Dyke-impounded fresh groundwater resources in coastal and island volcanic aquifers: Learning from the Canary Islands (Spain)

Freshwater in coastal and island aquifers is a valuable resource whose availability is strongly conditioned by
heterogeneity. More than 80 % of the Earth’s surface is of volcanic origin, but the effect of volcanic dykes on the
geometry of the saline interface that separates freshwater from seawater is still underexplored. This paper analyzes
the impact of volcanic dykes on the depth of the saline interface in coastal and island aquifers and,
subsequently, on the availability of fresh groundwater. Hydrogeological and hydrochemical data from a gallery,
perpendicularly crossing several tens of dykes, were integrated with numerical modeling on the volcanic island
of El Hierro (Canary Islands, Spain). Measured hydraulic heads demonstrated that the presence of dykes increased
the hydraulic gradient by more than an order of magnitude, with respect to an adjacent area not affected by
dykes. Numerical assessment confirmed that the lower the hydraulic conductivity of the dykes, the greater the
depth of the saline interface inland. This impact led to fresh groundwater reserves increasing inland, relative to a
hypothetical case without dykes. Numerical simulations also demonstrated that dykes can prevent salinization of
production wells in coastal and island aquifers, if they are correctly located. Locating production wells far
enough inland in an area affected by dykes allowed a higher freshwater extraction rate than if dykes did not exist;
near the coastline, the effect tended to be the opposite. These results will be key to improving the management of
fresh groundwater resources in coastal volcanic aquifers, and especially on volcanic islands such as the Hawaiian
Islands or the Macaronesian archipelagos.