A comparative analysis of different physically based modeling approaches to predict the fate of sewer leaks and their effects on urban aquifer recharge and contamination

Unintentional recharge by leaking sewers can have detrimental consequences on urban aquifers, like the spreading of pathogens, soil and groundwater contamination, and the occurrence of dangerous interactions with shallow and deep anthropogenic alterations. Due to the complexity of the urban subsurface, the scale of the problem, the high nonlinearity of the phenomena involved, and the computational burden, a series of simplifications are generally introduced. Through surface and subsurface flow modeling of a realistic case study, the aim of this work is to show that a wise use of powerful but conventional numerical models can allow to overcome these simplifications, thus enabling a more physically consistent prediction of the effects of sewer leaks on urban aquifer recharge and contamination. Three high resolution physically based subsurface flow modeling approaches (referred to as VWS-C, VWS-S, and WS-S) with a different level of detail are compared. Results show that modeling these phenomena is possible and feasible at large scales (with computation times in the order of hours for 2 years long simulations), even including all aspects that are typically only accounted for separately in other modeling studies. The comparative analysis also highlights how crucial it is to include the presence of the unsaturated zone and the accurate representation of pipe geometry. Neglecting these aspects can lead to inconsistent predictions of sewer leaks and contaminant plumes migration in the subsurface, and to incorrect estimations of exfiltration rates and contaminant fluxes, with inevitable consequences on the definition of potential monitoring and remedial actions of urban aquifers.