GEOCHEMICAL AND ISOTOPIC CHARACTERIZATION OF CONFINED AQUIFERS IN THE CENTRAL-SOUTH AREA OF CORDOBA PROVINCE-ARGENTINA

CABRERA, ADRIANA; MALDONADO, LUCIANA; BLARASIN, MONICA; DAPEÑA, CRISTINA

São Paulo - SP

Simposio; 9th SOUTH AMERICAN SYMPOSIUM ON ISOTOPE GEOLOGY - 9th SSAGI; 2014

Institute of Geosciences, USP

In the central-South region of Córdoba province people use mainly groundwater from confined aquifers for agricultural activities because of bad groundwater quality derived from the unconfined loessical aquifer (CE~3,900 µS/cm; max=15,600 µS/cm). The objective of this work is to formulate a hydrogeological model of the groundwater system using geological, geochemical and isotopic tools. A strong influence of geological regional faults was observed on the sedimentation paleoenvironments and geochemical groundwater features. The confined multilayered deep aquifers are formed by thin sand-gravel layers (1-5 m), situated between thick clay deposits (20-50 m). Lithological evidences show that the sediments are from continental (West) and marine origin (East), from the Miocene-Pliocene period. Two main systems were distinguished: a) the shallowest (120-230 m), with piezometric levels about -10 m; b) the deepest (250-400 m), with piezometric levels about + 5 m. These confined systems show areal changes in salinity and geochemical type (although sodium sulphate waters predominate) which are related to lithological variations, distance from recharge areas and layers depth. The shallow system has electrical conductivity (CE) around 2,000 μS/cm (max= 4,000 μS/cm) and isotopic composition similar to the unconfined aquifer and local precipitation (δ18O: -4.9 to -5.4 ?; δ2H: -24 to -29 ?), which would indicate autochthonous recharge and connection between systems. Due to the found hydraulic relationships (water table higher than piezometric level), the contribution from the unconfined aquifer toward the deep system is interpreted. The deepest system has freshest groundwater (CE~1.000 µS/cm), related to the granulometry and quartz composition of sediments (sand-gravels paleochannels). The groundwater of this system is the most isotopically depleted, especially to the East (δ18O: -5.6 a ? 6.5 ?; δ2H: -30 a -39 ?), similar to surface water and groundwater located in western mountain and piedmont areas away from this region, indicating that they recharge these deepest aquifers.