The overall objective of this investigation was to improve our current understanding of biofilm and colloidal biomass dynamics in shallow sandy aquifers, following consecutive accidental releases of phenols, which is a key step towards developing effective approaches to bioremediation of aquifers and risk management. The removal of phenol, 2-monochlorophenol, 2,4,6-trichlorophenol and pentachlorophenols was monitored in sand-bed columns. The results showed that the removal of these contaminants followed pseudo-first-order kinetics. Biofilm thickness was directly related to the hydrophobicity of the phenols, continuous in the sand-bed columns contaminated with less hydrophobic and discontinuous in the sand-bed columns contaminated with the more hydrophobic contaminants. Changes in size of biocolloids were observed following consecutive pulses of phenols in the sand-bed columns, but no relationship with any physicochemical property was encountered. (C) 2010 Elsevier Ltd. All rights reserved.