Effects of Railway and Port Infrastructures on the Quantitative and Qualitative State of an Urban Coastal Aquifer

Currently, approximately 40% of the world’s population is concentrated in coastal cities, and this figure is expected to continue increasing. In this context of demographic concentration in maritime cities, coastal aquifers constitute strategic water resources, particularly in arid or semi-arid regions and especially during drought periods. These aquifers are subjected to various anthropogenic and climatic pressures that affect the quantitative and qualitative state of their water resources. Among the anthropogenic actions, the increasing construction of underground infrastructure, such as tunnels for rail networks, stands out for its hydrogeological impact. Many of these structures are built between retaining walls and/or incorporate drainage systems that distort the natural flow network of the aquifer, while simultaneously reducing its resources. Additionally, the construction of inner docks involves a displacement of the coastline further inland. The combined effects of these actions on a coastal aquifer can exacerbate the advancement of saline intrusion, making it essential to quantify these impacts.

This study evaluates the combined quantitative and qualitative cumulative impact of infrastructure tunnels and an inner dock on the main aquifer of the Llobregat Delta (Spain) over the period 1966–2024. The conceptual and geological model of the aquifer was reviewed, followed by the construction and calibration of a 3D variable-density flow and chloride transport model in MODFLOW 6. The model discretization was designed to accurately reproduce the real geometry of the tunnels, their retaining walls, and the geological units.

Two simulations were conducted: one representing the current state with infrastructure and another reflecting a potential state without these structures. Differences were calculated between the mass balance, chloride concentration maps, and piezometric level maps of both scenarios. Preliminary results indicate that the construction of the dock in a geologically unfavorable area, combined with the piezometric depression caused by a high density of tunnel and basement drainage systems, were determining factors in the rapid salinization and high salinity levels of the western hemidelta.

The contribution of this study is a methodology for quantifying these effects in other coastal aquifers, while highlighting the importance of geological knowledge, the implementation of best construction practices, and the strategic location of such infrastructure to preserve the water resources of an urban coastal aquifer.