- Drinking Water 2025 – Summary of the Chief Inspector’s report for drinking water in England
- Foreword
- Water supplies and testing
- Compliance with water quality standards
- Learning from microbiological failures
- Learning from turbidity failures
- Learning from chemical, taste and odour failures
- Learning from pesticide failures
- Asset health and service reservoir integrity
- Consumer contacts
- Drinking water quality events
- Water safety planning and risk assessment
- Perfluoroalkyl and polyfluoroalkyl substances (PFAS)
- Audit programme completed by the Inspectorate
- Air valve audits
- Enforcement, transformation and recommendations
- Materials in contact with drinking water (regulation 31)
- Security and Emergency Measures Direction - SEMD
- Network and Information Systems - NIS
- Research publications
- Whistleblowers
- Working with stakeholders
- Annex A – Number of tests carried out by companies
- Annex B – Compliance with standards
- Annex C – Compliance failures and events
Learning from pesticide failures
| Pesticide name | Standard | Total samples | Total failures | Comments |
|---|---|---|---|---|
| Propyzamide | 0.1 µg/L | 5,927 | 2 | ICW (1), NES (1) |
| Dieldrin | 0.1 µg/L | 4,790 | 1 | LNW (1) |
Pesticides in drinking water are tightly regulated because of their potential risk to human health and their widespread use in agriculture and land management. Under the Regulations, individual pesticides are subject to a parametric value of 0.1 µg/L and total pesticides to a parametric value of 0.5 µg/L. These standards reflect a precautionary approach rather than substance-specific toxicological thresholds.
Pesticides are most commonly associated with surface water and shallow groundwater sources, where they may enter supplies through agricultural runoff, spray drift or leaching into aquifers. Their occurrence is closely linked to catchment activities, seasonal use patterns, rainfall and the condition and effectiveness of treatment barriers.
Pesticide exceedances are compliance failures and must be investigated in accordance with regulation 18. Companies are expected to establish the source and extent of contamination, assess any risk to consumers, review the adequacy of treatment and catchment controls, and take corrective action to restore compliance and prevent recurrence. Detections below the parametric value also remain important because they may indicate increasing catchment pressure or deterioration in source water quality.
Companies are expected to apply a preventative, risk-based approach through regulation 27 risk assessments, catchment management, engagement with land users and appropriate treatment processes, including granular activated carbon where required. Effective monitoring, timely laboratory analysis and early operational intervention are essential to maintain compliance and ensure that water supplied remains wholesome and safe for consumers.
During 2025, three pesticide failures were reported from 20,831 samples, giving an overall compliance rate of 99.986%. Failures of the pesticide standard have reduced year on year since 2014. This improvement reflects the effect of catchment management initiatives, regulatory undertakings and continuing treatment improvements, including the use and maintenance of granular activated carbon.
Some pesticides remain difficult and costly to remove using available treatment technology. Companies must therefore continue to work with catchment stakeholders to reduce pesticide inputs at source, while maintaining effective treatment and operational controls. Small changes in catchment conditions, rainfall or raw water blending can materially affect pesticide concentrations in drinking water sources, and companies must remain vigilant in areas where these risks are known.
Propyzamide
Last Mile Water reported an exceedance of the individual pesticide standard after propyzamide was detected at 0.111 µg/L in a sample collected in November 2025. The company receives a bulk supply from Northumbrian, Essex and Suffolk Water, which had recorded a propyzamide exceedance at Horsley works a few days earlier. Northumbrian, Essex and Suffolk Water altered the raw water source blend, and subsequent samples showed concentrations reducing to below the regulatory standard. Further samples collected within the inset zone in December 2025 were below the limit of quantification.
A regulatory sample taken from Northumbrian, Essex and Suffolk Water’s Horsley works in November contained 0.14 µg/L propyzamide. This was the third recent propyzamide exceedance at the works, following detections above the standard in December 2022 and January 2023. Two previous propyzamide exceedances had also been recorded at Whittle Dene works in December 2020 and November 2022. Horsley works and Whittle Dene works are both subject to a notice requiring the company to deliver improvements to propyzamide removal, including replacement of granular activated carbon media and implementation of an effective ongoing maintenance strategy.
Both works abstract from the same raw water source, within a catchment containing significant arable land where propyzamide is used on crops. Significant rainfall in the weeks preceding the detections increased raw water turbidity and colour. At Horsley works, the clarifier stream was treating reservoir water at the time of the detection, while the other two streams were supplied by the River Tyne.
The company’s root cause analysis concluded that increased propyzamide concentrations in the raw water system, management of the raw water blend and the condition of the granular activated carbon media were contributory factors. The Inspectorate also made a recommendation relating to laboratory turnaround times, as delayed analysis can reduce the opportunity for timely operational response and source management.
The company confirmed that improvements had been made to raw water source operation and to its root cause analysis process following pesticide detections. It also updated its catchment risk assessment and provided details of the refurbishment of granular activated carbon cells required by the notice. The Inspectorate expects the company to manage and optimise current treatment performance while longer-term improvements are delivered, ensuring that catchment risk, treatment capacity and monitoring data are used together to prevent recurrence.
Dieldrin
Leep Networks Water reported a dieldrin exceedance after a result was reported as less than 0.036 µg/L, above the prescribed concentration value of 0.03 µg/L. The result was automatically flagged as a failure because the laboratory limit of quantification had been raised above the regulatory limit.
Further investigation confirmed that the reported result arose from an analytical issue rather than deterioration in water quality. A quality control standard failed during analysis, resulting in a “less than” value being reported above the regulatory limit. Follow-up samples from nearby properties were satisfactory for dieldrin and related pesticides, and monitoring data from the surrounding supply area showed stable and low pesticide concentrations.
The investigation concluded that the exceedance was not caused by the public water supply and did not present a risk to public health. However, the incident reinforces the importance of robust laboratory quality assurance and clear reporting of analytical limitations. The company’s laboratory service provider strengthened its procedures so that analytical anomalies and raised limits of quantification are clearly flagged and promptly reported to the company before regulatory data is submitted.