Assessing exfiltration from an urban sewer by slug dosing a chemical tracer (NaCl)

Water resources in urban areas comprise both natural and man-made water bodies, including surface waters and groundwater. Groundwater aquifers can be recharged from several sources, some of which may transport high concentrations of organic and inorganic pollutants. Such sources include exfiltration from leaky sewer pipes that could pose a serious threat to groundwater quality because of large extent of urban drainage systems. When the shallow unconfined and confined aquifers are connected, the transport of organic and inorganic compounds and pathogenic organisms may cause serious pollution. Consequently, the deep confined groundwater aquifers could become polluted and their water would require advanced treatment if used as a source of drinking water. Several methods exist to evaluate the structural state of sewers. Some of these consist in direct surveying inside the sewer pipes (i.e., by closed circuit television, CCTV) and others in quantifying the exfiltration rates by the detection of wastewater markers in the groundwater [1]. The QUEST method [2] (QUantification of Exfiltration from Sewer with artificial Tracers) serves to assess directly the exfiltration from flowing sewers in dry weather. It is based on establishing a tracer mass balance for the investigated pipes. The solutions of a tracer (NaCl) are dosed in two manholes of the investigated reach, and at a downstream location, the conductivity of sewage is measured by in-line probes. The sources of errors affecting the exfiltration rate originate from the experimental results and data analysis. In particular, they are due to: flow rate, natural wastewater conductivity, shape of the tracer signals at the measuring point, transport of tracer and general disturbances in the sewer (caused for example by turbulence or solids). To minimise the errors in experiments and data analysis, preliminary measurements of flow rate, in-sewer background conductivity and tracer transport should be carried out. In the paper that follows, the results of application of the QUEST method to an urban sewer network in a suburban area of Rome and the importance of site-specific preliminary tests are presented.