigate anomalies before submission. Where errors are identified, companies must correct the data, assess whether previous returns may have been affected, and implement controls to prevent recurrence.

These incidents reinforce that strong data governance is a fundamental component of regulatory compliance and public confidence in drinking water quality. Accurate reporting is essential to ensure that regulatory scrutiny is directed to genuine risks and that published compliance information properly reflects the quality of water supplied to consumers.

Nickel

Nickel is a naturally occurring metal that may be present in drinking water from source waters or through leaching from metallic materials, particularly within distribution systems and domestic plumbing. Under the Regulations, nickel is a prescribed parameter with a parametric value of 20 µg/L at the consumer’s tap, reflecting the potential contribution from premise plumbing and fittings.

Exceedances of the nickel standard are compliance failures and must be investigated in accordance with regulation 18. Companies are expected to take prompt steps to establish whether the source lies within company assets, supply conditions or private plumbing, assess any risk to consumers, and identify appropriate corrective action. Elevated concentrations are most commonly associated with new or recently installed fittings, stagnation within internal plumbing, or water quality conditions that increase metal dissolution.

Companies should use investigation outcomes to understand whether failures are isolated to individual premises or indicate a wider risk associated with water chemistry, materials in contact with drinking water, or local distribution conditions. Regulation 27 risk assessments should identify where nickel may present a risk to wholesomeness and set out proportionate control measures, including advice to consumers where failures arise from premise plumbing.

In 2025, nickel represented nearly one fifth of all chemical failures for the second consecutive year, accounting for 19% of the total. Most failures were associated with consumer distribution systems and were commonly linked to new taps or fittings. The number of failures reported by new appointment and variation companies was notable relative to the size of the populations served, including Independent Water Networks, Last Mile Water, Albion Water and ESP Water.

Where nickel failures occur, companies are expected to provide clear and appropriate advice to affected consumers, including steps to reduce exposure where stagnation or premise plumbing is implicated. This may include flushing taps after periods of non-use and ensuring that any replacement fittings are suitable for contact with drinking water and installed correctly. Companies should also ensure that learning from individual failures is reflected in sampling strategies, risk assessments and engagement with developers, installers and consumers where appropriate.

Nitrite

In 2025, Thames Water reported four nitrite exceedances in four separate water supply zones. Each affected zone was supplied from treatment works using chloramination as part of the disinfection strategy.

Nitrite in drinking water is closely associated with chloraminated supply systems. Under the Regulations, nitrite is a prescribed parameter with a parametric value of 0.5 mg/L at consumers’ taps. The standard reflects potential health impacts, particularly for bottle-fed infants. In chloraminated systems, nitrite can form through nitrification, a biological process in which ammonia-oxidising bacteria convert ammonia to nitrite, particularly where disinfectant residuals are low, water residence times are long, or temperatures are elevated.

Nitrite exceedances are compliance failures and must be investigated in accordance with regulation 18. Where chloramination is used, companies are expected to maintain effective operational control to prevent nitrification, including the management of disinfectant residuals, ammonia and nitrite monitoring, and adequate turnover within the network. Elevated nitrite may indicate a loss of system control and requires prompt corrective action to ensure that supplies remain wholesome and safe for consumption.

Thames Water’s investigations indicated that there were no associated treatment issues at the supplying works at the time of the exceedances. Two of the four failures were associated with the internal distribution systems of the sampled properties. At one property, the investigation was inconclusive because resamples from the original property and neighbouring properties were below the limit of quantification. At the remaining property, the consumer refused access for resampling, although samples from neighbouring properties were also below the limit of quantification. The company provided appropriate flushing advice to affected consumers. These findings demonstrate the importance of considering both company distribution systems and premise plumbing when investigating nitrite failures.

Northumbrian, Essex and Suffolk Water reported a nitrite concentration of 0.51 mg/L in a sample from a consumer property in July 2025. Investigation samples from neighbouring upstream and downstream properties also recorded nitrite concentrations above the standard, at 0.52 mg/L. Subsequent resamples from the original property and one upstream neighbouring property were satisfactory. Further sampling identified nitrite concentrations below the regulatory standard but above the company’s internal trigger level of 0.35 mg/L.

The affected properties were located in a zone supplied from works using chloramination to maintain a residual disinfectant in the downstream network. The company’s investigation confirmed that there were no nitrite issues at the supplying works and that the chloramination dosing process was operating normally. Network flushing was undertaken, and the local network configuration was verified using pressure loggers and valve checks. The company concluded that elevated water temperatures, long residence times and reduced turnover in the local distribution network were probable contributory factors.

