Revisiting the hydrogeological water balance of Banyoles Lake (northeast Catalonia, Spain): Implications for sustainable groundwater exploitation

Abstract Balancing groundwater exploitation with ecological preservation requires robust conceptual models, particularly in karst systems where aquifer–lake interactions are complex and highly sensitive to human and climatic pressures. Banyoles Lake (northeast Catalonia, Spain) is a groundwater-fed karstic-tectonic system of exceptional ecological value, yet its hydrogeological functioning remains insufficiently understood. This study revisits its water balance by integrating meteorological, limnological, hydrochemical, and isotopic data into an updated hydrogeological analysis. Results indicate that regional groundwater inflow from the Banyoles–Alta Garrotxa system contributes ~11.4 × 10 6 m 3 /year, equivalent to ~54% of total inputs to the lake, while local recharge through runoff and shallow aquifers represents only ~23%. Outflows are dominated by drains (~16.2 × 10 6 m 3 /year, 77% of total) and evaporation (~2.3 × 10 6 m 3 /year 11%). The refined conceptual model highlights the predominance of upward vertical flows from confined limestone aquifers through a karstified gypsum formation, which govern lake recharge and explain the observed sulphate enrichment of lake water. This framework challenges conventional interpretations that treat the lake surface as a constant-head boundary, instead recognizing it as a dynamic hydrogeological unit controlled by potentiometric head of the aquifer beneath, which has seldom been addressed in the hydrogeological literature. By clarifying the processes sustaining Banyoles Lake hydrogeology, this report provides a novel conceptual framework essential for sustainable groundwater governance under increasing climatic stress.