BER Research Highlights

Search Date: June 14, 2021

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August 14, 2020

Microbial Communities in Floodplain Soils Remain Unchanged Throughout Seasonal Redox and Water Table Flux

Fine-grained sediments appear to host distinct microbial groups that are stimulated through water table rise and fall throughout the season.

The Science
Microbial communities play a crucial role in environmental systems, mediating biogeochemical reactions through metabolic processes that can vary depending on environmental conditions. Understanding these metabolic shifts is particularly important in soils closely connected to groundwater and surface water (e.g., those in floodplains), where redox transformations can determine whether contaminants are sequestered or mobilized. This study characterized microbial community diversity at the uranium- and molybdenum-contaminated Riverton, Wyo., site from spring to fall across multiple depths. Results indicate that communities are surprisingly stable over time, despite extreme seasonal geochemical changes (redox inversion) and hydrological changes (flood to drought). This finding suggests that these microorganisms oscillate between “active” and “dormant,” depending on current environmental conditions, and the products of their metabolism play a greater role in contaminant mobility than the organisms themselves. Future studies to investigate metabolic capacity and activity through metagenomic and metatranscriptomic sequencing will support further integration of hydrological, geochemical, and microbial data into models and other knowledge of contaminant transport and mobility.

The Impact
Biogeochemical changes in the subsurface are driven primarily through microbial life and microbes’ metabolic responses to environmental conditions around them. Soil moisture content, which varies throughout the season from spring flooding to summer drought, profoundly influences microbial communities and the availability of molecular oxygen (O2), thus affecting soil redox conditions and water quality. Microbial responses to hydrological changes therefore comprise keystone functionalities within subsurface ecosystems; however, it is not fully understood how large changes in soil moisture and O2 impact subsurface microorganisms themselves or their metabolism. In this study, surprisingly, microbial communities did not change significantly during seasonal flood-to-drought transitions. Rather, the research team found that depth within the soil profile and soil horizon characteristics were the strongest factors determining microbial diversity. These results provide insight into the complexity of coupling (or decoupling) microbial and geochemical trends in floodplain soils over space and time. For example, seasonal changes in soil moisture apparently are not sufficiently prolonged to displace established microbial groups.

Summary
Riparian floodplains are important regions, given the connectivity of groundwater with river water, biodiversity, presence of contaminants, and capacity to generate and recycle nutrients. In a given year, these floodplains experience changes in precipitation, river discharge (including flooding), and water content (including drought), all of which impact water quality. For example, in the western United States, the U.S. Department of Energy (DOE) manages several former uranium ore–processing floodplain sites, where contaminant concentrations change in response to changing sediment moisture and season. There is a need to understand how hydrology, geochemistry, and microbiology interact to drive these changes. In this study, the team collected floodplain soil samples through a full growing season at the uranium-contaminated Riverton, Wyo., DOE legacy site. These samples were analyzed for key geochemical species, water content, and microbial diversity through community DNA analysis. Findings showed that, despite clear seasonal shifts in geochemical and redox conditions corresponding to changes in hydrological conditions (e.g., flood and drought), microbial community diversity remained largely unaffected. The same microbial groups were present at a given depth throughout the year, indicating their ability to persist despite environmental change. The team did, however, observe slight differences in soil surrounding fine-grained, organic-rich layers (referred to as “transiently reduced zones,” or TRZs). This finding agrees with previously observed export of reducing conditions from TRZs but adds information about how these exports may also impact microbial diversity in adjacent soil layers through water table fluctuations. The results suggest that TRZ soil layers are even more important to floodplain functions than presumed by commonly observed redox transformations, because stable microbial communities drive geochemical changes while remaining relatively unchanged themselves.

Contacts
BER Program Manager
Amy Swain
U.S. Department of Energy Office of Science, Office of Biological and Environmental Research
Earth and Environmental Systems Sciences Division (SC-33.1) and Biological Systems Science Division (SC-33.2)
Environmental System Science and Biomolecular Characterization and Imaging Science
amy.swain@science.doe.gov

Principal Investigator
Chris Francis
Stanford University, Earth System Science Department
Stanford, CA 94305-4216
Caf@stanford.edu

Funding
Funding was provided by the Subsurface Biogeochemical Research (SBR) program of the Office of Biological and Environmental Research, within the U.S. Department of Energy (DOE) Office of Science, to the SLAC National Accelerator Laboratory Science Focus Area project under contract DE-AC02-76SF00515. Use of the Stanford Synchrotron Radiation Laboratory (SSRL) at SLAC is supported by the Office of Basic Energy Sciences, within the DOE Office of Science.

Publications
Tolar, B.B., Boye, K., Bobb, C. et al. “Stability of floodplain subsurface microbial communities through seasonal hydrological and geochemical cycles.” Frontiers in Earth Science – Biogeoscience 8, 338 (2020). [DOI:10.3389/feart.2020.00338]

Topic Areas:

Division: SC-33.1 Earth and Environmental Sciences Division, BER


February 20, 2020

Redox Interfaces Can Produce Toxic Arsenic Levels in Groundwater from Low Arsenic-Abundance Sediments

Sulfate-rich groundwater promotes formation of thioarsenates at fine-coarse sediment interfaces and increases arsenic solubility.

