Shallow riparian aquifers represent hotspots of biogeochemical activity in the arid western United States.
Researchers used a high-resolution sampling approach to track subsurface biogeochemical shifts during an annual period of water table fluctuation in a riparian aquifer near Rifle, Colorado. This work provided the first description of microbiological and geochemical responses to increased dissolved oxygen concentrations within the aquifer across a six-month period, revealing how local sediment heterogeneity was responsible for dramatically different shifts in both groundwater chemistry and microbial community structures. Simultaneously, the implementation of a computational framework for predicting biogeochemical behavior throughout the site indicated that seasonal “background” subsurface processes and responses could be successfully modeled.
This work resulted in a new understanding of how biogeochemical processes in riparian zones may respond to seasonal hydrologic fluctuations linked to snowmelt discharge. While subsurface regions previously were assumed to be relatively stable ecosystems, the new data indicated that dynamic geochemical and microbiological shifts occur on annual cycles, with implications for carbon cycling, metal mobility, and contaminant sequestration. Additionally, the aquifer site near Rifle is a potential template for riparian aquifers throughout the semi-arid inner mountain western United States, allowing these results to be extrapolated across larger scales. The successful implementation of a modeling framework offers one such approach for this up-scaling and will enable a greater understanding of potential shifts in biogeochemical processes under future climate change scenarios.
Various regions of the aquifer responded differently to the snowmelt-driven hydrologic perturbation based on redox state, with dissolved oxygen penetrating deeply into oxidized regions, and being rapidly consumed via abiotic reactions in naturally reduced regions, liberating Fe2+ and U6+ species. Microbial community composition varied across spatial and temporal scales. During periods of elevated river stage associated with increasing dissolved oxygen concentrations in the aquifer, microbial community composition favored putative chemolithoautotrophs and heterotrophs, while putative fermenters within the candidate phyla radiation (CPR) were greatly enriched (e.g., members of the Microgenomates and Parcubacteria) during water table fall. Reactive transport modeling was able to capture the dynamic behavior of both the geochemistry and microbiology at the site during the fluctuating hydrology, suggesting that a predictive framework can be developed to better understand biogeochemical responses to future hydrologic dynamics.
David Lesmes, SC-23.1, 301-903-2977
Paul Bayer, SC-23.1, 301-903-5324
Michael J. Wilkins
The Ohio State University
This work was supported as part of the Genomes to Watershed Scientific Focus Area at Lawrence Berkeley National Laboratory, which is funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research under Award Number DEAC02-05CH11231.
Danczak, R. E., S. B. Yabusaki, K. H. Williams, Y. Fang, C. Hobson, and M. J. Wilkins. 2016. “Snowmelt Induced Hydrologic Perturbations Drive Dynamic Microbiological and Geochemical Behaviors Across a Shallow Riparian Aquifer,” Frontiers in Earth Science 4(57). DOI: 10.3389/feart.2016.00057. (Reference link).
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