Generally, the Rio Grande is characterized by significantly lower δ66Zn compared to previously studied large river systems in North and South America, Europe, Asia and Africa ( Chen et al., 2008 and Chen et al., 2009; Little et al., 2014a and Little et al., 2014b). The results of this study show that major aqueous Zn sources are acidic drainage of hydrothermal deposits. Downstream changes in bedrock composition do not have a measurable effect on the dissolved δ66Zn in the Rio Grande watershed. Likewise, there appears to be little influence from anthropogenic Zn throughout the study area. Instead, changes of PETCM appear to exert a significant control on dissolved δ66Zn. In this study, an increase in pH from ∼7.0 to 8.5 correlates with a significant decrease in the dissolved δ66Zn. The average Δ66Znadsorbed-dissolved is +0.31‰+0.31‰. We explain this relationship as the result of the preferential adsorption of heavy Zn isotopes on suspended sediments and/or organic matter carried by the studied streams and river. This is in good agreement with the elevated sediment loads in the studied streams, because of a dry climate and long-term deforestation/overgrazing. Additionally, we suggest that the seasonal enrichment of the studied streams in light Zn isotopes might result from groundwater recharge affected by higher Zn adsorption and removal of heavy Zn isotopes during water infiltration and interaction with the bedrock.
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