INVESTIGADORES
GARCIA Maria Gabriela
congresos y reuniones científicas
Título:
The hydrochemical and isotopic signature of the Middle Paraná River downflow the confluence of Paraguay and Upper Paraná rivers: surface water mixing or groundwater inflow?
Autor/es:
CAMPODÓNICO, V; GARCÍA M. G; PASQUINI A. I
Reunión:
Congreso; III Reunión Argentina de Geoquímica de la Superficie; 2014
Resumen:
The
Paraná River
basin is one of the largest hydrological systems in South America (~2.6x106 km2). Most
large rivers exhibit transverse and longitudinal inhomogeneities downflow the
confluence of tributaries, which can be
detected for tens or even hundreds of kilometers (e.g., Yang et al., 1996).
A noticeable cross-sectional chemical asymmetry regarding the major dissolved
constituents was earlier distinguished in the Middle Paraná River, after the
confluence of its main tributaries, i.e., the Paraguay and Upper Paraná rivers
(Drago and Vasallo, 1980). In this
study we present new chemical and stable and radiogenic isotopic data of the
Paraná River drainage basin in order to explain the dissimilar composition of
water observed across the main channel of the Middle Paraná River.
Water
chemistry (major and minor dissolved constituents) and isotopic signature (δ18O,
δ2H
and 222Rn)
were analyzed in three cross-sections along the Middle Paraná River
(Corrientes-Resistencia, Goya-Reconquista and Paraná-Santa Fe). Waters
collected from main (Paraguay and Upper Paraná) and minor tributaries, as well
as groundwater from some deep (~105 m bs) and shallow boreholes (~15
m bs) located near both river
banks were also examined in order to assess the relative contributions
of these sources to the Middle Paraná River.
Downflow
the confluence of the Upper Paraná and Paraguay rivers a chemical and isotopic
asymmetry was confirmed, mainly through the values of electrical conductivity
(EC), major ions (Ca2+, Na+, Mg2+, Cl- and SO42-),
some trace elements (Fe, U, Th, Ba, Sr, As and REE) and stable isotopes (δ18O
and δ2H).
Towards the western margin of the middle strech, higher elemental
concentrations which resembled that of the Paraguay River were measured,
whereas at the eastern border, waters were more diluted and preserved the
chemical signature of the Upper Paraná River. We estimated, by means of Cl-
mass-balance, that after the confluence, at Corrientes-Resistencia, more than
80% of water in the western margin was supplied by the Paraguay River, whereas
at the East it accounted for less than 15% of the total water budget. This
chemical and isotopic variability remained detectable at least until ~225 km
downflow, where differences between western and eastern margins were less evident.
At ~580 km downflow the confluence, a slight inversion in the cross-sectional
chemical asymmetry was observed. This trend switch can be the result of the
input of solutes from minor tributaries that reach the main channel from the
East and/or may be due to higher groundwater discharges from the eastern bank.
A mass balance model was applied, as a first approach, to estimate the
groundwater inflow using the geochemical tracer 222Rn. The
results indicate that groundwater sources represent between 0.5% and 6% of the
total water inputs to the Middle Paraná River, revealing that the chemical
asymmetry is mainly due to the incomplete mixing of the main tributaries. Though the influence of groundwater is not a
determining factor in the chemical variability of the Middle Paraná River, it
may partially explain the higher concentrations of some trace elements (i.e.,
As, Fe, U, Ba and Sr) found in the eastern margin ~580
km downflow the confluence of Paraguay and Upper Paraná rivers.