In systems and methods for treatment of underground contamination, ferrous sulfide is provided as a substantially insoluble material in an underground formation. The ferrous sulfide accordingly may remain substantially in place, even over long periods of time, regardless of underground water movement or diffusion. As a result, the ferrous sulfide may act continuously to chemically reduce and remove contamination. When used for treatment of chromium ore processing residue contamination, the ferrous sulfide may remain in the pores of the soil or residue. As hexavalent chromium diffuses from the soil or residue, it is reduced by the ferrous sulfide. The ferrous sulfide may be injected as a liquid into the underground formation, and then change to a more solid form. Chlorinated solvent contamination, dissolved chromium from other than COPR contamination, and other dissolved metals may also be treated.