The future scenario of global climate change brings a major challenge for the groundwater resources sustainability. The evolution of groundwater in the coming years forecasts a progressive decline of the water table and a decrease in the streamflow, which implies a reduction of the water reservoir storage. This will have negative consequences not only on the associated natural ecosystems, but also on the resources focusing on water supply, among others.
Underground hydrological resources considered in river basin management plans include those belonging to aquifers that have a link with natural ecosystems, and those associated with aquifers that support the main supplies. The resources under the usual withdrawal depths are not taken into account because they are considered as non-renewable resources, in addition to the pumping cost or its assumed high salinity. In the case of surface aquifers with several hundred meters thickness, the use of groundwater resources is usually limited to the upper part of the saturated zone for the same reasons.
Therefore, under of this non stablished depth –variable according to the hydrogeological structure and the climatic and economic local factors– there exists an amount of unexploited groundwater resources, mostly unknown or not yet evaluated. Investigation of deep aquifers is extremely complex and has a high economic cost. Aspects like deep aquifer geometry, hydrogeological and hydrochemical characterization and resources estimation, need complex and specific techniques different from those used in the study of shallow aquifers.
There is a growing interest in the knowledge of deep aquifers because of the climate change context. In the present study, deep aquifers have been considered as deep groundwater reservoirs, regardless the quality or the hydraulic aquifer properties. The main goal is to define some aquifers –or geologic formations– like strategic reservoirs for human use. An adequate characterization of a deep aquifer could be integrated in the groundwater resources management and help to reduce the scarcity in extreme situations.
The Geological Survey of Spain –IGME– has performed the work of improving the knowledge and characterization of deep aquifers in this project. As a starting point, an inventory of deep aquifers has been carried out in the main river basins of Spain –Iberian Peninsula area– under specific selected criteria. The main characteristics have allowed a classification of Spanish deep aquifers. Some examples, representative from different typologies, have been selected for a detailed study as a “deep aquifer” with a proper methodology.