The geochemical filter of large river confluences

This study reports the behaviour and fate of major and trace elements in solid and solution in the Encontro das Aguas mixing zone in the Amazonian basin. This area results from the confluence of acidic, organic-rich Rio Negro and Andean sediment-rich Rio Solimoes. The differences existing between these two water masses are likely to modify the partitioning of elements between solid and solution or to induce losses by physical or chemical removal. To account for lateral and vertical heterogeneities in the river sections, the fluxes of elements in dissolved and solid loads were estimated as accurately as possible using an interpolation technique. A general loss of suspended particulate matter (SPM) of 23% is measured 80 km after the confluence but the various elements and their associated hosted minerals are not affected in a similar way. Si-SPM, Ca-SPM, Na-SPM or Sr-SPM, mostly associated with coarse phases (quartz and Ca-Na feldspars) are preferentially lost. Refractory elements (Co-SPM, Cr-SPM, Ti-SPM, Al-SPM, Ni-SPM), other alkali (A) and alkali earth (AE) elements (K-SPM, Mg-SPM, Ba-SPM), Fe-SPM or organic carbon (OC) are lost to a lesser extent due to their occurrence in finer fractions such as clays (smectite, illite or kaolinite), oxides and particulate organic matter (POM). In solution A-AE and Cu-diss, mostly originating from the Rio Solimoes behave conservatively. By contrast, refractory elements, coming mostly from the Rio Negro, exhibit important losses (from - 10% for DOC to - 60% for Co-diss). Changes in pH and ionic strength during the mixing may induce a sorption of these elements to particles supplied by the Rio Solimoes, a clustering and/or a settling of nanokaolinites phases or a sorption of Al-diss, Fe-diss, Co-diss, Mn-diss or Zn-diss by dissolved organic matter (DOM). The differential elemental settling at a river confluence could thus induce a phase shift between element supply and export out of a watershed. The physical and chemical processes reported here for a river confluence could be applied in other mixing zones such as estuaries or mining exhausts. (C) 2016 Elsevier B.V. All rights reserved.