Carbonate karst aquifers are strategic reservoirs of groundwater of good quality for drinking use, among others. Despite its importance, water resources management are often unfulfilled, resulting in aquifer overexploitation and subsequently groundwater quality impairment. The case of Sierra de Humilladero puts in evidence the impact of unsustainable groundwater abstractions during more than forty years with dramatic effects on groundwater hydrodynamics and quality. A detailed analysis of this negative effects on karst groundwater has been assessed from available hydrodynamic and hydrochemical datasets and updated field data. Furthermore, to preliminary address the mixing processes described earlier by other authors, an End-Member Mixing Analysis (EMMA) was performed to tentatively quantify the mixing ratios, as consequence of hydraulic interactions between Sierra de Humilladero carbonate karst aquifer with neighbour permeable lithologies.