Measurement of infiltration rates in urban sewer systems by use of oxygen isotopes

The paper presents the principle of a method to measure infiltration rates in sewer systems based on the use of oxygen isotopes and its application in Lyon (France). Investigations in the urban area of Lyon benefit from the isotopic differences between underground waters originating from the two rivers Rhône and Saône and from their associated alluvial aquifers. The oxygen isotopic composition of the Rhône water is roughly 3 ‰ lighter than that of the Saône river, due to the large differences in the mean altitude and topographic situation of their catchments. Large amounts of water are pumped from the Rhône aquifer for drinking water supply. As a consequence, this difference in the oxygen isotopic composition between wastewater and local groundwater can be used to evaluate infiltration in sewers in some parts of the city. The results obtained in a case study allow to evaluate the reliability and the uncertainties of this method.