Why must mine tailings be stored and disposed of carefully

Mine tailings be stored and disposed of carefully since they contain a lot of chemicals that are very toxic that leaches into aquifers, and they also runoff in the rainwater into streams and lakes.

Tailings, also called mine dumps, culm dumps, slimes, tails, refuse, leach residue or slickens, are the materials left over after the process of separating the valuable fraction from the uneconomic fraction (gangue) of an ore.