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Mixing as a driver of temporal variations in river hydrochemistry: 2. Major and trace element concentration dynamics in the Andes‐Amazon transition

Variations in riverine solute chemistry with changing runoff are used to interrogate catchment hydrology and to investigate chemical reactions in Earth's critical zone. This approach requires some understanding of how spatial and temporal averaging of solute‐generating reactions affect the diss...

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Bibliographic Details
Published in:Water resources research 2017-04, Vol.53 (4), p.3120-3145
Main Authors: Baronas, J. Jotautas, Torres, Mark A., Clark, Kathryn E., West, A. Joshua
Format: Article
Language:English
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Summary:Variations in riverine solute chemistry with changing runoff are used to interrogate catchment hydrology and to investigate chemical reactions in Earth's critical zone. This approach requires some understanding of how spatial and temporal averaging of solute‐generating reactions affect the dissolved load of rivers and streams. In this study, we investigate the concentration‐runoff (C‐Q) dynamics of a suite of major (Na, Mg, Ca, Si, K, and SO4) and trace (Al, Ba, Cd, Co, Cr, Cu, Fe, Ge, Li, Mn, Mo, Nd, Ni, Rb, Sr, U, V, and Zn) elements in nested catchments of variable size, spanning the geomorphic gradient from the Andes Mountains to the Amazon Foreland‐floodplain. The major elements exhibit various degrees of dilution with increasing runoff at all sites, whereas the concentrations of most trace elements either increase or show no relationship with increasing runoff in the three larger catchments (160–28,000 km2 area). We show that the observed main stem C‐Q dynamics are influenced by variable mixing of tributaries with distinct C‐Q relationships. Trace element C‐Q relationships are more variable among tributaries relative to major elements, which could be the result of variations in geomorphology, lithology, and hydrology of the subcatchments. Certain trace metals are also lost from solution during in‐channel processes (possibly related to colloidal size‐partitioning), which may exert an additional control on C‐Q dynamics. Overall, we suggest that tributary aggregation effects should be assessed in heterogeneous catchments before C‐Q or ratio‐Q relationships can be interpreted as reflecting catchment‐wide solute generation processes and their relationship to hydrology. Key Points: Tributaries of the Madre de Dios River exhibit distinct solute concentrations and concentration‐runoff relationships for many elements Tributary mixing proportions change systematically with runoff, affecting the concentration‐runoff relationships observed downstream Mixing effects on concentration‐runoff relationships are observed in catchments of 160–28,000 square kilometer area and vary from element to element
ISSN:0043-1397
1944-7973
DOI:10.1002/2016WR019729