Fate in soil of murine Norovirus : used as a surrogate for human norovirus present in wastewater reused in agricultural irrigation

Urban treated wastewaters may be heavily contaminated by human pathogenic enteric viruses that cause acute gastroenteritis, despite their treatment. In order to compare health risks of different wastewater management scenarios, we investigated (i) how to predict virus shedding from epidemiological data on acute gastroenteritis, (ii) the environmental fate of these viruses when wastewaters are discharged into rivers, and (iii) the fate of these viruses when they are brought to the soil by irrigation. Our study focused on a wastewater collection basin of 240,000 inhabitants near Clermont-Ferrand, on the Artiere and Allier Rivers and the Allier alluvial groundwater potentially contaminated by wastewater discharges, and on a soil in the irrigation perimeter reusing these wastewaters. Concentrations of various viruses were monitored over the same period in raw and treated wastewaters, as well as in surface and underground freshwaters. We proposed a method based on epidemiological data to estimate the daily number of new cases of acute gastroenteritis of viral etiology in 2015-2016; and we combined these estimates with a viral shedding model to estimate the quantities of enteric viruses arriving at the treatment plant. The fate of viruses has been simulated by taking into account the removal of viruses in the treatment plant and dilution-mixing in rivers. The fate of viruses brought to the soil by treated wastewater reused in agricultural irrigation was studied on a well-represented soil in the perimeter using a surrogate virus. Its fate has been described by a model combining transfer, reversible immobilization and removal; the model distinguished between mobile and immobile waters, as well as between free viruses and viruses adsorbed on colloids in suspension. The method for switching from epidemiology to virus shedding allowed us to accurately simulate virus inflows to the treatment plant, including a winter peak and the prominent role of norovirus GII in viral gastroenteritis cases. The simulation of virus removal in the treatment plant and subsequent dilution-mixing phenomena in rivers allow correctly simulating the viral load in the river downstream of the wastewater discharge, but their subsequent fate remains poorly characterized. When brought to the soil, the surrogate virus was progressively removed or irreversibly immobilized, according to a 0.38 log10 daily removal. The reversibly immobilized fraction could be estimated by a Freundlich isotherm. The addition of Mg2+ favored the immobilization of viruses, as well as their adsorption on colloids dispersed in mobile water. While sterilized wastewater had no major effect on virus immobilization compared to artificial soil solution due to the antagonistic effects of their organic compounds and mineral cations, groundwater rich in Mg2+ favored immobilization of viruses. An additional work, complementary to this PhD, showed the impact of irrigations on vegetable surface and internalized contaminations. After improvement, our study could be coupled with a quantitative viral risks assessment.Key words: enteric virus, sewage, discharge, reuse, irrigation, environmental fate, scenarios, assessment,