In methods for treatment of in ground chromium ore processing residue contamination, ferrous sulfide is provided as a substantially insoluble material in the residue deposit. The ferrous sulfide accordingly may remain substantially in place, in the pores of the soil or residue, 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. As hexavalent chromium diffuses from the soil or residue, it is reduced by the ferrous sulfide. The ferrous sulfide may be placed into the underground deposit by injecting a ferrous salt solution into the deposit, waiting for a precipitation to elapse, and then injecting a sulfide solution into the deposit.