Alluvial
aquifers serve as one of the main water sources for domestic,
agricultural, and industrial purposes globally. Groundwater quality,
however, can be threatened by naturally occurring and anthropogenic
metal contaminants. Differing hydrologic and biogeochemical conditions
between predominantly coarse-grained aquifer sediments and embedded
layers or lenses of fine-grained materials lead to variation in metal
behavior. Here, we examine processes controlling Zn partitioning within
a dual-pore domain-reconstructed 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. Following the introduction of Zn to the
groundwater source, Zn preferentially accumulated in the fine-grained
lenses, despite their small volumetric contributions. While the clay
fraction dominated Zn retention in the sandy aquifer, the lenses supported
additional reaction pathways of retentionthe reducing conditions
within the lenses resulted in ZnS precipitation, overriding the contribution
of organic matter. Zinc concentration in the groundwater controlled
the formation of Zn-clays and Zn-layered double hydroxides, whereas
the extent of sulfide production controlled precipitation of ZnS.
Our findings illustrate how both spatial and compositional heterogeneities
govern the extent and mechanisms of Zn retention in intricate groundwater
systems, with implications for plume behavior and groundwater quality.
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