The Science
Groundwater contamination by arsenic from natural and anthropogenic sources, though a worldwide concern, is primarily monitored in areas with elevated sediment arsenic concentrations. Sharp redox transitions over space and time are also common, particularly in alluvial aquifers, and can influence the molecular speciation of arsenic as well as arsenic release or retention. However, the impact of redox and sediment interfaces on arsenic release and groundwater quality remains largely unexplored, especially where sediment arsenic concentrations are low. In this study, the research team set up a laboratory column experiment with natural, low-arsenic sediments from the sandy aquifer and an organic-rich, sulfidic, clay deposit of an alluvial floodplain at the Riverton, Wyo., U.S. Department of Energy (DOE) Office of Legacy Management site. Through a combination of aqueous- and solid-phase arsenic, sulfur, and iron speciation analyses (including X-ray absorption spectroscopy), the team showed that substantial release of arsenic to the groundwater may occur where there is a consistent supply of aqueous sulfide, but iron reduction promotes iron-sulfide precipitation. High groundwater concentrations of arsenic in this experiment were coincident with the occurrence of thiolated arsenic species (making up £40% of aqueous arsenic), suggesting that elevated groundwater concentrations are caused by conditions promoting arsenic thiolation, which in this study translated into aqueous concentration ratios of sulfide:arsenic > 100 and sulfide:iron < 1.

The Impact
In this study, reducing conditions were exported from the small, sulfidic, organic-rich, clay lenses into the initially oxic aquifer sand. This promoted toxic levels of arsenic release to groundwater in the aquifer sand, effectively recruiting a much larger volume of aquifer to function as reducing arsenic-thiolating zones. Although there are multiple factors influencing the outcomes, such as relative spacing and sizes of fine-grained materials as well as flow and groundwater-pumping rates, these results suggest that there is a considerable risk of underestimating threats from geogenic arsenic (and likely other toxic trace elements) unless relatively small scale but drastic variation in sediment compositions are taken into consideration when installing groundwater wells.

Summary
Arsenic contamination of groundwater is a globally recognized concern but is most often considered in areas of extensive anthropogenic contamination (e.g., through mining operations) or naturally elevated geogenic concentrations (e.g., in the large river deltas of South and Southeast Asia). In this study, however, the team used natural floodplain sediments with arsenic concentrations below the global average (1.6 milligrams per kilogram of sediment) and examined the influence on arsenic concentrations in groundwater by the presence of fine-grained, organic-rich sediment lenses in groundwater aquifer sand. Results indicate that when sulfate concentrations in the groundwater are high, the export of reducing conditions from fine-grained, sulfidic lenses into aquifer sand can promote iron reduction that in turn leads to iron-sulfide precipitation and elemental sulfur formation. The elemental sulfur then reacts with arsenic to form thiolated arsenic species, which appear to have a higher solubility and mobility than other arsenic species. Thus, the combination of high-sulfate groundwater and heterogeneous sediment composition (e.g., fine-grained, organic-rich/coarse interfaces) can locally promote severely elevated arsenic concentrations, even when sediment arsenic concentrations are below the global average.

The findings from this study suggest that zones and lenses with differing redox regime and sediment composition that are small enough to be disregarded (or even completely missed) during evaluations for well installations could still generate concerning or even toxic concentrations of arsenic and possibly other contaminants.

Contacts
BER Program Manager
Amy Swain
U.S. Department of Energy Office of Science, Office of Biological and Environmental Research
Earth and Environmental Systems Sciences Division (SC-33.1) and Biological Systems Science Division (SC-33.2)
Environmental System Science and Biomolecular Characterization and Imaging Science
amy.swain@science.doe.gov

Principal Investigator
John Bargar
SLAC National Accelerator Laboratory, Stanford Synchrotron Radiation Lightsource
Menlo Park, CA 94025
Bargar@slac.stanford.edu

Funding
Funding was provided by the Subsurface Biogeochemical Research (SBR) program of the Office of Biological and Environmental Research, within the U.S. Department of Energy (DOE) Office of Science, to the SLAC National Accelerator Laboratory Scientific Focus Area under contract DE-AC02-76SF00515. Use of the Stanford Synchrotron Radiation Lightsource (SSRL) at SLAC is supported by the Office of Basic Energy Sciences, within the DOE Office of Science.

Publications
Kumar, N., Noël, V., Planer-Friedrich, B. et al. “Redox heterogeneities promote thioarsenate formation and release into groundwater from low arsenic sediments.” Environmental Science & Technology 54(6), 3237–3244 (2020). [DOI:10.1021/acs.est.9b06502]

Topic Areas:

Division: SC-33.1 Earth and Environmental Sciences Division, BER