Stanford Synchrotron Radiation Lightsource

Clay lenses, enriched in organic matter, reduced iron, and sulfide, contribute to the extent and variability in Zn retention in a simulated aquifer.

The Science                                 

Alluvial aquifers are an essential source of groundwater worldwide, particularly for water storage purposes. Fine-grained lenses of clay and organic matter, enriched in iron and sulfur, are abundant within aquifers and support cycling of nutrients, carbon, and toxic metals by providing chemical reducing conditions in otherwise oxygenated systems. Changes in redox status may have immense influences on the behavior (dissolved concentration, bioavailability, mobility) of heavy metals within the aquifer and resulting groundwater quality. Therefore, we investigated the retention of a simulated Zn plume in a model alluvial aquifer system containing fine-grained reduced clayey lenses. The fine-grained lenses were specifically responsible for significantly increased Zn retention (23%) resulting in both spatial and compositional differences in the uptake of Zn. 

The Impact

Reducing conditions sustained within fine-grained sediment lenses enhanced the extent and breadth of retained Zn species compared to the coarse sediments of a model alluvial aquifer system. Furthermore, Zn loading, and the distribution of Zn species differed between individual lenses, suggesting that unbalanced multiple driving forces vary in intensity along the flow-path. These results emphasize the complex nature of alluvial aquifer systems and behavior of metal contaminants residing within them, which has direct implications to the quality of the groundwater. Thus, the spatial and compositional heterogeneities of alluvial aquifers must be specifically taken into account when maintaining and managing groundwater systems.

Summary

Understanding the biogeochemical conditions in alluvial aquifers experiencing redox heterogeneities is essential to preserve the quality of the groundwater stored within them. The fate of metal contaminants within these complex systems is challenging to predict. Thus, we studied the retention pathways of Zn within a model dual-domain (clayey-sandy) alluvial aquifer. Natural coarse aquifer sediments from the Wind River−Little Wind River floodplain near Riverton, WY, were used in columns with or without fine-grained lenses to examine biogeochemical controls on Zn concentrations, retention mechanisms, and transport. Zn preferentially accumulated within the fine-grained lenses, which enhanced Zn uptake by 23%, despite only comprising 5% of the sediment mass in the model aquifer. We found that clay minerals and layered double hydroxides dominated Zn retention in the coarse sediments, whereas ZnS prevailed in the fine-grained lenses, emphasizing distinct differences in Zn species between the domains. Zinc resistance to solid-phase aqueous extraction but sensitivity to acid extraction suggests limited, but measurable, capacity for re-release and transport unless the pH decreases considerably. Our findings emphasize the importance of considering differences in sediment composition, and the size and distribution of heterogeneities, in evaluating potential threats of metal contaminants to aquifer groundwater.

Contacts

John Bargar
SLAC National Accelerator Laboratory, Stanford Synchrotron Radiation Lightsource
bargar@slac.stanford.edu

Maya Engel

SLAC National Accelerator Laboratory, Stanford Synchrotron Radiation Lightsource
mayaeng@stanford.edu

Funding

The United States-Israel Binational Agricultural Research and Development Fund (BARD; grant no. FI-569-2018) and the Israeli Council for Higher Education provided financial support for Engel through postdoctoral fellowships. The project was conceived and supported by the SLAC Floodplain Hydro-Biogoechemistry Science Focus Area project funded by the US Department of Energy, Office of Biological and Environmental Research, Earth and Environmental Systems Sciences Division (EESS) under contract no. DEAC0276SF00515SBR (SLAC National Accelerator Laboratory). Additional support (for Engel and Fendorf) was provided by SBR Project DE-SC0020205. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (contract no. DE-AC0276SF00515).

Publications

Engel, M., Boye, K., Noël, V., Babey, T., Bargar, J.R., Fendorf, S. (2021) Simulated Aquifer Heterogeneity Leads to Enhanced Attenuation and Multiple Retention Processes of Zinc. Environmental Science & Technology. https://dx.doi.org/10.1021/acs.est.0c06750