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A laboratory mixing apparatus used to prepare test specimens of columns of soil mixed with slurries consisting of clay, typically bentonite, and granular zero-valent iron (ZVI) is described. The slurries are injected and simultaneously mixed into the soil via a hollow-stem auger to mimic the process that occurs in the field associated with the remediation of source zones contaminated with chlorinated solvents (e.g., carbon tetrachloride and trichloroethylene) using large-diameter (e.g., 2.4 m or 8 ft) hollow-stem augers. The presentation includes descriptions of the testing apparatus and procedures for preparing the test columns, mixing the slurries into the base (host) soil, and determining the resulting vertical distributions in the physical properties (e.g., unit weight, water content, porosity, void ratio, clay content, ZVI content, and degree of saturation) of the post-mixed column specimens. Example results of the vertical distributions in physical properties as well as of treatability studies for base soils that were contaminated with chlorinated solvents are provided to illustrate the use of the laboratory apparatus and testing procedures.