Large scale screening of seawater intrusion risk in Europe : methodological development and pilot application along the Spanish Mediterranean coast

Seawater intrusion caused by overabstraction has become an important problem in coastal areas of the Mediterranean. Intrusion processes highly depend on complex local conditions and studies generally focus on specific problems of individual aquifers. Seawater intrusion problems have hardly been addressed at large scale due to the strong impact of local conditions and the problems related to data collection for large geographical areas. To fill this gap and to explore the potential use of readily available data sources, we developed a simple screening methodology for large scale assessment of seawater intrusion risk along the Mediterranean coast of the EU based on a two tiered assessment procedure. Tier 1 is a simple risk assessment based on the balance of groundwater recharge and water abstractions for coastal areas. A positive net groundwater recharge results in transition of the saltwater-freshwater interface to a new equilibrium state. No equilibrium exists with negative net recharge (over-abstraction) and seawater is drawn into the aquifer compensating freshwater losses. Tier 2 provides a quantitative characterization of seawater intrusion for standardized aquifers considering generalized local geological conditions: A simple analytical intrusion model calculates freshwater loss and seawater progression for specified combinations of aquifer properties, aquifer dimensions and boundary fluxes (recharge and abstractions). An unstressed quasi-natural state (recharge only) is compared with a stressed state adding the current level of abstractions. The estimation of groundwater recharge and the spatial disaggregation of national water abstraction data are still tentative and future improvement of the procedures is necessary. A pilot application was carried out for the coast of SE-Spain. As geological data did not support an assessment of individual 'true' coastal aquifers, we defined standardized aquifer domains of different size and applied the seawater intrusion model (Tier 2) to each aquifer domain using local geology and boundary fluxes. Freshwater loss and progression of the saltwater-freshwater interface illustrate the potential severity of potential intrusion processes. The approach supports screening of intrusion risk over large geographical areas based on local relation of abstraction and recharge. The methodology is principally promising, even though input data used for the pilot studies are still based on tentative approaches and need to be replaced by more detailed analysis of statistical information and modelling. Water demand and groundwater recharge as basic input data can be linked to model applications, allowing assessment of future intrusion problems in the context of scenario analysis (for example assessing changes of intrusion risk based on climate change, land use changes and changes in water demands).