Statistical study of the relationship between free chlorine levels and bacteriological checks on systems in the Paris area

The majority of French cities are supplied with treated surface water. The primary factor in determining the quality of this water concerns disinfection to ensure that the system is not contaminated with parasites, coliforms and streptococci; some of these organisms, especially coliforms, are capable of multiplying within the system. The presence of bacteria indicative of faecal contamination in water samples, which contains a significant amount of free chlorine, has revealed a possible link between the two. Based upon sampling points of the Parisian system Žand in the suburbs for purposes of comparison., we show the existence of a linear relationship between the results of analyses for coliforms on a given day, D, and the content of free chlorine in the water on days D-2 and D-3. Subsequent ground tests confirm the explanations proposed regarding these phenomena.