GEOLOGIC STRUCTURE OF VOLCANIC SPRINGS AN D AQUIFERS ON THE REUNION-ISLAND - HYDROGEOLOGIC IMPLICATIONS

Geologic investigations of four pilot study areas on the Reunion Islan d have permitted characterization of the hydrogeologic properties of v olcanic aquifers associated with the Piton des Neiges volcano. The goa l is to improve our general understanding of groundwater flow in volca nic terrains. The volcanic stratigraphy of Piton des Neiges indicates four main periods of activity from 2.1 million to 20,000 years before present. The two oldest stages (I and II) consist of flows of oceaniti c composition while the subsequent stages, III and IV, are more differ entiated. Horizons of pyroclastic and volcanic detrital material are i ntercalated with flows. The hydrogeologic setting of the Piton des Nei ges has been established in part based on comparison with studies of g eologically comparable areas: volcanic aquifers confined between basal tic dykes on the Hawaiian islands. a system of ''water bearing cells'' on the Tenerife Island, subdivided into upper and lower vadose zones. the existence of a piezometric surface steepens sharply towards the c enter of the island has been deduced from a detailed survey of water g alleries in the Canary Islands. The results of Fernadopulle and Custod io in the Canary Islands are consistent with field observations presen ted for the Reunion Island. A total of 42 springs observed in the pilo t study areas has been divided into three categories according to geol ogic and morphologic characteristics of water-bearing beds. Most of th e observed springs(23 of 42) are classified in a superficial zone rela ted to subsurface flow in aquifers of very limited extent. An intermed iate zone is characterized by perched water tables; flow is channelled through paleo-valleys or stretched on extensive zones of ash. Finally , a deep zone characterized by the presence of a ''basal water table'' is defined for springs (7 of 42) located in entrenched valleys of ''a mphitheater'' morphology. Discharge rates are high and result from reg ional drainage of a thick section volcanic rock.