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.
Landslide hazard and risk assessment (LHA & LRA) in the French West Indies is a big challenge, particularly in Martinique, where several factors contribute to high slope sensitivity to landslides. This sensitivity is particularly due to volcanic ground, hurricane seasons, and growing pressure from urban development. Thus, to protect future goods and inhabitants and avoid increased slope sensitivity to landslide, it is necessary to analyze by different ways and complementary approaches the future planned areas. This research focuses on a site the City Council of 'La Trinité' wishes to develop. The goals consist of locating landslide-prone areas and providing some recommendations/indications for future projects. The site is characterized by a hilly topography alternating steep slopes, gentle slopes, and eroded areas and is located on a complex lithology (i.e., andesite, basalt, and weathered materials). By combining several approaches and techniques (geology, geomorphology, geophysics, and modeling), it is demonstrated that some areas are particularly susceptible to landslide, notably where colluviums are juxtaposed to highly weathered materials. The different documents produced, based on modeling and expert knowledge, combined with indications should allow the definition of new susceptibility classes, taking into account probable anthropic influence and development. Even if the temporal probability of the experimental documents is not taken into account, they help with refining knowledge of landslide-prone areas and different types of instability. The documents should be discussed with end users for future planning.
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