Abstract: An apparatus for treating a medium in situ includes counter-rotating shafts and intermeshed blades arranged on the shafts in a helical array to define mixing zones along the length of the shafts. Reagent supply conduits are disposed in a housing surrounding the upper ends of the shafts for introducing reagents into the mixing zones. The housing is attached to a mobile excavator which moves the shafts horizontally and vertically. The shafts are lowered into the medium by the excavator and are counter-rotated to cause the blades to pump the medium and the reagents along the shafts. Once at the desired operating depth, the shafts are moved below the surface of the medium in the horizontal direction generally perpendicular to the plane of the axes of the shafts to define a volume in the medium.