Heterogeneity of hydraulic parameters and compartmenting of a volcanic aquifer revealed by long duration pumping tests: example of the Lamentin aquifer - Martinique FWI
Résumé
The Lamentin's aquifer has been highlighted in the 1970s and is considered as one of the most important of Martinique. Several studies were conducted to understand its structure and functioning. It is located close to the urban community of Fort de France, where lives half of the population of the island. This aquifer is thus strategic and its resource is expected to reduce or fill the drinking water deficit during dry seasons and to provide a backup resource during the rest of the year. Six boreholes were drilled between 1970 and 2006 and numerous geophysical investigations (SEV and electrical tomography) were carried to understand its structure and to evaluate its productivity. Between 2006 and 2009 two long-duration pumping tests were performed in the most productive borehole (LAF1): a 15-months pumping test during a complete hydrological cycle, and a 3-months test during the next dry season. Then a last pumping test was performed during a week in a borehole located downstream (BH n°202). Diagnoses of pumping tests based on the interpretation of the derivative curve of drawdowns were used to highlight the flow regimes. The tests performed in LAF1 show leakage effects from a semi-confining bed situated above the aquifer and also channelized flow between two impermeable boundaries. The one in the borehole 202 clearly shows flows toward a sub-vertical fracture and also a recharge boundary. The modeling of pumping tests shows that the hydrodynamic parameters are very heterogeneous, despite the relatively small extension of the aquifer (about 20 km²). The transmissivities vary by a factor of 10, from 2.10-4 to 3.10-3 m²/s; and the storage coefficients between 1 ‰ and 0.2 ‰. In addition, aquifer geometry is also constrained; LAF1 borehole is characterized by two impermeable boundaries reached after 3 months of pumping and the 202 borehole by a constant head boundary after 50 hours of pumping. The system heterogeneity probably has its origin in the compartmentalization of the aquifer due to tectonic activity that has affected the island. Geophysical data and piezometric maps made during the longest pumping test seem to confirm the presence of several geological discontinuities. Then the water abstraction into a compartment with productivity will be limited by the volume of stored water because of its compartmented geometry. As a consequence, a flow deficiency from the neighboring compartments with lower permeability will appear; the recharge will also be limited by the semi-confined bed.