The composition and potential impact of the colmation zone on sewer exfiltration

There is a body of research which asserts that exfiltration from urban sewers can be a major contributor to groundwater pollution. Test rig studies by the authors and others have demonstrated a capacity for defects in sewer systems to be partially self-sealing due to sewer sediments and associated solids, wall slimes and biofilm growth. This sealing material has been described as the colmation layer but its composition and impact on long and short term patterns of exfiltration and on the nature of the exfiltrate are not fully understood. This paper reviews existing knowledge of the impact of the colmation layer on exfiltration rates and describes a laboratory based experiment aimed at providing a better understanding of the nature of the colmation layer and the potential contribution of biofilm growth to its formation and persistence.