Beneath the pavement: The hidden nexus of urbanization and groundwater quality in Krakow, Poland

Groundwater has historically played a pivotal role in the development of human civilizations, with wells serving as essential sources of drinking water. In Krakow, groundwater currently accounts for approximately 3% of the city’s total water supply, with the majority provided by surface water. Nonetheless, groundwater is utilised within an emergency system consisting of over 350 wells. Due to poor water quality, the groundwater extracted from these wells is used exclusively for non-consumptive purposes.

This study investigates groundwater pollution in Krakow, based on the research conducted between April and May 2023. A total of 91 wells were examined, of which only 64 were active. All wells are located within the Krakow area. Electrical conductivity was measured in situ immediately after sampling, and subsequent laboratory analyses were conducted to determine the concentration of major ions (Ca²⁺, Mg²⁺, Na⁺, K⁺, Li⁺, F^-, Br^-, HCO₃⁻, SO₄²⁻Cl⁻) and nutrients (NH₄⁺, NO₂⁻, NO₃⁻, and PO₄³⁻).

The results revealed considerable spatial variability in groundwater chemistry, closely linked to the geological structure of the region. Much of the Krakow area is underlain by Jurassic limestones, Cretaceous marls, and Miocene clays overlain by Quaternary sediments. Physiochemical parameters, such as electrical conductivity, salinity, and water temperature, exhibited substantial variability, influenced by intense anthropogenic activities, particularly affecting shallow groundwaters within Quaternary sediments. The highest groundwater salinity was observed in the historical city centre, where extensive impervious surfaces contribute to runoff-related contamination. Elevated nitrate concentrations, indicative of long-term pollution, are likely caused by leaks in the combined sewage system, particularly in central part of Krakow. Furthermore, increased temperatures in shallow groundwater in the urban core underscore the impact of the urban heat island effect, highlighting the intricate relationship between urbanisation and groundwater quality.