Interpretation of Drinking Water Monitoring Data for Environmental Health Professionals

As part of a larger study investigating the role of public health agencies in assuring drinking water safety in Canada, a questionnaire survey of practicing Public Health Inspectors/Environmental Health Officers (PHI/EHO) in Western Canada was conducted. The results from a portion of this questionnaire dealing with the interpretation of water quality monitoring data are presented and analysed. These reflect, in common with other professional disciplines surveyed, evidence of an over confidence in the accuracy of monitoring data in cases, such as that found in effectively-treated drinking water, where the occurrence of target contaminants should be rare. A hypothetical monitoring scenario was presented with characteristics sufficient to estimate what level of confidence was warranted in a positive result. The respondents' stated requirements for a level of accuracy that should be provided by environmental monitoring to support making a risk management decision, implied a capability that will be difficult to achieve. The respondents' own estimates for confidence in the hypothetical scenario presented greatly exceeded the realistic, but poor level of confidence that was warranted for a positive result. More effective monitoring strategies and appropriate responses to detected contamination will inevitably depend upon better understanding of the practical quantitative limitations of treated water monitoring.