The hydrodynamic characteristics of a new type of reactor, the immobilized soil bioreactor, were studied. This apparatus is a practical new engineering concept (soil immobilization) based on entrapment of soil particles, which contain pollutant-degrading microorganisms, in the pores of a geotextile to activate the indigenous microorganisms. The soil immobilization is the third on the size scale of immobilization processes, coming after (1) that of molecules in heterogeneous catalysis (in Angstrom) and (2) that of microbial cells and their fragments in immobilized cells and enzymes biocatalysis (in micron). The size of immobilized soil particles is in the range of a millimeter. A mathematical model of liquid flow within the reactor is proposed, which qualitatively explains the distribution of the immobilized soil in space. The dynamics of soil immobilization within the bioreactor has been studied as a function of the particle size, initial slurry concentration and ail flow rate. A mathematical model of the process of soil immobilization was proposed based on deep filter mechanics. The process can be described by a second-order kinetic model. This study will be of great importance for the design of immobilized soil bioreactors for degradation of recalcitrant soil pollutants.