Simulated Aquifer Heterogeneity Leads to Enhanced Attenuation and Multiple Retention Processes of Zinc

Abstract

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 retentionthe 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.

Export of Organic Carbon from Reduced Fine-Grained Zones Governs Biogeochemical Reactivity in a Simulated Aquifer

“…Newly formed reducing coarse-grained zones are therefore presumably more susceptible to hydrologic perturbations (e.g., major influx of oxygenated water through rainwater infiltration, drought with subsequent desaturation, and aeration of sediments) than fine-grained zones. It is possible that these differences contribute to the apparent discrepancy between previous field-scale modeling studies showing a strong influence of oxygenation on groundwater chemistry − and empirical field/laboratory experiments ,,, indicating strong resilience of the reduced, fine-grained zones to oxygen intrusion. Previous modeling studies focused on chemical differences between reduced zones and a “general” aquifer and typically did not account for physical differences between fine- and coarse-grained zones.…”

“…Artificial aerated groundwater was prepared to mimic the composition of groundwater measured at the Wind River site 4 (Table S3). The groundwater at this site has much higher salt, bicarbonate, and sulfate concentrations than that at the Slate River site; hence, a single (Wind River) groundwater composition was chosen to facilitate comparisons with previous studies 4,36 and between the two types of sediments. However, this setting precludes any attempt to link results from this experiment to the current conditions at the Slate River floodplain.…”

Nitrate Controls on the Extent and Type of Metal Retention in Fine-Grained Sediments of a Simulated Aquifer

“…The ultimate ability of an aquifer to attenuate contaminants depends on the properties and spatial heterogeneity of the sediments comprising the aquifer. − While coarse-grained sands and gravels typically dominate aquifer composition, the presence of fine-grained sediments (as layers or lenses) is common, if not universal. , The physical characteristics of the fine-grained materialspermeability, dimension, composition, and distributionadd important complexity to the aquifer framework. , …”

“…Fine-grained sediments tend to be rich in clay, organic matter, and Fe and Mn oxides and hydroxides, as well as deficient in oxygen. , Because these sediments have low permeability, the interface with highly permeable sands and gravels is exceptionally important for contaminant transport. Such sediment interfaces are highly reactive “hotspots” that support microbial activity, redox cycling, and potential contaminant retention or liberation. , …”

“…To preserve groundwater quality, we must expand our understanding of contaminant behavior in aquifers with spatial and compositional sediment heterogeneities that promote dynamic hydrologic and biogeochemical processes. Therefore, the objectives of this study were to (1) evaluate Ni attenuation in a simulated aquifer having architectural complexity, (2) examine the impact of nitrate on the fate and transport of Ni, and (3) identify commonalities and/or differences in observed trends between Ni, Fe, and Zn (which was the focus of our previous study). Both Ni and Zn are common metal contaminants of groundwater and sediment samples globally. , …”