An environmentally acceptable leak sealant comprising lava rock aggregate (scoria) for sealing leaks in dams, holding ponds or other water-containing structures. The aggregate is selected and graded for particle size and density. Particles of different sizes co-operate to engage walls of the leak and interlock with each other to form a seal within a leak opening. The particles must sink at a suitable rate, and become entrained in water flow through the leak. Beneficially larger particles have a generally higher sink rate than smaller particles, and so arrive first at the leak to staunch a major part of the flow; intermediate and smaller particles follow to form an effective seal. Applications include sealing sluice gates, stop logs at hydroelectric dams to reduce water wastage for increased electricity productivity, for maintenance requiring de-watering of the turbine; or for sealing leaks in cracks in bedrock or concrete structures associated with dams.