The company presented its nitrate and nitrite management strategy to the Inspectorate. This includes enhanced nitrite monitoring at relevant service reservoirs, with an internal trigger level of 0.35 mg/L requiring investigation and flushing to reduce concentrations. The Inspectorate expects companies operating chloraminated systems to use monitoring data, operational knowledge and risk assessments to identify locations vulnerable to nitrification and to take proactive action before regulatory standards are exceeded.

Copper

Copper in drinking water is primarily associated with leaching from internal plumbing systems, including pipework, fittings and storage tanks, rather than from source waters or treatment processes. Under the Regulations, copper is a prescribed parameter with a parametric value of 2 mg/L at the consumer’s tap, reflecting its predominant origin within premise plumbing. Exceedances are compliance failures and must be investigated in accordance with regulation 18 to establish whether the cause lies within company assets, water chemistry, local distribution conditions or private plumbing. In 2025, two copper exceedances were reported, which is lower than in previous years. Companies are expected to assess the effectiveness of corrosion control measures, consider any wider implications for regulation 27 risk assessments, and provide appropriate advice to affected consumers where failures arise from premise plumbing or fittings.

Wessex Water – Copper

In October a sample collected from a Wessex Water’s consumer’s property in the Shepherds Shore zone contained a pre-flush copper concentration of 2.1mg/l. A flushed sample collected at the same visit contained copper at a concentration of 0.04mg/l. As part of the company’s investigations a resample from the original property was satisfactory as were samples collected from a neighbouring property. A water fittings inspection identified that new copper pipework had been fitted during a kitchen refurbishment earlier in the year. Wessex Water wrote to the consumer and advised them to flush their tap before drinking for a period of time and offered to return to the property at a later date to take further samples to demonstrate the risk from the new copper pipework has reduced once the internal pipework has become conditioned with a build up of scale.

Anglian Water – Copper

A pre-flush sample taken from a motor garage in August contained copper at 2.995mg/L, and also contained a petrol/solvent odour. The results of the flushed sample were satisfactory for copper. Anglian Water’s investigation found organic compounds typical of fuel in samples at the premises, while samples from neighbouring properties were satisfactory. Advice not to drink the water was provided and work was carried out to replace the communication pipe with a barrier pipe which prevents migration of hydrocarbons from the soil, and the boundary box was also replaced with one suitable for contaminated land. The consumer’s supply pipe was found to be made of copper, and advice was provided to replace it to prevent further copper exceedances.

Sodium

Sodium in drinking water may arise from source water characteristics, treatment processes, or domestic water softening systems. Under the Regulations, sodium has a prescribed concentration value of 200 mg/L. Exceedances at consumer taps must be investigated in accordance with regulation 18 to establish whether the result is representative of the public supply or has been influenced by premise plumbing, including ion-exchange water softeners.

Domestic water softeners can significantly increase sodium concentrations in softened water and may also affect the corrosivity of water in premise plumbing. Regulatory samples should therefore be collected from points that are representative of the mains supply. Where a softening system is present, companies should confirm the sampling location, avoid softened outlets where possible, and ensure that sampling procedures include appropriate checks to prevent unrepresentative results.

In 2025, Northumbrian, Essex and Suffolk Water reported a sodium concentration of 290 mg/L from a consumer tap sampled in February. The investigation attributed the exceedance to a domestic water softener connected to the sampled tap. The zone is supplied by a bulk Anglian Water supply with elevated hardness, and previous sodium exceedances in this zone have also been associated with domestic softener installations. In response, the company instructed samplers not to sample from properties with domestic water softeners where this would compromise representativeness, introduced hardness checks at properties before sampling, and updated its sampling procedure. The Inspectorate also advised the company to consider proactive communication with consumers in the affected zone on the importance of ensuring that any water softener is correctly installed and does not supply taps used for drinking or cooking unless appropriate controls are in place.

Benzo(a)pyrene

Benzo(a)pyrene is a polycyclic aromatic hydrocarbon (PAH) and is regulated separately from the combined PAH parameter because of its toxicological significance. PAHs in drinking water are most commonly associated with the deterioration or disturbance of coal-tar lined cast iron mains. Exceedances of benzo(a)pyrene or the combined PAH standard are compliance failures and must be investigated in accordance with regulation 18 to establish the cause, extent and recurrence risk. Regulation 27 risk assessments should identify where coal-tar lined mains or other relevant materials present a risk and set out appropriate monitoring, control and replacement strategies.

