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[EN] Fully 3D and multidimensional geological models are key for energy, mining and water resources assessment and management, which has a great economic impact. 3D geological models provide critical information regarding geometric properties of the geological bodies of interest, such as ore deposits or reservoirs, allowing inference of volumes and availability to be exploited. However, defining a useful methodology to create a fully digital 3D geological model of volcanic islands is still very challenging due to the intrinsic complexity observed in the structure of volcanic systems. Two-dimensional geological maps, associated vertical geological cross-sectional diagrams and borehole and water-mining tunnels (galleries) logs were examined on El Hierro Island. A geological modelling procedure using the software GeoModeller obtained the first 3D geological model of El Hierro volcanic island, as a basis for volcanic formation definitions and geometric analysis. Surface, volume and center of mass estimations of the differentiated sub-units allowed a better understanding of the spatial distribution and evolution of the volcanic cycles of El Hierro Island over time. The 3D model provided geometry information correlatable with volcanic activity cycles and lava/pyroclast deposition mechanisms that ultimately defines the geometry of the volcanic units. This information provides new variables valuable for the investigation of the existing relationships between rift zones and giant lateral collapses. In addition, the obtained 3D geological model of a volcanic island will support the building of the first hydrogeological and geothermal model of El Hierro Island, allowing advancement of sustainability of isolated systems.

[EN] 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.

[EN] Contaminated groundwater is a serious problem in developed countries. The abandonment of industrial waste may lead to acid drainage affecting groundwater and severely impacting the environment and urban infrastructure. We examined the hydrogeology and hydrochemistry of an urban area in Almozara (Zaragoza, Spain); built over an old industrial zone, with pyrite roasting waste deposits, there were acid drainage problems in underground car parks. Drilling and piezometer construction, and groundwater samples revealed the existence of a perched aquifer within old sulfide mill tailings, where the building basements interrupted groundwater flow, leading to a water stagnation zone that reached extreme acidity values (pH < 2). A groundwater flow reactive transport model was developed using PHAST to reproduce flow and groundwater chemistry, in order to be used as a predictive tool for guiding remediation actions. The model reproduced the measured groundwater chemistry by simulating the kinetically controlled pyrite and portlandite dissolution. The model predicts that an extreme acidity front (pH < 2), coincident with the Fe (III) pyrite oxidation mechanism taking dominance, is propagating by 30 m/year if constant flow is assumed. The incomplete dissolution of residual pyrite (up to 18 % dissolved) predicted by the model indicates that the acid drainage is limited by the flow regime rather than sulfide availability. The installation of additional water collectors between the recharge source and the stagnation zone has been proposed, together with periodic pumping of the stagnation zone. The study findings are expected to serve as a useful background for the assessment of acid drainage in urban areas, since urbanization of old industrial land is rapidly increasing worldwide.