This paper presents considerations for the application of the natural water isotope method on catchment areas. For the estimation of the amount of infiltration and inflow in sewer systems the paper shows two applications in the Netherlands: one successful application on a relatively small catchment area with a simple geo-hydrological groundwater system and one unsuccessful application in an area that shows a large heterogeneity of δ18O values in groundwater. Also, the paper focuses on the validity of the assumption that the isotopic ratios of drinking water are equal to those of strict domestic wastewater. In the transition from drinking water to strict wastewater it is shown that changes in isotopic composition of the water due to evaporation in common household appliances and effects inside the human body are insignificant. However, the presence of high-efficiency condensing boilers in an area can significantly influence the δ18O value of strict wastewater, especially in winter months. This effect should be taken into account when applying the isotope method in such areas.

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