A Quantitative Risk Assessment of Verotoxigenic E. coli (VTEC) in Private Groundwater Sources in the Republic of Ireland

Based on the sampling of 262 private domestic wells in Ireland, mainly located in High/Extreme vulnerability areas, a quantitative model was developed for prediction of gastrointestinal infection risk attributable to verotoxigenic E. coli (VTEC) contamination of private wells in Ireland. Models are based on maximum likelihoods and predict an overall crude incidence rate of 28.3/100,000 well users per annum; approximately 5-6 times higher than crude incidence rates reported within the Irish population and up to 40 times higher than the current EU mean. Predicted crude incidence rates (CIR) of 21.6/100,000 and 88.7/100,000 were associated with borehole and hand-dug wells users, respectively, with predicted annual probabilities of 0.1% and 0.17%, respectively. Predicted annual infection risks associated with both (untreated) well types exceed the USEPA acceptable target of <1 infection/10,000 consumers/year. Where treatment systems are employed, models predict that a relatively small pathogen log reduction is necessary to meet health targets. Sensitivity analyses indicate that pathogen loading and dose-response are the input variables with the greatest contribution to model outputs. Using a more conservative assumption for pathogen contribution ratio, models predict a lower annual CIR of 5.56/100,000. Due to limitations in inherent assumptions, model results, while useful, should be treated with caution.