2H and 18O BEHAVIOR IN PRECIPITATIONS, SURFACE AND GROUNDWATER SYSTEMS OF THE PAMPEAN PLAIN IN THE SOUTH 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

Groundwater is the most important resource destined to agricultural activities in the South of Cordoba province (Pampean Plain region). The objective of this study is to evaluate the 2H and 18O fractionation in precipitation, surface and groundwater bodies in order to improve the hydrogeological model reached so far with geochemical and hydrodynamic conventional techniques. The isotopic composition of the precipitation is depleted in East-West direction due to a continental effect, taking into account that its origin is linked to humid air masses from the Atlantic Ocean. The high deuterium excesses observed are supported by the origin of air masses related to recycling water from Amazon?s basin and the occurrence of the low level jet (LLJ), the Intertropical Convergence Zone (ITCZ) and the ENSO (El Niño Southern Oscillation). In western mountain areas, the local isotopic fractionation linked to lower temperatures due to higher frequency of snow events is added. The average isotopic composition of the plain unconfined aquifer (18O: -4,4 ?; 2H: -28,4 ?; d=14 ?) is similar to rainfall in the plain area, indicating local meteoric recharge. The confined aquifer systems show different hydraulic and geochemical behavior, depending on their depths and lithostratigraphic and structural features associated to the Neogene-Quaternary geological history of the Pampa Plain. The shallowest confined aquifer system (120-220 m) show a similar isotopic composition to the unconfined aquifer (18O: -5,0? to -4,6?; 2H: -32? to -28?), indicating hydraulic connection between them. The deeper confined aquifer system (250-400 m) has a more depleted isotopic composition (18O ≈ -6,6?; 2H≈ -43?), indicating its hydraulic disconnection with the overlying aquifer systems. Therefore, it is interpreted that the recharge area of these deeper layers is allochthonous, that is, the perimountain western belt linked to Comechingones Mountains, where losing streams which infiltrate recharging groundwater have the same isotopic signature.