Fixing a leaking concrete slab typically involves identifying the source of the leak and applying an appropriate sealant or waterproofing material. This may involve excavating around the perimeter of the slab and applying a waterproof membrane or injecting a specialized polyurethane foam sealant into any cracks or gaps in the concrete. It's important to consult with a professional contractor or plumber to ensure proper diagnosis and repair of the leak to prevent further damage or potential safety hazards.