Application of geochemistry and radioactivity in the hydrogeological investigation of carbonate aquifers (Sierras Blanca and Mijas, southern Spain)

The chemical characteristics, 3H contents and radioactivity of groundwaters from the Sierras Blanca and Mijas (Southern Spain) have been studied in relation to the chemical composition and radioactivity of the aquifer host rocks, and the residence time of the water. The Sierras Blanca and Mijas are made up of calcitic and dolomitic marbles of Triassic age. The groundwaters that drain the calcitic marbles (which outcrop principally in the western Sierra Blanca) have less mineralization, which decreases quickly with recharge (as does the 3H content), and the gross alpha and beta activities are below detection limit. This is due to the short residence time of water inside the aquifers which are conduit flow systems. The waters of the dolomitic marbles (eastern Sierra Blanca and Sierra Mijas) have higher and less variable mineralization and contain greater concentrations of Mg2+, SiO2 and SO42- (ions normally associated with slow flows). The 3H contents are more uniform with time (indicating an older age) and there is detectable natural radioactivity, because the waters have a longer residence time in the aquifers, which are diffuse flow systems.