Dr. Chaojie LI - PhD Defense - August 25, 2023

© 2023 EPFL

© 2023 EPFL

Professor Tamar Kohn, thesis director, and the whole LCE'team, congratulate Dr. Chaojie Li for his thesis on "Numerical simulation of the fate, transport and public health risks of enteric viruses in Lake Geneva".


Lakes are receiving water bodies of wastewater effluents and sewage overflows that can contain various types of pathogens. These pathogens travel with the lake current and may enter recreational water zones and infect people who swim or conduct other activities near the shore. Human viruses are among the most ubiquitous waterborne pathogens. This thesis investigates the fate, transport and infection risk of waterborne viruses by employing a coupled hydrodynamic, water quality and quantitative microbial risk assessment model, using Lake Geneva as a study site.
The first step towards a risk assessment scheme for waterborne viruses in lakes is to establish and validate a water quality model. In this thesis we demonstrate that remote sensing (satellite) data can be used to inform water quality models for a large lake. We used total suspended matter (TSM) as a parameter that can be both estimated from the backscattering in satellite images and modeled in terms of particle abundance. Specifically, we compared TSM concentrations in Lake Geneva deduced from images taken by Sentinel-2 and Sentinel-3 satellites to those estimated from Delft3D hydrodynamic and particle tracking models. The results demonstrate that the model was able to capture both the position of a TSM cloud arising five days after an instantaneous point source release and the direction of particle transport and TSM plume size resulting from a continuous source. When simulating the whole lake domain, model results closely approximated the satellite-derived TSM concentrations along lake transects within 9%. In return, the particle tracking model was able to complete partially impaired satellite images and fill in a four-day image gap between satellite revisits. The synergy of remote sensing techniques and particle tracking modeling allows a rapid, continuous and more accurate analysis of solute transport in lakes.
Solving the transport of waterborne viruses in lakes would not inform individuals of the risks posed by the pathogens. A link between the virus concentration and infection risk must be established. In this thesis, we propose to use a coupled water quality and quantitative microbial risk assessment (QMRA) model to study the transport, fate and infection risk of four common waterborne viruses (adenovirus, enterovirus, norovirus and rotavirus) in Lake Geneva. The measured virus load in raw sewage entering the lake was used as the source term in the water quality simulations for a hypothetical scenario of
discharging raw wastewater at the lake surface. After discharging into the lake, virus inactivation was modeled as a function of water temperature and solar irradiance that varied both spatially and temporally during transport throughout the lake. Finally, the probability of infection while swimming at a popular beach was quantified and compared among the four viruses. Norovirus was found to be the most abundant virus that caused an infection probability that is at least 10 times greater than that of the other viruses studied. Furthermore, environmental inactivation was found to be an essential determinant in the infection risks posed by viruses to recreational water users. We determined that infection risks by enterovirus could be up to 1000 times, and by rotavirus up to 50 times lower when virus inactivation by environmental stressors was accounted for compared with the scenarios considering hydrodynamic transport only. In addition, the model highlighted the role of the wind field in conveying the contamination plume and hence in determining the infection probability. Our simulations revealed that for beaches located west of the sewage discharge, the infection probability under eastward wind was 43% lower than that under westward wind conditions.
Climate change influences lake hydrodynamics, temperature and ground radiation levels and thus affects the fate and transport of waterborne pathogens in lakes. We compared virus concentrations and associated public health risks for 2019 and 2060, using the above mentioned risk assessment scheme. Long-term hydrodynamic simulations suggested that although the annual hydrodynamic transport pattern of Lake Geneva will remain relatively stable, a 1.9 °C increase in the lake surface water temperature can be expected by 2060, while a slight decrease in lake current velocity may occur. The subsequent effects on the fate and transport of the four enteric viruses of concern varied by season. Increased temperature and solar radiation in warm seasons could compensate for the additional sewage-borne virus load brought about by population growth. In cold seasons, this offset is unlikely to happen, resulting in infection risks that increase proportionally to the increase in population and hence the virus load discharged into the lake. In addition, current estimates of environmental virus inactivation rate are highly variable, and this variability was found to have a larger impact on enteric virus concentrations in the lake than changing environmental conditions. A more accurate
estimation of the environmental inactivation of viruses is essential in predicting the fate of enteric viruses in aquatic systems.
Although wastewater treatment reduces viral concentrations, incomplete removal may pose health threats to people who are exposed to the water receiving wastewater effluent. However, frequent and long-term environmental surveillance on waterborne viruses is lacking, because less attention was drawn on viruses rather than other pollutants and due to the limitations in virus detection technologies. Consequently, very few studies have addressed the characteristics and features of enteric virus concentrations in sewage in different places and the prediction of virus concentrations based on measurement data. Here, we collected enteric virus concentration measurements in eight WWTPs in three different countries, to characterize the virus concentrations and fit the measurement data with different mathematical distributions, while considering the effect of data correction by normalization to internal standards and the influence of sample down-scaling. It was found that the Poisson lognormal distribution led to the best fit for most viruses detected in most wastewater treatment plants. Accounting for the recovery efficiency of the laboratory measurements of enteric pathogens by using an estimate of recovery based on spiking an external surrogate virus led to different results depending on the distribution and quality of the recovery data. Thus, recovery test results need to be carefully examined before being employed for the correction of enteric virus measurement. Sampling frequencies have a noticeable impact on distribution fitting for virus concentration measurement, high measurement frequencies could largely reduce the uncertainties in predicting mean virus concentrations and virus concentrations at low exceedance probabilities.
The work in this thesis highlighted the potential of combining water quality simulation and virus-specific risk assessment for a safe water resources usage and management and revealed the key components for a more accurate estimation of the fate and transport of waterborne viruses in lakes in the future.

Key words: water quality simulation; hydrodynamic simulation; waterborne viruses; risk assessment; human health; climate change; remote sensing; Lake Geneva; sewage characterization