Private Water Supplies - Case Study 2011/11

The implications of not establishing that a laboratory is accredited and using only an approved or authorised method of analysis

This case study relates to a spring source, which provides the water supply to a food premises (cheese manufacturer). The supply is not treated. A planned sample, collected by the local authority, was reported as being positive for E.coli. As part of the investigation, the local authority reviewed the original risk assessment and noted that previously recommended improvement works to prevent source contamination had not been completed. It was concluded that the supply was a potential danger to human health and the local authority put in place a Regulation 18 Notice restricting the supply. However, at this stage, the local authority noticed that the laboratory certificate recorded the E.coli result as presumptive, not confirmed, and contacted the Inspectorate for advice.

When testing drinking water samples for compliance with the regulations, laboratories must follow either the specified method or an authorised method (Schedule 3). The specified method for E.coli is set out in Microbiology of Drinking Water1 published by the Standing Committee of Analysts. The method requires confirmation of presumptive results. An authorised method is one that has been approved by the Inspectorate. Approval is on the basis of rigorous testing in comparison to the specified method and authorised methods are notified to the EU Commission.

The Inspectorate advised the local authority that the laboratory had not followed the specified method because confirmation had not been carried out. The implications of this error were significant because the presumptive result could not be used as reliable evidence in support of the local authority’s Regulation 18 Notice. Fortunately, in this case the local authority had other evidence through the risk assessment (incomplete remedial work to safeguard the source) demonstrating a potential danger to human health.

This case study highlights the importance of local authorities taking steps to secure that all samples of drinking water are submitted only to laboratories that are accredited for the purpose. For each sample and every parameter, local authorities are under a duty to ensure that the accredited laboratory follows the specified method or an authorised method. Whereas it is desirable and sensible for a local authority to ask the laboratory to notify them immediately about a presumptive E.coli result (because this enables a rapid investigation and action to safeguard public health) all such results must be confirmed and local authorities must only use confirmed results in annual data returns to the Inspectorate and in support of any enforcement.

1The Microbiology of Drinking Water (2009) – Part 4 – Methods for the isolation and enumeration of coliform bacteria and Escherichia coli (including E.coli O157: H7) available at -223MAYh.pdf