Monitoring groundwater infiltration and gas dynamics in the sewer with the portable mass spectrometer "MiniRuedi"
Creators: name: Zhu, Mengqi
Publisher: name: ETH Zurich
Year: 2022
Subjects: subject: Noble gas
subject: portable mass spectrometer
subject: groundwater infiltration
subject: Sewer
subject: Groundwater
subject: info:eu-repo/classification/ddc/620
subject: info:eu-repo/classification/ddc/500
subject: Engineering &amp; allied operations
subject: Natural sciences
subject: FOS: Natural sciences

description: The extraneous inflows and infiltration, particularly groundwater infiltration, have long been regarded as a primary problem of urban sewer systems. The detection and quantification of groundwater infiltration is the first and crucial step towards the more efficient, cost effective and sustainable sewer rehabilitation and management. The tracer based methods are a promising option to directly differentiate and quantify various water sources in the sewer systems. However, due to the harsh environment of the sewer systems, it is generally difficult to find the appropriate tracers and the corresponding analytical methods that can avoid the reliance of the labor-intensive and time-consuming lab-based techniques.

In this thesis, a new tracer method for the groundwater infiltration quantification was proposed and investigated, using the noble gases as tracers coupled with a recent developed portable mass spectrometric system called miniRuedi. The mechanistic modelling was first implemented to better understand the gas-related processes in the sewers and provide guidance for the field experiments. A case study with the field monitoring campaign was then set up in the sewer system of the municipality of Dübendorf (CH) in Dec 2021 to Jan 2022 and Mar 2022. The noble gases were quasi-continuously sampled and analysed on site by miniRuedi from two locations: i) the headspace of a trunk sewer subjected to sewer air transport and ii) the headspace of a pump sump in the experimental hall of Eawag, which is diverting a side-flow of wastewater from the sewer. The local groundwater infiltration was quantified by using several assumptions and a lumped parameter gas loss factor. The accuracy and applicability of the proposed method were analysed via sensitivity and uncertainties analysis. Further, an exploratory data analysis was conducted to extract the information regarding the potential drivers of the sewer gas dynamics in the context of groundwater infiltration. Finally, the general potential of applying the miniRuedi in the urban drainage networks was assessed.

The results of mechanistic modelling showed that the steady state mass balance could be a good approximation to the complete dynamic mass balance. The noble gases signals in terms of concentrations were greatly subject to the sewer air transport and ventilation. In the case study, the groundwater infiltration ratio estimated by noble gases tracer method are around 40 to 80%, and the 24h integrated groundwater infiltration ratio is 57.7% (± 2.7%). These values are in similar range as "night-minimum" estimates, though not completely aligned in terms of temporal dynamics. The accuracy of the estimation results from noble gases tracer method greatly depends on the uncertainties in water/noble gases sampling and analysis with miniRuedi. The presented groundwater infiltration ratio might be systematically biased due to the strong assumption regarding the gas exchange and corresponding gas loss factor. The knowledge foundation in the fields of gas-related processes in the sewers are generally lacking.

Overall, despite the noble gases tracer method for groundwater infiltration quantification suffering from uncertainties and potential systematic bias, the miniRuedi per se still shows great potentials in investigating other sewer-related processes, such as oxygen turnovers, in-sewer activities, and sewer air transport and ventilation.