The invention relates to a method for immobilising bacteria in a material permeable to the bacteria, in particular a geological body, comprising forming a first zone comprising the bacteria in the material and allowing the first zone to move through at least part of the material; forming a second zone comprising an effective amount of a flocculating agent to the material and allowing the second zone to move through at least part of the material, wherein the zones are allowed to move such that the zones become at least partially overlapping and at least part of the bacteria flocculate, thereby becoming immobilised.