Geochemical and Isotopic Composition of Deep Thermal Aquifers. Southern Córdoba province Argentina. Preliminary

CABRERA, ADRIANA; DAPEÑA, CRISTINA; BLARASIN, MONICA; PANARELLO, HECTOR; MATTEODA, EDEL

Bariloche, Río Negro, Argentina

Simposio; VI South American Symposium on Isotope Geology; 2008

INGEIS (CONICET-UBA)

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 preliminary hydrogeochemical 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 (Q mode) 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-ranging texture, 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 (l< 1800 µS/cm), hypothermal and mesothermal (29ºC - 36ºC) and sodium sulphate type waters, the first (120 m - 200 m) and second (225 m - 290 m) levels displayed the most saline waters (l > 1800 µS/cm) and the greatest concentrations of As and F- and values of δ18O between -5,4? and -4,6? and of δ2H between -32? y -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 (l < 1800 µS/cm) and different isotopic composition (δ18O≈ -6.6?; δ2H≈ -43?) allow us to assume recharge sectors located outside the studied area, probably in the perimountain western region.