PNNL

The Science

Global change processes modify the extent and functions of the transitional zones between wetlands and uplands—terrestrial–aquatic interfaces (TAIs)—in both marine and freshwater ecosystems. How fluctuating water levels alter groundwater biogeochemistry remains to be explored at these TAIs. This study found that short-term water level changes can significantly alter the redox state of groundwater, especially in transitional areas. Higher groundwater redox potential was observed in uplands than in wetlands. Further, this study revealed more frequent oxidizing states in transitional areas and wetlands than in uplands. Rapid and infrequent water table fluctuations due to the lower elevation of transitional areas and wetlands caused more temporary increases in redox potential, significantly increasing the overall redox state. The redox-oxygen relationship in this study displayed redox hysteresis. This means the rate at which redox potential increased was more rapid with oxygen inputs than the rate at which it decreased when oxygen was consumed. These findings help improve models predicting how climate change affects groundwater chemistry.

The Impact

This research explores how changes in groundwater levels affect the chemistry of underground water, especially in areas where land meets water, like wetlands. This is important because it helps us understand how climate change could affect water quality and the environment. This study is one of the first to use high-resolution data to capture rapid instances of significant redox changes in real time. This highlighted permanent and activated ecosystem control points for modeling ecological functions of the TAI. Permanent control points show redox states from upland to wetlands, representing landscape variance in a model using nested sub-grid areas. Activated control points are temporary, arising from processes and mechanisms that dominate biogeochemical transformations. The diverse range of redox values can support reactive transport models and validate recent advancements in predicting groundwater redox status. This research allows other scientists to better predict environmental changes and could affect fields like environmental science, hydrology, and climate science.

Summary

Researchers investigated how short-term fluctuations in groundwater levels influence subsurface redox potential across TAIs from wetlands to uplands. High-temporal-resolution data were collected along wetland-to-upland gradients and during fluctuating water levels in freshwater coastal areas to examine spatiotemporal patterns of groundwater redox potential. The findings reveal that topography influences groundwater redox, with higher values observed in uplands than in wetlands. However, high variability within transitional TAIs complicated the redox zonation. Although declining water levels reduced groundwater redox in most locations, there was an increase in redox variability. This was likely due to rare, short-term water level fluctuations introducing oxygen and other oxidant agents like nitrate. The redox-oxygen relationship displayed redox hysteresis, maintaining more oxidizing states longer than dissolved oxygen. Unexpectedly, this study found more frequent oxidizing conditions in transitional zones and wetlands than in uplands. These observations suggest that occasional oxygen inputs at wetland–upland boundaries serve as critical control points for biogeochemical processes. High-resolution data can capture these rare but essential biogeochemical events, contributing to redox-informed models and improving our ability to predict climate change feedback.

Contacts

• Vanessa L. Bailey, Pacific Northwest National Laboratory, vanessa.bailey@pnnl.gov, COMPASS Principal Investigator
• Fausto Machado-Silva, University of Toledo, fausto.silva@utoldeo.edu, Corresponding Author

Funding

This research is supported by Coastal Observations, Mechanisms, and Predictions Across Systems and Scales, Field, Measurements, and Experiments (COMPASS-FME), a multi-institutional project supported by the Department of Energy (DOE), Office of Science, Biological and Environmental Research program as part of the Environmental System Science Program. Pacific Northwest National Laboratory led this project, operating through Battelle Memorial Institute for DOE. This work was also supported by the Smithsonian Environmental Research Center. Thank you to the team staff of the University of Toledo, Old Woman Creek National Estuarine Research Reserve, and Ottawa National Wildlife Refuge.

Related Links

COMPASS Project page