Recharge and redox processes drive urban groundwater quality in Vienna’s shallow aquifers

The quality of urban groundwater is subject to a complex interplay of various processes influenced by hydrogeology, anthropogenic activities, and infrastructure. Understanding these factors and their spatial and temporal dynamics is essential for the sustainable management of urban water resources. To date, data sets containing sufficient key parameters and an appropriate spatial resolution have often been lacking for urban aquifers. Here, we present a high-spatial resolution multi-tracer approach to assess the controlling factors of shallow urban groundwater quality. Water samples were collected from 144 to 154 groundwater wells during warm (fall 2021) and cold (spring 2022) conditions in Vienna. A comprehensive set of parameters (major ions, nutrients, heavy metals, water stable isotopes and nitrate isotopes, wastewater indicator compounds and greenhouse gases) was analyzed to evaluate spatial and seasonal variations. Statistical analysis revealed that the dominant processes affecting groundwater quality are recharge, redox processes, and rock-water interactions. A combined tracer interpretation indicated zones influenced by surface water - groundwater interactions, pollution from wastewater leakage, and different redox processes, such as denitrification. Further anaerobic processes, including iron and manganese reduction, sulfate reduction, and methanogenesis, were shown to impact groundwater quality. Groundwater temperatures of up to 27 °C were observed near urban subsurface infrastructure, indicating that subsurface buildings and surface sealing act as additional stressors to shallow groundwater. The presented high-resolution spatial sampling enabled the creation of maps of urban groundwater recharge, redox processes, which provide a better understanding of groundwater quality dynamics and serves as basis for developing strategies to sustainably manage urban water resources.