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

In order to better understand the factors influencing zinc (Zn) isotope composition in hydrological systems, we analyzed the delta Zn-66 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 delta Zn-66 (-0.57 to +0.41 parts per thousand relative to the JMC 3-0749-Lyon standard) than groundwater samples (-0.13 to +0.12 parts per thousand) and than samples from streams that are in close proximity to abandoned mining sites (+0.24 to +0.40 parts per thousand). Regional changes of bedrock geology, from primarily igneous rocks to primarily sedimentary rocks, have no resolvable effect on the delta Zn-66 of aqueous samples. Instead, an increase in water pH from 7.5 to 8.5 corresponds to a considerable decrease in the delta Zn-66 of dissolved Zn (R-2 = -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 Delta Zn-66(adsorbed-dissolved) = +0.31 parts per thousand). 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 delta Zn-66, and may overlap with the pH/adsorption effect on delta Zn-66, 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. (C) 2015 Elsevier B.V. All rights reserved.