This study assesses the risk of fluoride contamination to karst aquifers from coal-fired power plant ash disposal sites in semi-arid regions, using Shanxi Province, China, as a case study. These vital water sources face increasing pressures from reduced recharge and elevated pollutant loads. Fluoride leaching from ash poses a particular threat; however, the coupled dynamics of the vadose zone and aquifer in such settings remain inadequately studied. To address this, we integrated HYDRUS-1D and MODFLOW to simulate fluoride transport and its spatiotemporal distribution. Simulations show that with an intact impermeable layer, fluoride migration is limited to a depth of 20 cm over 30 years. Once the layer is breached, fluoride reaches the deep K-1 sandstone aquifer after 55 years, with concentrations peaking at 1.57 x 10(-4) mg.L-1 after 60 years-three orders of magnitude below China's Class III groundwater standard (GB/T 14848-2017). Natural attenuation confines the plume within 300 m. This modeling framework confirms the natural barrier effect of the vadose zone, supports karst groundwater protection, and offers guidance for environmental management in the coal power industry, thereby supporting China's "Dual Carbon" strategy. This study provides a robust framework for risk assessment, while future work incorporating adsorption processes and fracture networks could further enhance predictive accuracy.