Hydrogeological conceptual model of andesitic watersheds revealed by high-resolution heliborne geophysics

Abstract

Abstract. We conducted a multidisciplinary study at the watershed scale of an andesitic-type volcanic island in order to better characterize the hydrogeological functioning of aquifers and to better evaluate groundwater resources. A heliborne Time Domain ElectroMagnetic (TDEM) survey was conducted over Martinique in order to investigate underground volcanic structures and lithology, characterized by high lateral and vertical geological variability and resulting in a very high heterogeneity of their hydrogeological characteristics. Correlations were made on three adjacent watersheds between resistivity data along flight lines and geological and hydrogeological data from 51 boreholes and 24 springs, showing that the younger the formations, the higher their resistivity. Correlation between resistivity, geology and transmissivity data of three aquifers is attested: within the interval 10–100 ohm m and within a range of 1 to 5.5 Ma, the older the formation, the lower its resistivity, and the older the formation, the higher its transmissivity. Moreover, we demonstrate that the main geological structures lead to preferential flow circulations and that hydrogeological watersheds can differ from topographical watersheds. The consequence is that, even if the topographical watershed is small, underground flows from an adjacent watershed can add significant amounts of water to such a catchment. This effect is amplified when lava domes and their roots are situated upstream, as they present very high hydraulic conductivity leading to deep preferential groundwater flow circulations. We also reveal, unlike basaltic-type volcanic islands, that hydraulic conductivity increases with age in this andesitic-type volcanic island. This trend is interpreted as the consequence of tectonic fracturing associated with earthquakes in this subduction zone, related to andesitic volcanic islands. Finally, our approach allows characterization in detail of the hydrogeological functioning and identification of the properties of the main aquifer and aquitard units, leading to the proposition of a hydrogeological conceptual model at the watershed scale. This working scale seems particularly suitable due to the complexity of edifices, with heterogeneous geological formations presenting high lateral and vertical variability. Moreover, our study offers new guidelines for accurate correlations between resistivity, geology and hydraulic conductivity for volcanic islands. Finally, our results will also help stakeholders toward a better management of water resources.

Earthquakes and extreme rainfall induce long term permeability enhancement of volcanic island hydrogeological systems

“…We can also assume that weathering processes and earthquakes act together to enhance aquifer permeability, the first altering the rock and the second cleaning the pathway and creating new fractures. Our results then confirm the hypothesis 57 , 58 that the aquifer permeability increases with geological age on the andesitic type volcanic island, such as the one of Martinique, is the consequence of earthquakes seismic waves. In addition, we demonstrate the cumulative effect of extreme rainfall, to our knowledge the first such observations in earthquake hydrogeology.…”

Mountain‐Block Recharge: A Review of Current Understanding

“…Boreholes drilled for deep well installation should be logged using standard downhole geophysical logging tools and should be completed with multilevel well screens to better enable discrete-depth groundwater sampling. Airborne and surface geophysical surveys capable of discerning deep subsurface geologic framework (and potentially porosity and permeability) such as airborne time domain electromagnetic surveys (Vittecoq et al, 2019) and nuclear magnetic resonance (Legchenko et al, 2002) should be performed. These methods become yet more powerful when combined with downhole geophysical logs and drill core from a local deep borehole (Flinchum et al, 2018;Orlando et al, 2016;Vittecoq et al, 2019).…”

“…Airborne and surface geophysical surveys capable of discerning deep subsurface geologic framework (and potentially porosity and permeability) such as airborne time domain electromagnetic surveys (Vittecoq et al, 2019) and nuclear magnetic resonance (Legchenko et al, 2002) should be performed. These methods become yet more powerful when combined with downhole geophysical logs and drill core from a local deep borehole (Flinchum et al, 2018;Orlando et al, 2016;Vittecoq et al, 2019). Because installing wells and performing such surveys can be prohibitively expensive for individual researchers or institutions, we advocate for funding strategies that involve pooling investment in observational infrastructure and that lever existing infrastructure to the greatest degree possible.…”

Hydrogeophysical Characterization in a Volcanic Context From Local to Regional Scales Combining Airborne Electromagnetism and Magnetism

“…In order to improve regional groundwater management, it is mandatory to improve the characterization of inland aquifers and include their possible impact on the recharge rates of basal aquifers. To this end, the vadose zone needs to be studied on both local and regional scales so as to define the aquifer geometries and groundwater watersheds (Vittecoq et al., 2019).…”