GEOCHEMICAL AND ISOTOPIC COMPOSITION OF DEEP THERMAL AQUIFERS. SOUTHERN CÓRDOBA PROVINCE ARGENTINA. PRELIMINARY REPORT.

CABRERA, A.; DAPEÑA C,; BLARASIN, M.; PANARELLO, H.O.; MATTEODA, E.

San Carlos de Bariloche

Simposio; VI SOUTH AMERICAN SYMPOSIUM ON ISOTOPE GEOLOGY (VI-SSAGI); 2008

INGEIS-CIC-CONICET

The studied zone is situated in the Pampean Plain (Córdoba, Argentina), where the groundwater resources of major interest are confined-deep aquifers, which are often used due to the bad quality of the unconfined aquifer. The objective of this work is to present a preliminaryhydrogeochemical and isotopic model of deep aquifers in the studied area. The hydrogeological and isotopica analyses were made according to conventional methodologies. Hydrochemical maps, Schoeller diagrams, box plots and dendrograms (Qmode) were obtained. The isotopic analysis (18O and 2H) were very significant in the assessment of these deep aquifers since the few sedimentological and hydraulic data of the deepest layers made difficult a hydrogeologic classification. The deepest sediments (120 m to 350 m), fluvial materials with wide-rangingtexture, were assigned to Upper Tertiary units. Three deep multilayered aquifer systems were defined taking into account depths and groundwater quality: a semiconfined lower deep system, a confined system with low confinement grade and a confined sytem with high confinement grade. Although all of them contain fresh waters (CE< 1.800 μS/cm), hypothermal and mesothermal (29 ºC36 ºC) and sodium sulphate type waters, the first (120 m-200m) and second (225 m-290 m) levels displayed the most saline waters (L > 1.800μS/cm) and the greatestconcentrations of As and F- and values of d18O between -5,4 and -4,6 and of d2H between -32 and -25 ,which are similar to the phreatic system, being able to suppose a feasible hydraulic connection between aquifers. The isotopic composition of local and western allochthonous streams and the characteristics of deepest confined levels (225 m-340 m), with always artesian wells, better groundwater quality (C.E. < 1.800 μS/cm) and different isotopic composition (d18O= -6,6 ; d2H= -43 ) allow us to assume recharge sectors located outside thestudied area, probably in the perimountain western region.