Quantification of sewer leakage by a continuous tracer method V. Prigiobbe and M. Giulianelli ABSTRACT Water authorities interested in the evaluation of the structural state of a sewer must quantify leakage to plan strategic intervention. However, the quantification of the exfiltration and the localisation of structural damage are challenging tasks that usually require expensive and time-consuming inspections. Herein, we report one of the first applications of the QUEST-C method to quantify the exfiltration in a continuously operating sewer by dosing two chemical tracers, sodium bromide (NaBr) and lithium chloride (LiCl). The method was applied at the catchment scale in a 14-year-old sewer in Rome, Italy. Preliminary laboratory tests, field measurements, and numerical simulations showed that reliable results require the QUEST-C method to be applied to sewers without lateral inflows, during periods of quasi-steady flow, and that the travel time of the NaBr tracer is minimised. Three sewer reaches were tested and the estimated exfiltration, as a fraction of the dry weather flow (DWF), increased from 0.128 in the agricultural area to 0.208 in the urban area. Although our estimates are at the lower end of the range given in the literature (0.01–0.56 DWF), the exfiltration was not negligible, and interventions should focus on the sewers in urban areas. This illustrates the capability of the QUEST-C method to guide strategic intervention at low cost and without an interruption of sewer operation. However, careful interpretation of the results is recommended for sewers with many lateral inflows, where leakage may be overestimated. V. Prigiobbe* (corresponding author) M. Giulianelli Water Research Institute, Via Salaria km 29,300, 00015 Rome, Italy E-mail: giulianelli@irsa.cnr.it *Current address: Petroleum and Geosystems Engineering, University of Texas at Austin, 1, University Station C0300, Austin, TX 78712-0228, USA Key words | exfiltration, leakage, rehabilitation, sewer, strategic intervention, urban drainage INTRODUCTION Sewer systems may experience major deterioration of infra- structure due to inadequate and insufficient maintenance and rehabilitation, allowing sewage to exit these systems and contaminate the adjacent ground and surface waters with high levels of pathogenic microorganisms, oxygen- demanding organic compounds, oil, and other pollutants, such as endocrine disrupters (Barrett et al. ). Studies at the catchment scale suggest that the fraction of sewer flow leaking into the environment by exfiltration can range from 1% to 56% of the dry weather flow (DWF) of the sewer (Rutsch et al. ). The impact of exfiltration is largest during the DWF period, when the natural recharge of groundwater is low in comparison with sewer leakage. In cities served by ageing sewer systems, concerns about sewer leakage are increasing. To mitigate exfiltration impacts effectively, strategic interventions to rehabilitate damaged sewers must be considered and should be based on reliable leakage quantification and risk assessment. Quantification methods can be classified into two categories (Rutsch et al. ): 1. Indirect methods used at the catchment scale to evaluate the impact of exfiltration based on groundwater monitor- ing for water and solute balances. 2. Direct methods applied at the pipe and reach scale that focus on sewer damage, such as pressure tests and tracer mass balances. The indirect methods are based on transport modelling of the sewage contaminants in the groundwater and must be validated by an extensive experimental dataset of groundwater velocities and compositions. These methods have been applied in several cases to monitor various indi- cators of sewer contamination such as stable nitrogen 132 © IWA Publishing 2011 Water Science & Technology | 64.1 | 2011 doi: 10.2166/wst.2011.639