Independent Water Networks reported a benzo(a)pyrene exceedance in the Tendring underground zone in January 2025. Repeat samples from the original property and investigational samples from neighbouring properties did not detect benzo(a)pyrene above the limit of quantification. The company concluded that the likely source was deterioration of coal-tar lining in older cast iron mains within the upstream network used to provide the bulk supply. The bulk supplier confirmed the presence of older mains upstream of the company’s water supply zone. Fluoranthene, another PAH, was detected in all samples collected as part of the investigation. On this basis, the Inspectorate did not agree with the company’s conclusion that recurrence was unlikely. Recommendations were made to strengthen monitoring protocols and to ensure that the PAH risk assessment for the zone was reviewed in a timely manner.

In July 2025, Thames Water reported an exceedance of the benzo(a)pyrene standard in the Epping Forest zone. The company attributed the failure to temporary mobilisation of mains material following a local burst main. Investigatory samples confirmed elevated PAH concentrations close to the burst location. Coal-tar lined mains are present in the area, and this risk had already been identified by the company and was subject to enforcement action by the Inspectorate. Following repair of the burst main and return to the usual supply arrangements, sample results returned below the regulatory standard. The event demonstrates the importance of understanding where legacy mains materials remain in service and ensuring that operational disturbances, including bursts and flow changes, are considered within company risk assessments and response procedures.

Severn Trent Water reported a benzo(a)pyrene result of 0.02496 µg/L. Although this individual result exceeded the benzo(a)pyrene standard, the combined PAH concentration remained below the regulatory limit. The company investigated the local distribution network, including additional sampling from nearby properties, key network points and the original property. All follow-up samples were satisfactory, with no further detections of benzo(a)pyrene or related compounds. Reviews of network operation, recent maintenance activity, flow and pressure conditions did not identify a likely cause, and there was no evidence of network materials commonly associated with PAH failures. Targeted flushing and boundary box sampling were undertaken as a precaution and confirmed satisfactory water quality.

Taste and Odour

Taste and odour are important indicators of consumer acceptability and confidence in drinking water. Under the Regulations, taste and odour are indicator parameters and water must be acceptable to consumers with no abnormal change. Although failures are not usually associated with direct health effects at the levels typically encountered, they can indicate deterioration in source water quality, treatment performance, distribution system condition or operational control.

Unacceptable taste or odour may arise from algal activity in source waters, naturally occurring organic compounds, treatment inefficiencies, changes in disinfection practice, or mobilisation of deposits and biofilm within the distribution system. Chlorine and chlorinous tastes may also occur where dosing is elevated, poorly controlled, or where residual management is affected by long residence times or changes in demand.

Taste and odour failures are compliance failures and must be investigated in accordance with regulation 18. Companies are expected to establish the cause and extent of the issue, assess whether there is any wider risk to water quality or consumer acceptability, and take appropriate corrective action. Persistent or widespread complaints may indicate weaknesses in treatment optimisation, asset performance, network management or customer response arrangements.

Companies are expected to manage taste and odour proactively through effective source water monitoring, treatment process control, maintenance of distribution systems and timely response to customer contacts. Regulation 27 risk assessments should identify where taste and odour risks may arise and set out appropriate control measures so that water remains not only compliant, but acceptable to consumers.

Anglian Water

For instance, a failure for taste and odour as well as copper in Billingborough zone of Anglian water in August was from a petrol station. Following this failure the company made contact with the property and issued a do not drink. The following day they replaced the communication pipe with barrier pipe and also replaced the boundary box for one suitable on contaminated land. The property were issued with further advice about replacing taps, and the company will resample once this work is complete from the customers side to remove the restriction of use.

United Utilities

A quantitative taste and odour exceedance was identified in a sample taken in June 2025 in the Ashton West supply zone, supplied by Buckton Castle works. The Inspectorate noted that between January 2020 and June 2025 the company had reported 13 quantitative odour and 11 quantitative taste exceedances from four supply zones supplied from Buckton Castle works. Despite repeated incidents, no root cause had been identified. 2,4,6-trichloroanisole (TCA), linked to musty odours, was detected four times in 2022 and 2025 from within the treatment process, at the final water, from service reservoirs, and at consumer taps. Following the June 2025 exceedance confirming persistent taste and odour issues, enforcement was initiated requiring the company to carry out a full risk review and investigation. This Notice requires the company to carry our a full review of the taste and odour risk assessment and mitigation at Buckton Castle works.

