Abstract: The study systematically identified groundwater recharge sources and the contribution rate of leachate to groundwater both within the slag yard and in downstream areas. The diffusion pathways and attenuation patterns of the contaminant plume were delineated, the groundwater pollution mechanism of the manganese slag yard were revealed based on the spatial distribution characteristics of hydrochemical and contaminant by applying methods such as hydrogeological analysis, hydrogen and oxygen isotope tracing, and a binary mixing model in a typical manganese slag stockpile in a karst area of Xiushan, Chongqing. The results indicated that the groundwater in the study area was primarily recharged by infiltration of atmospheric precipitation and leachate seepage. The contribution ratio of leachate at monitoring points within the slag yard reached 54% to 96%. The hydrochemical type of groundwater evolves from the HCO₃-Ca·Mg type towards the SO₄-Na type along the groundwater flow. The concentrations of manganese and ammonia nitrogen were low in the upstream area, attained their maximum at the slag site, and decreased in the downstream area. Groundwater pollution can be characterized as a multi-source, multi-pathway transport model characterized by “release from the slag yard - fissure migration - subsurface diffusion - discharge into surface water”. Based on these results, a triple-protection strategy is proposed, including containment via permeable reactive barriers (PRBs) deployed downstream, proactive hydraulic control through interception and drainage systems installed upstream and around the manganese slag yard, and early-warning capacity built on an optimized monitoring network. This study offers a systematic approach to preventing and controlling groundwater pollution in similar karst mining areas.