The dissolved chemical and isotopic signature downflow the confluence of two large rivers: The case of the Parana and Paraguay rivers

JOURNAL OF HYDROLOGY

卷:528

Article

Agustina Campodonico, Verena¹ 

[1] Consejo Nacl Invest Cient & Tecn, Ctr Invest Ciencias Tierra CICTERRA, Cordoba, Argentina

关键词: Middle Parana River;    South American large rivers;    Mixing of waters;    Mass balance model;    Water isotopes;    Hydrochemistry;   

DOI  :  10.1016/j.jhydrol.2015.06.027

来源: Elsevier

【 摘 要 】

The Parana River basin is one of the largest hydrological systems in South America (similar to 2.6 x 10(6) km(2)). Downflow the confluence of tributaries, most large rivers exhibit transverse and longitudinal inhomogeneities that can be detected for tens or even hundreds of kilometers. Concordantly, a noticeable cross-sectional chemical asymmetry in the dissolved load was distinguished in the Middle Parana River, after the confluence of its main tributaries (i.e., the Paraguay and Upper Parana' rivers). Water chemistry and isotopic signature in three cross-sections along the Middle Parana River, as well as from main and minor tributaries, and some deep (similar to 105 m bs) and shallow boreholes (similar to 15 m bs) located near both river banks, were analyzed in order to define the extent of mixing and identify possible contributions from groundwater discharges. Downflow the confluence of the Upper Parana and Paraguay rivers a chemical and isotopic asymmetry was observed, mainly through the values of EC, major ions (Ca2+, Na+, Mg2+, Cl- and SO42-), some trace elements (Fe, U, Th, Ba, Sr, As and REE) and stable isotopes (delta O-18 and delta H-2). Toward its western margin, 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 Parana' River. This variability remained detectable at least until similar to 225 km downflow the confluence, where differences between western and eastern margins were less evident. At similar to 580 km downflow the confluence, a slight inversion in the transverse 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 East bank. A mass balance model was applied, as a first approach, to estimate the groundwater inflow using the geochemical tracer Rn-222. The results indicate that groundwater contributions represent between similar to 0.5% and 6% of the total water inputs to the Middle Parana River under baseflow conditions. This implies that the chemical asymmetry in the Middle Parana River is mostly 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 river, it may partially explain the higher concentrations of some trace elements found in the eastern margin similar to 580 km downflow the confluence. (C) 2015 Elsevier B.V. All rights reserved.

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