Source tracking of leaky sewers: A novel approach combining fecal indicators in water and sediments

•Microbial and chemical fecal indicators were analyzed both in water and sediment.•Wastewater infiltration was quantified in an urban canal during dry and wet weather.•WWMPs concentrations in sediments are a tracer of cumulative wastewater discharges.•Leaky sewers are successfully identified with E. coli in water and WWMPs in sediments.

In highly urbanized areas, surface water and groundwater are particularly vulnerable to sewer exfiltration. In this study, as an alternative to Microbial Source Tracking (MST) methods, we propose a new method combining microbial and chemical fecal indicators (Escherichia coli (E. coli)) and wastewater micropollutants (WWMPs) analysis both in water and sediment samples and under different meteorological conditions. To illustrate the use of this method, wastewater exfiltration and subsequent infiltration were identified and quantified by a three-year field study in an urban canal. The gradients of concentrations observed suggest that several sources of fecal contamination of varying intensity may be present along the canal, including feces from resident animal populations, contaminated surface run-off along the banks and under bridge crossings, release from contaminated banks, entrainment of contaminated sediments, and most importantly sewage exfiltration. Calculated exfiltration–infiltration volumes varied between 0.6 and 15.7 m3/d per kilometer during dry weather, and between 1.1 and 19.5 m3/d per kilometer during wet weather. WWMPs were mainly diluted and degraded below detection limits in water. E. coli remains the best exfiltration indicator given a large volume of dilution and a high abundance in the wastewater source. WWMPs are effective for detecting cumulated contamination in sediments from a small volume source and are particularly important because E. coli on its own does not allow source tracking.