Multi-Isotope Characterization of Water in the Water Supply System of the City of Ljubljana, Slovenia

Urban water supply systems (WSS) are complex and challenging to manage since the properties of water in the WSS change from source to the end user over time. However, understanding these changes requires a more profound knowledge of the WSS. This study describes the urban water cycle within the WSS of Ljubljana, Slovenia, where different water parameters such as temperature, electrical conductivity, total alkalinity, δ2H, δ18O, and δ13CDIC were monitored from September to November 2018. Altogether 108 samples were collected, including from the source (3) and at different levels of the WSS: wells (41), joint exits from water pumping stations (7), reservoirs (22), water treatment locations (2), drinking fountains (13), taps (19) and wastewater system (1). The data show that although the ranges of δ2H and δ18O values were small, each well is represented by a unique fingerprint when considering additional parameters. A statistically significant difference was observed between sampling months, and temperature and most parameters showed higher variability within the wells than across the WSS, suggesting a more unified WSS. Finally, based on δ13CDIC values, a distinction could be made between river/groundwater interactions within the WSS and between shallower and deeper wells and their distance from the river bank.