Effect of underground structures on groundwater residence time distributions in confined aquifers

Abstract

Anthropic pressure and property economics lead to the vertical development of urban space, but groundwater residence time can be influenced by the urban underground structures constructed in it. Meanwhile, groundwater residence time is critical to the renewability of aquifers and the vulnerability to contamination. The effects of underground structures on local and regional residence times for confined aquifers remain unclear. This paper presents novel analytical solutions for groundwater residence time distributions (RTDs) affected by impervious underground structures in confined aquifers under three boundary conditions (BCs), namely the fixed-flux BC, the fixed-head BC, and the diffuse-leakage BC, respectively. The analytical solutions show a good agreement with numerical results for barriers fully obstructing the aquifer. For barriers partially obstructing the aquifer, discrepancies can be found between the analytical and the numerical RTDs due to the hydrodynamic dispersion of flow pathlines. Nevertheless, the calculated mean residence time (MRT) presents a good correspondence with numerical results for all parameter realizations, indicating that analytical solutions can be used to estimate regional MRT even for three-dimensional flow fields. Global sensitivity analysis (GSA) using Sobol’ method identifies influential parameters for residence times and underlines the importance of boundary conditions in determining the MRT. Finally, two successful applications of the analytical solutions in Barcelona aquifers indicate that the modification in residence times does not correspond to the modification in hydraulic heads. Overall, the proposed analytical solutions successfully quantify the barrier effect on residence times and prove to be useful in supporting underground space exploitation, and aiding the environmental risk assessment.