Occurrence of pathogenic microorganisms in the Saint Lawrence River (Canada) and comparison of health risks for populations using it as their source of drinking water

A 300-km portion of the Saint Lawrence hydrological basin in the province of Québec (Canada) and 45 wa-ter treatment plants were studied. River water used by drinking water treatment plants was analyzed (6-L sample vol-umes) to determine the level of occurrence of bacterial indicators (total coliforms, fecal coliforms, and Clostridiumperfringens) and pathogens (Giardia lamblia, Cryptosporidium, human enteric viruses). Pathogens and bacterial indica-tors were found at all sites at a wide range of values. Logistic regression analysis revealed significant correlations be-tween the bacterial indicators and the pathogens. Physicochemical and treatment practices data were collected frommost water treatment plants and used to estimate the level of removal of pathogens achieved under cold (0°C–4°C) andwarm (20°C–25°C) water temperature conditions. The calculated removal values were then used to estimate the annualrisk of Giardia infection using mathematical models and to compare the sites. The estimated range of probability of in-fection ranged from 0.75 to less than 0.0001 for the populations exposed. Given the numerous assumptions made, themodel probably overestimated the annual risk, but it provided comparative data of the efficacy of the water treatmentplants and thereby contributes to the protection of public health.