A novel analytical model for the transit time distributions in urban groundwater systems

Abstract

With the ongoing rapid urbanization across the globe, its interference with groundwater resources is critical to freshwater sustainability. The groundwater transit time distribution (TTD) lumps the flow and transport processes of a regional groundwater system and therefore characterizes the aquifer’s resilience to nonpoint-source contamination. However, the influence of large-scale urban areas on the regional groundwater TTD is not clear. This study proposed a novel analytical model for groundwater TTDs accounting for the effects of impervious urban structures. After the verification against results from particle tracking, we apply this analytical expression to investigate how the position and spatial extent of the urban area change the TTDs from the pre-urban ones. The sensitivity analysis suggests that urban areas tend to increase both the mean and the variance of groundwater transit times. Moreover, modeling results highlight the importance of the spatial relationship between the urban area and the aquifer in determining the urban groundwater TTDs. For aquifers intersected by a local urban area, mean transit time (MTT) is dominated by the horizontal extent of the urban area (i.e., the impervious area), whereas for aquifers intersected by a regional urban area, MTT is strongly controlled by the vertical extent of the urban area in addition to its horizontal size. Being computationally efficient, the proposed analytical model can aid decision-making in urban freshwater resources management and urban planning.