Inexpensive, highly effective methods for the in-situ reduction of hexavalent chromium to the non-toxic trivalent oxidation state in soil and groundwater containing hexavalent chromium are provided, which may reduce hexavalent chromium concentration in soil to as low as 5 mg/L. The methods involve sampling soil comprising hexavalent chromium to determine a reaction amount of a reducing agent, providing a frame on a top surface of the soil, wetting the soil, spreading a reducing agent on the top surface of the soil, and flushing the soil with water to dissolve the reducing agent. The reducing agent may be a chemical agent, a biological agent, or a combination of chemical and biological agents. The biological reducing agents may include sludge from wastewater, return activated sludge, waste activated sludge, leachate from landfill or from composting operations, or compost material from municipal wastewater, industrial wastewater, or solid waste operations.