South West and Bournemouth Water

A routine compliance sample collected by South West Water from a property on the Isle of Tresco in February was tested by laboratory panellists who reported salty, oily and astringent descriptors. South West Water carried out an investigation following notification of the result. Operational checks confirmed that the supplying Vane Hill Works was operating normally and no consumer complaints relating to taste or odour had been received. Follow-up sampling from the affected property, the supplying service reservoir, the treatment works and elsewhere within the distribution network did not identify any further failures for the taste parameter.

The company investigation concluded that the most likely cause of the taste detection was elevated sodium and chloride concentrations resulting from saline intrusion into the Isle of Tresco’s groundwater sources. These conditions are long-standing and reflect the limitations of the existing treatment process on the island, which does not remove sodium or chloride. Historical monitoring shows these parameters have remained relatively stable over recent years, with no evidence of deterioration in raw water quality or the quality of water supplied to consumers.

South West Water has legal instruments in place for each of the Isle’s of Scilly habited islands to secure long term upgrades to the legacy systems. The Isle of Tresco is subject to Notice ISC-2021-00009, which includes actions to mitigate taste, sodium and chloride parameters.

Portsmouth Water

In February Portsmouth Water collected a random compliance sample from a hotel which contained a salty taste. Subsequent samples from the original and neighbouring properties returned satisfactory results with no taste detected.

A water fittings inspection completed as part of Portsmouth Water’s investigation confirmed the presence of a water softener system, which was not identified at the time of sampling. This was despite the use of hardness test strips being a requirement in the company procedure to alert the sampler to the potential for the presence of a water softener, which can impart a salty taste. The sampling team has since been re-briefed on the importance of using hardness test strips and evidence of this was provided by the company.

The hotel owner was advised to maintain a mains-fed unsoftened supply for drinking water, however the Inspectorate noted that sodium was not tested in the during the original investigation, preventing adequate risk assessment for hotel guests and staff. The company received a recommendation to further investigate sodium concentrations at the hotel and consider whether further action was needed, including under Section 75(2) of the Water Industry Act. The response to the recommendation included details of the sodium concentration investigation which did not highlight any concerns. A suggestion was also made to Portsmouth to strengthen the company sampler auditing process.

Thames Water

An unusual onion/indole odour was detected from a supermarket within Thames Water’s Keston and Forestdale water supply zone in August. The sample was collected from a tap in the staff canteen which was not routinely used as a drinking water outlet.

Following notification of the failure, the company undertook an extensive investigation that included repeat sampling from the original tap, an alternative tap within the supermarket, neighbouring properties and the upstream service reservoir. Laboratory assessments continued to identify a low-level onion odour in samples collected from both taps within the affected premises. In contrast, all samples collected from neighbouring properties and the upstream supplying asset showed no evidence of the unusual taste or odour. GCMS analysis did not identify hydrocarbons or other contaminants of concern.

A water fittings inspection undertaken at the affected property identified several non-compliances requiring remedial action, including deficiencies in backflow protection arrangements associated with appliances in the staff canteen and bakery areas. The company considered these findings, together with the persistence of the odour within the affected premises and the absence of evidence elsewhere in the supply system, to indicate that the issue was isolated to the property. The water fittings breaches were promptly rectified.

ESP Utilities

In May, ESP Utilities detected a petrol/chemical odour in a sample taken from a show home on a newly constructed housing estate in Bromsgrove that was still under development at the time of sampling.

The company investigated the failure, however resamples from the affected show home and two neighbouring occupied properties were satisfactory for taste, odour and hydrocarbons. Site inspections, discussions with site personnel, and a water fittings inspection did not pinpoint any specific issue and no evidence of fuel spills, construction-related contamination or operational issues affecting the supply was found. The bulk supplier, Severn Trent Water, also confirmed that there were no upstream operational issues at the time of the failure.

Whilst no evidence of contamination or wider network impact was identified, the Inspectorate considers that petrochemical tastes or odours warrant a precautionary approach. The Inspectorate therefore recommended that consumers should be provided with appropriate precautionary do not drink advice pending the outcome of investigations into petrochemical-related failures.

Table of contents