Abstract: Laboratory tests with synthetic acid mine water show the sealing effect of the gelatinous precipitate that forms when hydrated lime or powdered limestone is added in a simulated mine entry closed by a porous barrier.^Field tests were conducted in a recently abandoned coal mine.^Hydrated lime and limestone slurries were pumped into the mine water behind rubble barriers through 2-inch steel pipes to test the laboratory findings.^The outflow was observed at weirs attached to the ends of two 12-inch diameter drain pipes.^The results indicated that only temporary sealing of the outflow was achieved and that neutralization took place when the interior water flow conditions were favorable.