The Altered Water Balance: An analysis of urban shallow groundwater level dynamics using a high-density groundwater level observation network for Aalborg, Denmark

Study Region : The research is conducted in the Kærby district of Aalborg, Denmark (0.6 km 2 ). Study Focus : Urban areas with shallow groundwater levels face increasing challenges related to groundwater flooding, infrastructure damage, and sewer system infiltration under changing anthropogenic climatic conditions. This study aims to quantify and analyze components of the water balance through a comprehensive, observation-based assessment of shallow urban groundwater variability, using high-resolution data from both citizen science initiatives and automated groundwater level monitoring. New Hydrological Insights for the Region : Over a 4.5-year period, groundwater levels are monitored, revealing seasonal fluctuations of approximately ± 0 . 5 m, and rapid dynamic responses to individual rainfall events, with observed groundwater level increases up to 6 times the corresponding rainfall depth. Cross-correlation analysis links fluctuations to antecedent precipitation and groundwater recharge dynamics. Hydraulic characterization indicates high infiltrability and a narrow vadose zone (0–2 m), explaining rapid saturation and groundwater response. The study quantifies groundwater infiltration into combined sewers using a water budget, showing that in some subcatchments, annual infiltration matches precipitation and exceeds recharge, suggesting lateral inflow and anthropogenically transformed flow regimes. Geospatial interpolation reveals inverted gradients and lowered groundwater levels driven by infrastructure-induced drainage. The study reveals a natural water balance altered by urbanization. The findings highlight the importance of understanding natural and anthropogenic hydrological processes to support climate-resilient urban water management and inform solutions mitigating groundwater impacts.