Misjudging drinking water quality risk: Adopting a barrier approach for meaningful risk assessment to address latent risks.

The water quality risk assessment is the foundation for every drinking Water Safety Plan. The need to conform with a 'Corporate' risk assessment model commonly dominated by asset management frequency data can cause misjudgement of microbial risk. Well-performed risk assessments should identify potential risks. Risk assessments which place too much emphasis on historical evidence to demonstrate 'likelihood' of microbial contamination are unsuitable for drinking water quality because they fail to recognise 'latent' risks associated with absent or underperforming barriers to contamination. Most outbreaks occur when these 'latent' factors align to create a failure. Good risk assessments need to provide foresight. This is achieved if drinking water quality risk assessments are based on a 'barrier' approach. Where adequate and reliable multiple barriers to contamination are present the likelihood of a hazardous event should then be categorised as rare. Where barriers are absent, inadequate, or unreliable, then a higher likelihood is appropriate, depending on the nature and extent of the barrier shortfall. Practical examples show how the 'barrier' approach can be applied. The barrier risk assessment directly informs the operational monitoring programme, enabling regular confirmation that the challenge and barrier performance are consistent with the predictions of the risk assessment.