Abstract: A composite bioreactor and a method of using same is described. The bioreactor (10) comprises a combination of a sulfur oxidizing bacteria (SOB) bioreactor (12) directly on top of a sulphate reducing bacteria (SRB) bioreactor (14); the SOB bioreactor (12) comprising a heap of said soil to be rernediated and having suitable microorganisms therein to react with degrade contained contaminants; said SRB bioreactor comprising trough means (18) having support means to support said soil heap associated with an opening thereof, said support means having permeable membrane (16) associated therewith to allow passage of liquid but substantially no soil and sulfur reducing bacteria within said trough means (18); and liquid and nutrient supply means to promote growth of said sulfur oxidizing bacteria and said sulphate reducing bacteria within said composite bioreactor.