Abstract
Riparian floodplains represent an interaction zone between the terrestrial subsurface and rivers, where regional groundwater flows, infiltration, and evapotranspiration drive mixing of water and import/export of nutrients and contaminants. These dynamics create seasonally transient redox conditions that drive biogeochemical transformations, which strongly modify groundwater quality. Microbial responses to changing hydrological conditions are perhaps the critical step connecting hydrology to geochemical transformations and groundwater quality, yet are not well understood. We aimed to address this knowledge gap by monitoring seasonal transitions at the U.S. Department of Energy legacy uranium ore processing site in Riverton, WY, through spring-summer-fall hydrological transitions. Our goal was to characterize the microbial community throughout the soil profile, down to the saturated aquifer, and observe its response to wet-dry transitions across a full season and compare to changes in geochemistry and hydrology. Next-generation sequencing was employed to identify biogeochemically-relevant microbial taxa based on the 16S rRNA gene; we found a broad diversity of microbial clades including taxa involved in sulfur and metal cycling, as well as nitrification. These data were paired with measurements of soil moisture, major nutrients and cations, and trace elements. Overall microbial community composition was dependent on soil depth or type, with seasonal effects only observed in the topsoil or subsurface aquifer. This finding indicates that microbial communities in the transiently-reduced center of the soil profile at the Riverton, WY site are remarkably stable, despite moisture and redox inversions. In addition, these communities likely impact the communities in surrounding soil horizons through export of metabolites and solutes as the water table rises and falls throughout the season.
Highlights
Alluvial aquifers are profoundly important as water supplies and for underground water storage, in arid and semiarid regions such as the Western United States, notably including the Colorado River Basin (Rumsey et al, 2015; Miller et al, 2016)
Most taxa cluster together toward the center – indicating a more cosmopolitan presence throughout the soil column throughout the season – eight Proteobacteria operational taxonomic units (OTUs) appear to have been impacted by increased moisture or metal (Fe, U) concentrations. Four of these OTUs are affiliated with Deltaproteobacteria – including Desulfobulbaceae and Desulfuromonadaceae families – which we found to be most abundant in deeper layers (TRZ, Clay, Aquifer; 21–24% of total community versus 7–8% in the upper soil layers; Figures 3B, 5A)
Through this work at the U-contaminated floodplain at Riverton, we found that depth and soil type were the strongest determinants of microbial community composition in floodplain soils (Figures 3, 5A, 6, 8), rather than the time of sample collection (Figure 5B and Supplementary Figure S6)
Summary
Alluvial aquifers are profoundly important as water supplies and for underground water storage, in arid and semiarid regions such as the Western United States, notably including the Colorado River Basin (Rumsey et al, 2015; Miller et al, 2016). Floodplains are sites of intermittent and sometimes intense hydrological shifts including seasonal water table fluctuations, flooding, and drought (Burt et al, 2002; Dam et al, 2017; Noël et al, 2017a; Chudasma et al, 2018), similar to changes that may occur in deep alluvial aquifers in response to pumping/recharge or climate-driven changes in water levels Such hydrological changes drive precipitation and dissolution of solids and control the diffusion of oxygen and the mobility/availability of aqueous nutrients and contaminants (Carlyle and Hill, 2001; Lair et al, 2009; Schulz-Zunkel et al, 2015). A key element to this understanding is increased knowledge about how microbiological and geochemical processes couple (or decouple) in response to hydrological transitions, as this should provide clues about which biogeochemical processes are active at different stages
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