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

Abstract

Study region: Brussels (Belgium) and Nantes (France), two cities located in oceanic settings. Study focus: The article assesses the capabilities of detecting the presence of groundwater in sewers and waste-water in storm sewers (also called parasitic waters), which increase sewer-overflows risk, augment the water-treatment plant operation costs and pollute downstream rivers. This is done by means of Laser Absorption Spectroscopy (LAS) instruments that favor the development of stable isotopes (δ18O and δD) as tracer of urban waters. The study first describes the factors affecting isotopic composition of urban waters. It also describes how to optimize the use of stable isotopes, either alone or coupled with other tracers to detect parasitic waters. New hydrological insights: 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. Keywords: δ18O, δD, Stable isotopes, Parasitic water, Groundwater, Waste-water inflows, Sewers