Advancing micromodel and imaging capability to detect hotspots in the hyporheic zone
EMSL Project ID
51880
Abstract
Many microbially-driven reactions in groundwater only occur when dissolved oxygen concentrations are sufficiently low. In shallow, oxygen-rich groundwater systems like the hyporheic zone, microzones of localized anoxia can give rise to reactive hotspots. Current upscaled models of river corridor biogeochemistry assume average system parameters and consider these hotspot reactions to be thermodynamically possible only when bulk oxygen concentrations fall below the threshold for anoxia. Nevertheless, byproducts of anoxic and anaerobic reactions are commonly detected in bulk oxic sediments within the hyporheic zone, suggesting that bulk-unfavorable biogeochemical reactions are underpredicted when the system is assumed to be uniform and described well by its average (bulk) conditions. This evidence implies that river corridor-scale estimates of biogeochemical fluxes fail to capture a significant source of anoxic reaction products, including potent greenhouse gases and reduced metal contaminants. Unfortunately, little is known about the mechanisms that drive microzone formation and persistence, largely due to the challenges of directly measuring microscale heterogeneity within pore waters. This large-scale research proposal will support efforts to better understand the mechanisms controlling microzone formation in hyporheic sediments, in an effort to bridge the scale gap between difficult to observe microzone activity and continuum models needed to accurately estimate biogeochemical fluxes in river corridors. We propose to develop leading edge microfluidic devices (micromodels) representing oxygen saturated hyporheic sediments. These models will provide us with the unique ability to experimentally (a) track the spatial and temporal distribution of oxygen-depleted microzones (hot spots and hot moments), and (b) identify the processes controlling these distributions. Advanced microfluidic experiments will be developed at the single pore- and pore network-scales to quantify the intermittency of flow obstruction by biofilms (bioclogging) within the pore network. We will also integrate custom-designed oxygen sensors (optodes) into all micromodels to non-destructively determine the spatial distribution of anoxic microzones, as well as their temporal distribution in the pore network. These results will allow us to distinguish, for the first time, how biofilms dynamically modulate the formation of hotspots/hot moments across multiple scales in saturated sediments. Building upon current conceptual models that suggest that sediment heterogeneity and flow variability control anoxic microzone abundance, we hypothesize that microzones also evolve from the dynamics of bioclogging. The feedback mechanisms between fluid flow and biofilm growth promotes the formation of temporally unstable preferential flow paths that limit the distribution of oxygen in pore spaces until they shift to a new location. The continuous, yet temporally fluctuating, export of anoxic reaction products is therefore an emergent feature of many saturated sediments supporting biofilm forming bacteria. The experimental capability developed in partnership with EMSL will allow us to identify the biophysical processes and scales controlling reaction heterogeneity within the hyporheic zone. Additionally, it will position our team to provide physically-based parameterizations for analytical upscaled models capable of representing microheterogeneity at the sale of river corridors. The proposed work furthers EMSL's mission by providing biological and physical data associated with fine-scale hydrobiogeochemical heterogeneity within shallow groundwaters.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2021-10-01
End Date
2023-10-01
Status
Closed
Released Data Link
Team
Principal Investigator
Co-Investigator(s)
Team Members