EVALUATION OF GROUNDWATER RESOURCES AVAILABILITY IN LA SELVA BASIN (NE SPAIN)USING HYDROGEOCHEMICAL DATA

Groundwater resources in many Mediterranean areas becomes depleted during dry periods because of the continuous extraction to fulfill all demands. The aquifer of la Selva basin (NE Spain) has experienced a significant decrease of the water level, which has dried up shallow wells and springs. In this work, potentiometric, hydrochemical, and isotopic data are used to evaluate the capability of the whole aquifer system; that is, the detritical arkosic non consolidated tertiary aquifers of the basin and their recharge areas in the surrounding crystalline massifs.

Potentiometric measurements indicate the flow paths between the recharge areas and the basin. They also show the progressive decay of water levels during the study period. Dominant hydrochemical type in the area is calcium bicarbonate, except for deep wells in granite where is calcium chloride. Water chemistry from granite and arkosic areas is attributed to silicate dissolution by CO2 charged waters, suggested by the HCO3:Ca molar ratio, and a general subsaturation in calcite. Arkosic aquifers show high SO4 and NO3 contents attributed to anthropogenic origin. Deep wells in granite have high chloride and cationic concentrations. Long term dissolution of albite, oligoclase and chloride may account for the build up of Cl and Mg, among others. Tritium contents are nil for the deepest wells, and show consistency between shallow wells in granite and those in the arkosic aquifers.

Results indicates a close relationship between the recharge areas and the exploited aquifers of la Selva basin. Main fractures provide an efficient connection between both aquifers. Geochemistry of deep wells in granite suggest a poor and slow recharge towards this exploited levels.

Finally, groundwater resources availability is restricted to the arkosic aquifer in the basin, and is controlled by annual rainfall recharge. Exploitation of the granite terrains may thus impair the overall recharge of those aquifers, and also of the deepest granite levels; resulting of a loss of supplying capability to actual demands.