A Case Study on Factors Influencing Escherichia coli Concentrations in an Urban River Draining a Fully Sewered Area

Escherichia coli is an important indicator microorganism of fecal contamination in water. However, routine government monitoring often fails to capture the actual state of pollution, because E. coli concentrations in urban rivers are highly variable. This study presents a case study on factors influencing E. coli concentrations in an urban river draining a fully sewered area. An approximately 70-fold higher concentration compared with the average dry-weather concentration (1.9 CFU/mL) was observed under wet-weather conditions, probably due to the effects of combined sewer overflows. A very short survival of E. coli (less than one day) was expected in the unfiltered overlying water, due to the contributions of bacteriophages, protozoan predation, and bacterial competition, whereas a longer survival was expected in the sediment. Such a short survival may be a characteristic of the target watershed, where treated wastewater accounted for approximately 75% of the total flow. The highly variable antimicrobial resistance among E. coli populations under dry-weather conditions was possibly caused by the regrowth of a limited number of E. coli individuals in the sediment. Rising temperatures due to global warming are expected to decrease the concentration of E. coli in the target watershed, where E. coli populations are strongly suppressed by predation and competition.