A method is provided for treating contaminants in the vapors and shallow soils in an area below a slab of a building such that the slab is located above a first layer of gravel and a second layer of soil. The method includes providing an ozone unit, a riser pipe, and tubing. An area below the slab where contaminants need treated is first detected and then a hole is formed in the slab to define an injection location. The method also includes positioning the riser pipe in the hole at the injection location and coupling the tubing between the ozone unit and the riser pipe. The method further includes dispensing ozone from the ozone unit to flow through the tubing and the riser pipe to treat contaminants in the vapors and shallow soils in the area below the slab.