Significance of the Coastal Shelf Shape on the Saline Intrusion

Abstract:

A large number of people live in coastal areas using the available water resources, which in (semi-)arid regions are often taken from groundwater resources. Compared to surface water, these usually provide a safe water supply due to the remediation and retention capabilities of the subsurface, their high yield, and potentially longer term stability. With a water withdrawal from a coastal aquifer, coastal water management, however, has to ensure that seawater intrusion is retained in order to keep the water salinity at an acceptable level for all water users (e.g. agriculture, industry, households).

Besides monitoring of water levels and saline intrusion, it has become a common practice to use numerical modeling for evaluating the coastal water resources and projecting future scenarios. When applying a model, it is necessary for the simplifications implied during the conceptualization of the setup to include the relevant processes (here variable-density flow and mass transport) and sensitive parameters (for a steady state commonly hydraulic conductivity, density ratio, dispersivity).

Additionally, the model’s boundary conditions are essential to the simulation results. In order to reduce the number of elements, and thus, the computational burden, one simplification that is made in most regional scale saltwater intrusion applications, is to represent the shelf with a vertical boundary, contrary to the natural conditions, that usually show a very shallow decent of the interface between the aquifer and the open sea water.

We use the scientific open-source modeling toolbox OpenGeoSys [1] to quantify the influence of this simplification on the saline intrusion. Using an ensemble of different shelf shapes for a steady state setup, we identified a significant dependency of saline intrusion length on the geometric parameters of the shelf. With these simplified setups (homogeneous aquifer, steady state), the drawn conclusions provide valuable insight for real-world applications that potentially overestimate the saline intrusion length.

[1] Kolditz, O., et al. (2012). OpenGeoSys: an open-source initiative for numerical simulation of thermo-hydro-mechanical/chemical (THM/C) processes in porous media. Environmental Earth Sciences, 67(2), 589–599. doi:10.1007/s12665-012-1546-x