Hydrogeology, aquifers and geothermal energy

Recharge and flow

Recharge is the water that enters the aquifer from the surface, mainly through rainfall and, to a lesser extent, through runoff in barrancos and zones of fractured lava. In the Canaries, recharge varies enormously between the windward side of the high islands, where trade-wind clouds bring constant moisture, and the arid eastern zones, where rainfall is scarce and irregular.[1][2]

Once infiltrated, water flows gravitationally towards areas of lower potential energy, following fractures, contacts between lava flows and the most permeable levels. The direction of flow does not always coincide with the topographic surface, because volcanic layers may dip or be truncated by faults that divert water towards the sea or into other hydrogeological units.[1][2]

Porosity and permeability

Porosity determines how much water a rock can store. In Canarian volcanic materials, porosity can be intergranular, occurring between scoria or pyroclast fragments, or fractured, opening in cracks and joints. Massive basaltic lavas have low primary porosity, but can host high permeability if fractured.[1][2]

Permeability, in turn, controls the speed at which water moves and therefore the productivity of an aquifer. Two rocks can have the same porosity but different permeability if one has connected pores and the other isolated pores. This difference explains why some areas of similar appearance produce very different yields.[1][2]

Aquifers and hydrogeological catchments

An aquifer is a geological formation capable of storing and transmitting water in exploitable quantities. In the Canaries, aquifers are found mainly in fractured basaltic lava flows, scoria debris and pyroclastic deposits. Each island may have several distinct aquifers, separated by low-permeability levels or by lithological changes.[1][2]

The hydrogeological catchment is the recharge area that feeds an aquifer. It does not always coincide with the visible catchment defined by barrancos, because groundwater can cross topographic boundaries through deep fractures. Knowing the real extent of the catchment is essential for calculating available recharge and planning extraction.[1][2]

Variation between islands

The western islands, younger and higher, have deep aquifers with relatively high recharge from rainfall on the summits and condensation of trade-wind clouds. The eastern islands, older and lower, depend on coastal aquifers and small catchments where recharge is limited and historical exploitation has lowered water tables.[1][2]

This difference explains why some islands have developed systems of horizontal galleries to capture water from inside cliffs, while others depend more on desalination and reuse. There is no single hydrogeological solution for the whole archipelago; each island requires a plan adjusted to its climate, geology and demand.[1][2]

Planning, sustainability and cautions

The Canarian hydrological plans establish the water body, recharge, permitted extraction and qualitative status of each aquifer. Their aim is to prevent overexploitation, which can cause falling water tables, reduced ecological flows and marine intrusion in coastal aquifers.[1]

Sustainability depends not only on the amount extracted, but also on water quality. Agricultural, urban or saline contamination can make an aquifer that is numerically balanced unusable for certain purposes. That is why hydrochemical monitoring is as important as measuring levels and flows.[1][2]

Overexploitation of aquifers not only reduces available resources, but can also induce marine intrusion and degrade ecosystems associated with water tables. Geothermal energy, if not managed with sustainability criteria, can locally deplete available heat and compete with other uses of groundwater. Both resources require continuous monitoring and regulation based on up-to-date data.[1][2][3]

Geothermal energy

Geothermal energy uses heat stored in rocks and groundwater. In the Canaries, the geothermal gradient is higher in areas with recent volcanic activity, where residual heat from magmatism can raise the temperature of water at accessible depths. The geothermal manuals of the Government of the Canary Islands set out the methodology for evaluating this resource.[1][2]

Low-enthalpy geothermal use, for heating and hot water, is the most developed in the archipelago. High-enthalpy projects require deep drilling and detailed knowledge of permeability and temperature at depths of several kilometres, fields in which available information remains fragmentary.[1][2]