High‐resolution conceptual hydrogeological model of complex basaltic volcanic islands: a Mayotte, Comoros, case study

Abstract

International audienceWe present a high-resolution conceptual hydrogeological model for complex basaltic volcanic islands based on Mayotte Island in the Comoros. Its geological structure and hydrogeological functioning are deduced from a large dataset: geological mapping, geophysics, some forty new boreholes, piezometric data, hydraulic conductivity, hydrochemical data, etc. We describe previously unknown deep cut-and-fill palaeovalleys. The resulting conceptual geological and hydrogeological model of the island is very different from the Hawaiian model, in that it lacks a low-elevation basal aquifer and dyke-impounded high-level aquifers. It is closer to the Canary Islands model, which has, however, not yet been described at a high-resolution scale. It does not have a continuous aquifer, but rather a discontinuous succession of perched aquifers separated by aquicludes and aquitards. This results more from the complex geological structure of the island, which has experienced several phases of volcanism, erosion and weathering, than from its age, but is also a result of the high-resolution scale of the model. High-resolution conceptual modelling is now necessary to solve problems of applied geology and hydrogeology

Evidence for tectonic, lithologic, and thermal controls on fracture system geometries in an andesitic high‐temperature geothermal field

“…Geometrical models in andesite‐hosted geothermal reservoirs can then support flow model development in reservoirs [ Berkowitz , ]. Findings from the Rotokawa reservoir also inform on fracture systems in andesitic volcanoes [ Lachassagne et al , ; Conway et al , ] or epithermal deposits [ Brathwaite et al , ] which form two possible end‐members of the evolution of andesitic formations.…”

“…Non sedimentary rocks are increasingly recognized as reservoirs able to produce fluids. Volcanic formations host geothermal [ Arnórsson , ; Nemčok et al , ; Wilson and Rowland , ], groundwater [ Lachassagne et al , ], hydrocarbon [ Feng , ], and mineral [ Spörli and Cargill , ] resources, while also being capable of storing unwanted CO 2 [ Pollyea et al , ; Matter et al , ] and radioactive waste [ Chen et al , ], and capable of supporting surface engineering projects [ Alemdag , ]. The geometry of vesicles and fractures in volcanic units has been extensively studied in outcrops [e.g., DeGraff and Aydin , ; Gudmundsson , ; Conway et al , ] but can be difficult to quantify in reservoirs due to sparse core and wireline logging measurements.…”

“…Non sedimentary rocks are increasingly recognized as reservoirs able to produce fluids. Volcanic formations host geothermal [Arnórsson, 1995;Nemčok et al, 2004;Wilson and Rowland, 2016], groundwater [Lachassagne et al, 2014], hydrocarbon [Feng, 2008], and mineral [Spörli and Cargill, 2011] resources, while also being capable of storing unwanted CO 2 [Pollyea et al, 2014;Matter et al, 2016] and radioactive waste [Chen et al, 2001], and capable of supporting surface engineering projects [Alemdag, 2015]. The geometry of vesicles…”

Contaminant transfer and hydrodispersiveparameters in basaltic lava flows: artificial tracertest and implications for long-term management

“…Laboratory experiments showed that hydraulic conductivity K may vary in a range com-prised between 10 −1 and 10 −15 m/s with some basalts presenting 10 ranges of magnitudes in the same formation [21] with strong variability on the vertical axis due to interflow spaces and horizontal fractures [22]. Consequently, groundwater pathways, storage and discharge may be very variable, as was pointed out in numerous systems such as the Golan heights in Israel [5], India [3], Reunion Island [2,23,24], Mayotte Island in the Comoros archipelago [1,12], Madeira Island [25] Galapagos Islands [8]. These systems may behave as fine grained systems with poorly variable discharge [9,26,27] associated with slow groundwater transit.…”

“…Exploitation of volcanic groundwater is vital in many locations around the world [1][2][3][4][5][6][7][8][9][10][11][12] therefore management and international policy (e.g., [13]) implementation of resource and related ecosystem protection [14][15][16][17] require the assessment of volcanic aquifer vulnerability to pollution in the context of local and global anthropogenic pressures.…”