Isotope variations of dissolved Zn in the Rio Grande watershed, USA: The role of adsorption on Zn isotope composition

- Variations of Zn isotopes are investigated in the Rio Grande watershed.
- Stream Zn isotope composition is significantly controlled by pH.
- pH increases cause decreases of dissolved δ66Zn.
- Bedrock composition has negligible control on dissolved δ66Zn.
- Anthropogenic Zn sources do not control dissolved δ66Zn.

In order to better understand the factors influencing zinc (Zn) isotope composition in hydrological systems, we analyzed the δ66Zn of dissolved Zn in the streams and groundwater of the Upper and Middle Rio Grande watershed in Colorado and New Mexico, United States. The stream water samples have a wider variation of δ66Zn (−0.57 to +0.41‰ relative to the JMC 3-0749-Lyon standard) than groundwater samples (−0.13 to +0.12‰) and than samples from streams that are in close proximity to abandoned mining sites (+0.24 to +0.40‰). Regional changes of bedrock geology, from primarily igneous rocks to primarily sedimentary rocks, have no resolvable effect on the δ66Zn of aqueous samples. Instead, an increase in water pH from 7.5 to 8.5 corresponds to a considerable decrease in the δ66Zn of dissolved Zn (R2=−0.37, p=0.003, n=22). Consequently, we link the observed Zn isotope variations to the process of adsorption of Zn onto suspended sediment and bedrock minerals (average Δ66Znadsorbed-dissolved = +0.31‰). Our results are in good agreement with previous experimental and empirical studies suggesting that Zn adsorption leads to a residual dissolved pool enriched in light Zn isotopes. Given that anthropogenic Zn sources can also be responsible for lowering of δ66Zn, and may overlap with the pH/adsorption effect on δ66Zn, the latter needs to be carefully considered in future studies to differentiate between natural and anthropogenic factors influencing Zn isotopes in this and other aquatic systems.