Modeling the spatial and temporal distribution of coastal groundwater discharge for different water use scenarios under epistemic uncertainty: case study in South Portugal

The following paper presents a discussion of spatial and temporal distribution of groundwater discharge estimates at a regional scale for two coastal aquifer systems in the Algarve region. A finite element distributed parameter groundwater flow model is applied to analyze the effect of uncertainties regarding definition of model boundary conditions and seasonality on estimated values. Finally, estimates of sustainable yields are provided based on predefined sustainability criteria and the importance of well location is analyzed. Steady-state simulations indicate a range of average annual coastal discharge between 6.5 × 10⁶ and 15 × 10⁶ m³/year from the Albufeira-Ribeira de Quarteira aquifer, and 4.5 × 10⁶ and 11.3 × 10⁶m³/year from the Quarteira aquifer, as well as significant spatial variation. Transient simulations show that seasonal variability inherent to these systems can lead to inversions of hydraulic head gradient during short periods. Model results indicate that coastal discharge rates are between 1.5 and 2 times higher during the peak winter months than during the minimum in the summer, and as such seasonality has a larger impact on discharge rates than BC conceptualization. Up to 3.31 × 10⁶ m³/year could be abstracted from existing well fields without causing seawater intrusion problems. Historical levels of abstraction are not within the selected sustainability criteria. However, by placing abstraction further from the coast sustainable yield increases. This work is part of ongoing research that aims to identify and characterize groundwater flow from the coastal strip towards the continental platform, taking into account structural geology, marine geology and the effects of the hydrological/hydrogeologic conditions on the associated ecosystems.