A method and apparatus for achieving in situ bioremediation of volatile contaminants in the vapor phase and dissolved or dispersed contaminants in the groundwater phase at a contaminated site. The apparatus of this invention is configured to be used within a remediation well of the type employed with vapor extraction and dewatering techniques, and serves to biologically treat soil contaminants in situ with microbes and any suitable microbial nutrients. As such, the present invention is adapted to be employed in conjunction with vapor extraction and dewatering remediation techniques to treat vapor and water-borne contaminants more effectively. The apparatus securely retains the microbes in a manner that serves to prevent leaching into the surrounding soil while simultaneously protecting the microbial population from predatory, competitive and inhibitory microorganisms.