Validation of tracer dilution methods for the quantification of exfiltration from sewer systems through laboratory tests

The importance of exfiltration phenomena in sewer systems has captured growing attention within the scientific community in the last recent years. The structural and hydraulic performance of urban drainage systems as well as groundwater quality are clearly threatened by liquid accidentally flowing into or out of the conduits. This paper examines well known field applications developed to quantify exfiltrations from sewer systems using NaCl as an artificial tracer. The method, called QUEST (Rieckermann et Gujer, 2002), is based on the calculation of a tracer mass balance over a certain sewer length. In order to validate the fundamental assumptions of the method, as well as to assess the uncertainty related to the different sources of error which normally affect field methods, a 140m long (200mm diameter) pipe circuit has been built in laboratory. The aim was to test the reliability of the QUEST under different hydraulic conditions (flow, velocity), different exfiltration rates and different amount of tracer injected, assessing therefore the range of applicability and the maximum expected accuracy of the method itself.