Background and objectives
The construction of the Salazie Amont deep water-transfer tunnel (GSAM) on Réunion Island provided a unique opportunity to investigate groundwater flow in a highly heterogeneous volcanic aquifer. Crossing the flank of the Piton des Neiges shield volcano, the tunnel encountered large groundwater inflows under high hydraulic pressure, revealing the complex structure and hydraulic behaviour of the volcanic formations.
The study combined field observations, geological investigations, hydrochemical and isotopic data, temperature measurements and analytical modelling to understand groundwater circulation and estimate the long-term discharge into the tunnel.
Main hydrogeological findings
The analysis of groundwater inflows identified three major aquifer systems along the tunnel alignment. Their hydraulic behaviour reflects strong geological heterogeneity, particularly the contrast between highly permeable vacuolar basalts and less permeable, zeolitised formations.
Stable isotope data (δ¹⁸O and δ²H) provided evidence of recharge at high elevations, while temperature measurements revealed significant hydrothermal gradients and contrasted flow regimes.
In particular, a negative correlation was observed between groundwater discharge rates and water temperatures in one of the main aquifer systems. This relationship provides insights into the mixing between relatively cold, recently infiltrated groundwater and warmer water circulating through deeper formations.
Analytical modelling of groundwater inflows
A major challenge was to estimate the long-term discharge into the tunnel despite limited hydraulic data and the strong spatial heterogeneity of the volcanic formations.
An analytical modelling approach was therefore developed, based on transient groundwater-flow solutions for both confined and unconfined aquifers. The progressive advance of the tunnel was explicitly taken into account using convolution and superposition methods.
The heterogeneous geological structure was represented as a succession of hydrogeological sectors with different hydraulic properties. The model incorporated the actual excavation history, including changes in tunnel geometry and drilling methods.
Model calibration was supported by comparison with permeability measurements obtained from horizontal boreholes.
Main results and implications
The modelling provided estimates of the expected steady-state groundwater discharge under different recharge scenarios. The probable scenarios yielded total tunnel inflows ranging from approximately 0.42 to 0.55 m³/s, highlighting the considerable groundwater resources intercepted by the tunnel.
The study demonstrated how deep underground infrastructure can provide valuable information on groundwater circulation in fractured volcanic formations. It also showed the potential of analytical solutions to investigate complex, heterogeneous aquifers when conventional numerical modelling is limited by the scarcity of available data.
This work contributes to a better understanding of groundwater flow, recharge and hydraulic heterogeneity in volcanic aquifers, with implications for both water-resource management and the assessment of groundwater inflows during tunnel construction.
