12 th International Conference on Urban Drainage, Porto Alegre/Brazil, 10-16 September 2011 Blackwood et al. 1 The composition and potential impact of the colmation zone on sewer exfiltration. D. J. Blackwood * , A. J. Spiers, R. Peter and D. J Gilmour Urban Water Technology Centre, School of Contemporary Sciences, University of Abertay Dundee, Bell Street, Dundee, DD1 1HG, UK *Corresponding author, e-mail d.blackwood@abertay.ac.uk ABSTRACT 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. KEYWORDS Exfiltration; Biofilms; Sewer defects. INTRODUCTION Rutsch et al. (2008) observed that 10 years of intensive interdisciplinary research had not resolved a debate on the magnitude and impact of sewer exfiltration. There is general agreement that sewer defects have a capacity for self-sealing thus reducing volumes of exfiltrate. This, together with the potential biodegradation and absorption of pollutants during the passage of exfiltrate through soil, may explain a lack of evidence of widespread pollution arising from exfiltration (Blackwood et al., 2002). However, there is evidence from experimental test rigs and from long term field monitoring of leakage from sewers (Wolf et al., 2005), that self-sealing of individual sewer defects is incomplete and hence exfiltration will continue in the long term and that these exfiltration rates will be variable. Furthermore, a number of studies (e.g. Rutsch et al, 2008) have identified an impact of exfiltration on groundwater in specific locations. This suggest a need to identify and protect vulnerable groundwater from sewer exfiltration but any risk assessments methodology to identify vulnerable locations will require a more accurate understanding of the long term leakage rates from defects. Theoretical estimates of leakage from individual defects have been based on Darcy’s Law: Q ex = A leak . k ƒ . (h / ∆s), where Q ex is the exfiltration rate, A leak is the area of the defect, k ƒ is the coefficient of permeability, h is the water depth, and ∆s is the depth of a soil. The soil in the immediate vicinity of a sewer pipe in the case of exfiltration comprises a mix of material transported by the exfiltrate from the sewer which forms a clogging layer, known as the colmation layer, in the pipe wall and in the media beneath the pipe. Rauch and Stegner (1994) refined the Darcian approach to modelling leakage from sewers by the introduction of