Potential and limits of stable isotopes (δ18O and δD) to detect parasitic water in sewers of oceanic climate cities

In Nantes, groundwater intrusions were detected above 8% of the sewer-water flow and waste-water inflows above 14% of the sewer-water flow. In Brussels, the stable isotopes may trace the presence of parasitic groundwater in a small part of the city territory (9 km2). This study concludes that stable isotopes can be used as tracer of parasitic waters in low altitude cities close to the ocean, when domestic water originates from catchments extending into the continent. In this case, tracing studies may take advantage of the seasonal variation of the isotopic composition of waste-waters, on the condition of assessing the other factors that impact the isotopic composition of urban waters, as done here.