Résumé
To overcome water scarcity issues, Managed Aquifer Recharge (MAR) structures are currently developed in many parts of the world, including poorly permeable terrain like weathered crystalline rocks. In such geological context, characterized by relatively limited groundwater storage mainly associated with fractures located at the interface between the upper weathered layer (saprolite) and the fractured bedrock, the efficiency of MAR is poorly known. To address this question and better understand the factors that control recharge dynamics, an artificial recharge basin was implemented at the Experimental Hydrogeological Park in Telangana (South India), a well-equipped and continuously monitored site situated in Archean granitic terrain. The thickness of the saprolite and hydraulic properties are relatively well known all over the site from previous geophysical surveys and hydraulic tests.To characterize recharge dynamics, recharge has been monitored in different boreholes surrounding the infiltration basin. Infiltration rates and water level data are interpreted by both a volume balance approach and different analytical solutions. In addition, a simple numerical model was used to show how the depth of the permeable interface between saprolite and granite controls recharge dynamics and observed water levels variations. Results show that the permeability of the saprolite/bedrock interface is sufficiently large to allow an efficient recharge that propagates laterally throughout the aquifer through this well connected interface. However, the variable depth of this permeable pathway controls the water level response, acting as a semi-impervious boundary, leading to remarkable water level variations. Thus, our findings show how the characteristics of the most permeable pathways control recharge dynamics in weathered crystalline rocks. In addition, we show how the depth variations of the permeable interface between saprolite and granite may be inferred from the monitoring of water level during recharge events.