Uranium Retention in a Bioreduced Region of an Alluvial Aquifer Induced by the Influx of Dissolved Oxygen

Virus production in shallow groundwater at the bank of the Danube River

Viruses play a crucial role in regulating prokaryotic populations, yet their impact on subsurface environments, specifically groundwater habitats, remains poorly understood. In this study, we employed the virus-dilution approach to measure lytic virus production rates in shallow groundwater located near the city of Vienna (Austria) during the period from July-November 2020. Physico-chemical parameters (pH, electrical conductivity, water temperature, concentration of dissolved oxygen), prokaryotic, and viral abundance, and viral decay rates were monitored as well. Our findings revealed low virus-to-prokaryote ratios varying between 0.9-3.9 throughout the study period and a lack of correlation between prokaryotic and viral abundance in groundwater. Virus production rates varied between 9-12% of viral abundance h-1 in July-August and between 34-36% of viral abundance h-1 in October-November. Seasonal variations in virus production rates were found to be correlated with electrical conductivity, revealing ~3.5 times higher virus production rates during periods with high electrical conductivity and low groundwater recharge in October-November compared to July-August with higher groundwater recharge and lower electrical conductivity. Our data indicate that groundwater recharge disrupts the balance between virus and prokaryotic host communities, resulting in a deficiency of suitable prokaryotic host cells for viral proliferation.

Reduction capacity in the transmissive zones fueled by the embedded low-permeability lenses: Implications for contaminant transformation in heterogeneous aquifers

Redox conditions play a decisive role in regulating contaminant and nutrient transformation in groundwater. Here we quantitatively described and interpreted the temporal and spatial variations of aquifer reduction capacity formation in lens-embedded heterogeneous aquifers in 1-D columns. Experimental results indicated that the aquifer reduction capacity exported from the low-permeability lens permeated into the downstream sandy zones, where it subsequently accumulated and extended. Reactive transport modeling suggested that reduction capacity within the lens preferentially diffused to the transmissive zones around the lens-sand interface, and was then transported via convection to downstream transmissive zones. A low-permeability lens of the same volume, but more elongated in the flow direction, led to less concentrated reduction capacity but extended further downgradient from the lens. The increased flow velocity attenuated the maintenance of aquifer reduction capacity by enhancing mixing and diluting processes in the transmissive zones. The reduction zones formed downstream from the low-permeability lens were hotpots for resisting the oxidative perturbation by O2. This study highlights the important role of low-permeability lenses as large and long-term electron pools for the transmissive zones, and thus providing aquifer reduction capacity for contaminant transformation and remediation in heterogeneous aquifers.

Nitrate-Stimulated Release of Naturally Occurring Sedimentary Uranium

Groundwater uranium (U) concentrations have been measured above the U.S. EPA maximum contaminant level (30 μg/L) in many U.S. aquifers, including in areas not associated with anthropogenic contamination by milling or mining. In addition to carbonate, nitrate has been correlated to uranium groundwater concentrations in two major U.S. aquifers. However, to date, direct evidence that nitrate mobilizes naturally occurring U from aquifer sediments has not been presented. Here, we demonstrate that the influx of high-nitrate porewater through High Plains alluvial aquifer silt sediments bearing naturally occurring U(IV) can stimulate a nitrate-reducing microbial community capable of catalyzing the oxidation and mobilization of U into the porewater. Microbial reduction of nitrate yielded nitrite, a reactive intermediate, which was further demonstrated to abiotically mobilize U from the reduced alluvial aquifer sediments. These results indicate that microbial activity, specifically nitrate reduction to nitrite, is one mechanism driving U mobilization from aquifer sediments in addition to previously described bicarbonate-driven desorption from mineral surfaces, such as Fe(III) oxides.

Reclaimed Water Reuse for Groundwater Recharge: A Review of Hot Spots and Hot Moments in the Hyporheic Zone

As an alternative resource, reclaimed water is rich in the various nutrients and organic matter that may irreparably endanger groundwater quality through the recharging process. During groundwater recharge with reclaimed water, hot spots and hot moments (HSHMs) in the hyporheic zones, located at the groundwater–reclaimed water interface, play vital roles in cycling and processing energy, carbon, and nutrients, drawing increasing concern in the fields of biogeochemistry, environmental chemistry, and pollution treatment and prevention engineering. This paper aims to review these recent advances and the current state of knowledge of HSHMs in the hyporheic zone with regard to groundwater recharge using reclaimed water, including the generation mechanisms, temporal and spatial characteristics, influencing factors, and identification indicators and methods of HSHMs in the materials cycle. Finally, the development prospects of HSHMs are discussed. It is hoped that this review will lead to a clearer understanding of the processes controlling water flow and pollutant flux, and that further management and control of HSHMs can be achieved, resulting in the development of a more accurate and safer approach to groundwater recharge with reclaimed water.

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

Sediment interfaces in alluvial aquifers have a disproportionately large influence on biogeochemical activity and, therefore, on groundwater quality. Previous work showed that exports from fine-grained, organic-rich zones sustain reducing conditions in downstream coarse-grained aquifers beyond the influence of reduced aqueous products alone. Here, we show that sustained anaerobic activity can be attributed to the export of organic carbon, including live microorganisms, from fine-grained zones. We used a dual-domain column system with ferrihydrite-coated sand and embedded reduced, fine-grained lenses from Slate River (Crested Butte, CO) and Wind River (Riverton, WY) floodplains. After 50 d of groundwater flow, 8.8 ± 0.7% and 14.8 ± 3.1% of the total organic carbon exported from the Slate and Wind River lenses, respectively, had accumulated in the sand downstream. Furthermore, higher concentrations of dissolved Fe(II) and lower concentrations of dissolved organic carbon in the sand compared to total aqueous transport from the lenses suggest that Fe(II) was produced in situ by microbial oxidation of organic carbon coupled to iron reduction. This was further supported by an elevated abundance of 16S rRNA and iron-reducing (gltA) gene copies. These findings suggest that organic carbon transport across interfaces contributes to downstream biogeochemical reactions in natural alluvial aquifers.

Complexation by Organic Matter Controls Uranium Mobility in Anoxic Sediments

Uranium contamination threatens the availability of safe and clean drinking water globally. This toxic element occurs both naturally and as a result of mining and ore-processing in alluvial sediments, where it accumulates as tetravalent U [U(IV)], a form once considered largely immobile. Changing hydrologic and geochemical conditions cause U to be released into groundwater. Knowledge of the chemical form(s) of U(IV) is essential to understand the release mechanism, yet the relevant U(IV) species are poorly characterized. There is growing belief that natural organic matter (OM) binds U(IV) and mediates its fate in the subsurface. In this work, we combined nanoscale imaging (nano secondary ion mass spectrometry and scanning transmission X-ray microscopy) with a density-based fractionation approach to physically and microscopically isolate organic and mineral matter from alluvial sediments contaminated with uranium. We identified two populations of U (dominantly +IV) in anoxic sediments. Uranium was retained on OM and adsorbed to particulate organic carbon, comprising both microbial and plant material. Surprisingly, U was also adsorbed to clay minerals and OM-coated clay minerals. The dominance of OM-associated U provides a framework to understand U mobility in the shallow subsurface, and, in particular, emphasizes roles for desorption and colloid formation in its mobilization.