- A laboratory flag is the start of a response, not proof that one component has failed.
- For regulatory failures, the local authority must investigate the cause under Regulation 16.
- Simply taking another sample to obtain a different result is not, by itself, an investigation.
- If the supply is considered a potential danger to human health, users must be informed and the Regulation 18 process applies.
- Keep the original report, advice, investigation, corrective actions and follow-up evidence connected.
1. First establish what actually failed
Start with the laboratory report, not an assumption about the treatment system. Confirm the supply, sampling point, sample date and time, parameter, reported result, unit and any standard or indicator value shown. Check whether the sample was part of the local authority's regulatory monitoring programme, an investigation, commissioning work, routine operational checking or a private sample arranged by the owner.
A laboratory report may use words such as fail, exceedance, non-compliant or may simply highlight a value. Those labels need context. The legal and operational response depends on why the sample was taken, which parameter is involved and what the local authority concludes. If you arranged the sample yourself and it suggests the water may be unsafe, contact the local authority promptly rather than waiting for the next routine visit.
2. Immediate action and user protection
Where a result may indicate a health risk, the priority is protecting people who use the supply. Keep a written record of who was contacted, when, what advice was given and how users were informed. That is especially important for supplies serving guests, tenants, staff, customers, children, older people or other groups who may not know the supply is private.
Under Regulation 15, if the local authority considers the supply to be a potential danger to human health, it must promptly take appropriate steps so people likely to consume the water are informed of the danger, where possible its nature, and advice that allows them to minimise it.
3. Regulation 16 investigation
DWI guidance is explicit: when a local authority suspects that a private supply is unwholesome, or an indicator parameter has breached the prescribed value, it must investigate the cause under Regulation 16. A repeat sample on its own is not an adequate investigation merely because the later result is satisfactory.
The investigation should establish why the result occurred and what action is necessary. The local authority will consider the laboratory result alongside the supply arrangement, previous results, risk assessment, treatment, source conditions and other relevant evidence.
For an owner or manager, the practical role is to make accurate information available quickly: recent maintenance, treatment alarms, power interruptions, UV intensity or lamp status where recorded, filter changes, tank inspections, rainfall or flooding events, source works, plumbing alterations and any complaints from users.
4. Investigate source to tap
A failed sample does not automatically mean the component nearest the sample point caused it. Work systematically from source to tap. Use the supply schematic and risk assessment to identify possible routes by which the problem could have arisen.
- Source: changes after heavy rainfall, flooding, agricultural activity, drainage defects, surface-water ingress or damage to a spring chamber, borehole headworks or well.
- Raw-water transfer and storage: damaged covers, vents, overflows, sediment, ingress, cross-connections or prolonged stagnation.
- Treatment: exhausted or blocked filtration, incorrect flow, UV faults, lamp or sleeve condition, dosing faults, bypasses, unsuitable treatment sequence or inadequate maintenance.
- Treated storage and distribution: contamination after treatment, dead legs, poorly protected tanks, pressure loss, repairs, backflow risks or local plumbing materials.
- Sampling point and sampling event: whether the point was representative, correctly identified and suitable for the purpose of the sample.
Do not alter equipment simply to make the next sample pass before the cause has been properly considered. Unrecorded changes can destroy useful evidence and make the investigation harder.
5. Common failure patterns
Microbiological results
Microbiological failures may be associated with faecal contamination at the source, ingress to storage, inadequate disinfection, treatment bypass, distribution contamination or sampling issues. The correct response depends on the organism, result and site circumstances. A satisfactory repeat result does not by itself explain the original failure.
Chemical parameters
Chemical exceedances can arise from geology, land use, pollution, treatment performance, plumbing materials or changes in source conditions. Some parameters are chronic rather than immediately acute risks, but they still require proper investigation where the regulatory standard is not met.
Indicator parameters
An indicator-parameter breach does not necessarily mean the water is an immediate danger to health, but it can indicate a deterioration or treatment/process problem. DWI states that indicator breaches must also be investigated to establish the cause.
6. Corrective actions
Corrective actions should follow the evidence from the investigation rather than a generic maintenance checklist. Record each action separately, with an owner, target date and evidence of completion. Examples may include protecting or repairing the source, cleaning and disinfecting storage, restoring treatment, repairing a dosing or UV fault, removing an unauthorised bypass, changing filters, correcting a cross-connection or replacing problematic plumbing.
Where temporary measures are required—such as alternative water, restrictions on use or interim treatment—keep them distinct from the permanent remedy. A temporary control is not the same thing as restoring and maintaining a wholesome supply.
7. Repeat sampling and verification
Repeat sampling is often part of verification, but its timing, location and parameters should follow the investigation and competent advice. Keep the repeat result linked to the original sample and record why that location and timing were chosen.
A useful close-out trail explains the sequence: original result → notifications → investigation → identified or suspected cause → corrective action → verification → authority decision or other competent close-out evidence. Avoid treating a single later sample as proof that a recurring or intermittent problem cannot return.
