ASSESSING EXFILTRATION FROM SEWERS WITH DYNAMIC ANALYSIS OF TRACER EXPERIMENTS Jörg Rieckermann 1 , Vojtěch Bareš 2 , Daniel Braun 3 , Oliver Kracht 1 , Valentina Prigiobbe 4 , Willi Gujer 1 Abstract A direct approach to quantify exfiltration from sewers is the QUEST-C tracer method. In this study, the authors present a novel approach for the dynamic analysis of a tracer experiment, considering information on varying sewer flow. An assessment of uncertainty is developed that accounts for systematic and random errors in the measurements and the sampling scheme. It is shown that the precision of the exfiltration measurement with tracers can be significantly improved by the dynamic analysis. Keywords: Exfiltration, sewers, tracer, QUEST-C Introduction If sewer leaks are situated below the groundwater table, clean ground water may infiltrate sewer pipes; if the leak lies above the groundwater table, raw sewage may exfiltrate into the surrounding soil. In order to assess the magnitude of exfiltration, several attempts have been undertaken to develop measurement methods in the last decades. Indirect methods try to deduct information on exfiltration from groundwater monitoring or a catchment-wide water balance. Direct measurements perform pressure testing on cracks or use tracer substances to identify leakage. Vollertsen et al. (2002) conclude: “The indirect methods for determination of the magnitude of the exfiltration are assessed to be based on such large a number of assumptions that they are considered non-conclusive. The direct determinations are based on measurement of very small differences in flow and are consequently difficult to perform with precision.” However, no detailed analysis of uncertainty of the computed exfiltration is reported in literature. In this paper, the QUEST-C tracer method is presented together with experimental results. As the traditional assumption of steady discharge during the experiment is rather weak, a novel approach for the analysis of QUEST-C experiments is presented that accounts for dynamic flow. The quantification of exfiltration from sewers with tracers The basic principle of exfiltration measurements with tracers is to dose a well-known amount of tracer to the sewer under investigation and to apply a mass balance on the investigation reach. 1 Swiss Federal Institute of Aquatic Science and Technology (EAWAG) ) and Swiss Federal Institute of Technology (ETH) 8600 Dübendorf, Switzerland, e-mail: willi.gujer@eawag.ch 2 Czech Technical University in Prague (CTU), Laboratory of Ecological Risks in Urban Drainage(LERMO), Thakurova 6, Prague 6, 166 29 3 Swiss Federal Institute of Technology (ETH), 8093 Zürich, Switzerland 4 Università degli Studi di Roma ”Tor Vergata” Rome and IRSA CNR, Rome, Italy 1