Applying a risk-based approach to investigate the cause
Introduction
The Inspectorate commonly receives enquiries from local authorities requesting advice following a breach of a specific parameter standard. This case study provides one such example in relation to a coliform detection from a sample taken from a spring supply. Coliforms are indicator parameters and detections in private water supplies are not uncommon. They are not necessarily indicative of a health risk, and their presence in isolation does not render the water unwholesome. Nevertheless, the regulations require that the presence of coliforms must be investigated to establish a cause. This will determine any underlying risk to the wholesomeness of the water to facilitate appropriate mitigating measures to address the risk.
Enquiries to the Inspectorate of this kind indicate that this is not always recognised or embedded in local authority practice. A common approach for example, is for a single “confirmation resample” to be taken in response, then the matter is brought to a close when a satisfactory result has been obtained. This compliance-based approach was once common in water industry practice but has long since been replaced by modern risk-based methodology as it does not address root cause.
The following case study provides an example of a common enquiry concerning the presence of coliforms in a private supply.
Coliform detection enquiry
In October 2025, the Inspectorate was contacted by a local authority in Wales asking what action they should take following a coliform result of 24 per 100ml from a private water supply in their area. They explained that they intended to restrict the supply by requiring the consumers to boil their water but did not explain any risk-based reasoning for this. The main purpose of their enquiry was to establish whether it was necessary for them to take any other immediate action.
It was noted that in this instance the coliform isolate from the sample had been typed and found to be Kluyvera intermedia. These bacteria occur in untreated water, soil and sewage and can cause opportunistic infections in immunocompromised patients. Whether it was the intention by the local authority to restrict the supply because of this was not stated in the enquiry. For this reason, it was assumed that it was the result, not the species that prompted this.
Inspectorate response
The Inspectorate explained that any decision resulting in the need for consumers to boil their water must be risk based and proportional to the risk. This should take account of health-based advice sought from Public Health Wales.
Advice to temporarily restrict the use of a private water supply suggested that the local authority considered the sample result to constitute a potential danger to human health. If this was the case, they were duty bound by regulation 17 to inform the consumers of this, as well as the nature of it. They were further advised to investigate the cause of the failure and if this determined that the risk this posed did indeed pose a potential danger to health, they were obliged to serve a regulation 20 enforcement notice. This must compel appropriate remediation to mitigate the risk in an appropriately timely manner.
It was explained that the root cause of the sample breach may be due to several reasons, a common cause being ingress of water, soil or other environmental debris into chambers, tanks or other part of the system that were not fully protected. Furthermore, that results of this kind indicated ineffective treatment notably suitable disinfection. Remediation must apply a multi barrier approach to address any hazards from source to tap that the investigation revealed.
Learning
This case study illustrates the need for local authorities to make risk-based decisions to inform appropriate subsequent actions on a case-by-case basis following parameter breaches. It sets out the fundamental requirement of the regulations to establish the cause through an investigation, irrespective of the parameter that has been breached. The outcome of this will determine what next steps must be taken to protect consumers and address the root cause. This will depend on the nature of the cause and the risk that this presents.
The primary aim of any investigation is to determine whether the supply presents a potential danger to human health.
The sample result and the circumstances of the situation must also inform the need to notify consumers to protect health until any risks are better understood by the investigation. It is also important that the response to any compliance breach is proportionate to the risk posed by the parameter at the concentration detected, especially when considering temporary restrictions of use.
In this instance the local authority intended to advise consumers to boil their water in response to the coliform breach in the absence of faecal indicators. Given the coliform type this seems to have been prudent in this instance, although this may have been coincidental. This does however illustrate that an understanding of the coliform type can be helpful in making risk-based decisions.
In many cases where coliforms are detected in the absence of faecal indicators, advice to boil (or any other temporary restrictions of use) are generally not necessary. Any decision to do so, irrespective of the coliform count, must be risk based, taking account of the circumstances on a case-by-case basis. This should include reviewing the most recent supply risk assessment and consider other relevant sample results where they are available (for example, turbidity), and whether there are any vulnerable consumers. The supply risk assessment must be updated following the investigation.