Microbiological assessment of private groundwater-derived potable water supplies in the Mid-West Region of Ireland

Determining the likelihood that groundwater contains faecal coliforms can aid water resource management in facilitating the protection of drinking water supplies. This study assesses the incidence of the faecal indicator organism Escherichia coli in 125 private water supplies (PWSs) serving individual houses in the Mid-West Region of Ireland. Two factors, aquifer type and rainfall (mm), were chosen as independent variables that can affect the vulnerability of a groundwater body. Using a geographical information system, the relative hydrogeological and climatological features unique to each sampling location were derived. Utilising this information, a logistic regression (LR) model was used to predict the probability of contamination of PWSs with E. coli. The model contained two independent variables: rainfall (mm; p < 0.001) and aquifer characteristics (p = 0.001). The full model, containing both predictors, was statistically significant at p < 0.001, indicating that the model distinguished between the independent variables' relationship to the incidence of contamination. The likelihood of E. coli contamination is greater with increased rainfall and in areas where a bedrock aquifer is dominant. The LR model explained between 27.4% (Cox and Snell R squared) and 36.8% (Nagelkerke R squared) of the variance in contamination and correctly classified 75.2% of cases.