Conceptual hydrogeological models of oceanic intraplate volcanic small islands

The origin of oceanic intraplate volcanic islands is attributed to hotspots rising from the lower Earth mantle. They evolve following patterns that are now known from the geological and oceanographic studies of different archipelagos and consist on submarine volcanics topped by subaerial effusions. They form some of the highest Earth's mountains, up to some thousand metres from the sea bottom to the top. Well-studied Archipelagos, as Hawaii and the Canary Islands, have provided geological models that can be applied to other islands and archipelagos, like Madeira, Cabo Verde, Reunion, Mauricius, Galapagos, Easter or Azores. Each island has its peculiarities, depending on the geodynamic setting, tectonic activity, age and degree of erosion. Groundwater is often the most important water resource, so the understanding of the hydrogeological behavior is the key for good management of water resources. The specific hydrogeological characteristics of the volcanic materials depend on several variables. Lithology is dominantly basaltic in most of the islands in its first stages of growth but differentiation to phonolitic and even rhyolitic materials can occur in late stages. The emplacement mechanisms condition if the pilled up rocks consists on lava flows (effusive eruptions) or pyroclastic materials (explosive eruptions). The geological structures, like the existence of calderas, dykes or interlayered sediments determine in great manner groundwater flow. The porosity and/or permeability of the aquifers highly decrease with the age due to compaction and endogenic effects. Several hydrogeological conceptual models have been proposed for different oceanic islands, such as the Hawaiian model and the Canarian model and more recently the models for Galapagos Islands, although not always results are well-supported by observations, hydrodynamics and natural tracers. Some numerical models have been implemented in the coastal areas of Hawaii and the Canary Islands as well for the whole island. Results are scale-dependent. All the conceptual models must consider the variables described above plus the climatology of the islands, which strongly conditions aquifer recharge. This recharge is a difficult to evaluate variable that generally cannot be calibrated due to the unknown, dominant discharge to the sea along the coast. The groundwater behaviour changes with groundwater exploitation. Exploitation has to be known, not only to close the water balance but because the results help in understanding the flow pattern. The hydrogeochemistry and environmental isotope studies can be important tools to define the conceptual model. There are so many variables that influence groundwater behaviour in volcanic intraplate oceanic islands that it is not recommendable to fit the data of a given island to the assumed model of other island or part of it. The real model consists simply in applying the general principles of island genesis and postdepositional effects, with the support of good monitoring data and careful observations.