8. Regulation 18 and Section 80
If the local authority concludes that the supply constitutes a potential danger to human health, Regulation 18 requires it to serve a notice on the relevant person or persons, subject to the conditions in that regulation. The notice sets out the grounds, any restriction or prohibition considered necessary and the corrective actions needed to protect health, restore acceptable water and maintain it.
If the water is unwholesome but does not constitute a potential danger to human health, DWI guidance explains that Section 80 of the Water Industry Act 1991 may instead be relevant, depending on the cause and circumstances. These are formal authority decisions; an operator should not label an internal action record a “Regulation 18 notice”.
Keep any formal notice, amendment and revocation with the incident evidence. The separate private water supply notices guide explains this enforcement pathway in more detail.
9. Feed the event back into risk assessment
A failure can reveal that an existing control was absent, ineffective or not being maintained. For supplies subject to Regulation 6 risk assessment, the local authority should consider the investigation findings when reviewing the risk assessment. DWI's FAQ specifically advises review as part of a regulatory-failure investigation, while noting that this does not apply in the same way to single-dwelling supplies that are exempt from routine Regulation 6 assessment.
Operationally, update the information that supports future assessment: schematic, asset list, treatment settings, maintenance frequencies, sampling points and any new hazards or controls identified.
10. What evidence to keep
- Original laboratory report and any laboratory comments.
- Supply and sampling-point identity, sample date/time and reason for sampling.
- Who received the result and when.
- Local-authority, laboratory and health advice received.
- User communications and any restriction/alternative-supply arrangements.
- Investigation notes, photographs, site observations and relevant treatment data.
- Named corrective actions, responsible person, deadlines and completion evidence.
- Contractor reports, invoices, service sheets and disinfection records where relevant.
- Repeat or investigative sample reports and why they were taken.
- Formal notices, amendments or revocation where applicable.
- Changes made to the schematic, maintenance plan or risk controls.
- A clear close-out note stating the evidence relied on.
11. When can an incident be closed?
Do not close an incident simply because a repair was completed or a later sample was satisfactory. Close it when the required investigation and corrective actions are complete, health-protection measures have been reviewed, verification evidence is available and any authority requirements have been satisfied. Where a Regulation 18 notice exists, the local authority must revoke it when it is satisfied that the health risk has been sufficiently mitigated.
12. Common mistakes
- “Just resample.” DWI specifically warns that resampling alone is not an investigation.
- Assuming the UV unit is always the cause. The problem may be upstream, downstream or unrelated to UV.
- Losing the original report. Keep the original laboratory evidence, not just manually typed results.
- Making undocumented changes. Record what changed, when and why.
- Confusing internal records with formal notices. Only the authority serves the statutory notice.
- Removing restrictions too early. Follow the authority's incident-specific advice.
13. Practical checklist
- Confirm exactly which result, parameter, unit and sample point are involved.
- Contact the local authority promptly where the result may indicate unsafe water or regulatory non-compliance.
- Record user-protection advice and communications.
- Preserve the original report and pre-repair evidence.
- Investigate source, treatment, storage, distribution and sample point.
- Record corrective actions separately from temporary controls.
- Arrange follow-up sampling as advised and link it to the original event.
- Update risk and operational records with lessons from the incident.
- Do not mark the event closed until the required evidence and authority position support closure.
14. Frequently asked questions
Does one failed sample mean the water is definitely unsafe to drink?
Not necessarily. The health significance depends on the parameter, result and circumstances, but a regulatory failure or concerning result must not be dismissed. Follow the local authority's advice and ensure the cause is properly investigated.
Can I just take another sample?
Repeat sampling may be part of verification, but DWI states that simply resampling to obtain a different result does not constitute the Regulation 16 investigation required after a regulatory breach.
Should I boil the water after a failed test?
Only if the local authority or appropriate health professional advises it for that incident. Boiling is suitable for some microbiological risks but is not the correct response to every type of failure.
Does a failed result automatically mean my UV system has failed?
No. The cause may be at the source, in storage, treatment, distribution, local plumbing or the sampling event. Investigation should consider the whole source-to-tap system.
When is a Regulation 18 notice required?
Where the local authority concludes that the private supply constitutes a potential danger to human health, Regulation 18 requires it to serve a notice on the relevant person or persons, subject to the regulation's conditions.
When should I close the incident record?
After the required investigation and corrective actions are complete, follow-up evidence supports the outcome and any local-authority requirements or restrictions have been formally addressed.
Keep the sample, investigation and corrective actions connected.
Private Water Supply Manager helps organise laboratory reports, corrective actions, maintenance evidence and follow-up records without pretending to decide whether water is safe.
- DWI: Local-authority FAQs — action after a parameter breach
- DWI: Regulation 15 — provision of information
- DWI: Regulation 18 — notices
- DWI: Serving notices — templates and examples
This guide applies to England. Regulatory decisions and restrictions are made from the facts of the incident; always follow the current local-authority and public-health advice for the supply.