Physcial Sciences Division
Research Highlights

June 2016

From Inches to Miles: The Science of Scaling the Subsurface

Researchers review factors that influence the scaling of subsurface reactions, devise formulas that link rates between the lab and the field

The micromodels show the concentrations of the chemical FMNH₂ (top) and the iron-laden mineral hematite (bottom). Scientists can use concentrations from these pore-scale models to determine how the two substances would react on a larger scale. Reprinted from Geochimica et Cosmochimica Acta, Vol 163, Yuanyuan Liu, Pore and continuum scale study of the effect of subgrid transport heterogeneity on redox reaction rates, 140-155, Copyright 2015, with permission from Elsevier. Enlarge Image.

Will a reaction that quickly releases carbon in a test tube work at the same rate over acres of farmland? The answer depends on the scaling factor, a number that is used to increase or decrease the reaction's rate depending on the scale of observation. Four geochemists at Pacific Northwest National Laboratory (PNNL) devised a new way to calculate the scaling factor. Their method predicts how rates change from the lab to the field. In creating the formula, they reviewed decades of scientific studies delving into situations, such as microbial growth, that influence reaction rates.

Why It Matters: Reaction rate models predict how fast a toxin will move or how long nutrients will remain in farmland. These models could be more precise if they included scientific findings at the pore scale, which involves the tiny gaps between and inside soil particles. This study offers a way to apply pore-scale data to larger areas - from inches to miles. In addition, the geochemists' systematic review of the scale-influencing factors highlights the state of the science and future directions.

Methods: Creating more accurate models of diverse subsurface areas means considering the pore scale and how it influences larger domains. The team found that the conventional modeling approach, using a numerical grid and the law of mass conservation, doesn't accurately "upscale" results. They went on to devise a formula that allows them to calculate reaction rates at different scales. The formula uses the concentration and distribution of the reactants (the reaction's starting materials). They can then factor in other aspects of scaling, such as microbial growth that plugs the pores.

What's Next? Researchers at PNNL are continuing to obtain insights into basic biogeochemical processes in complex subsurface environments.

Acknowledgments

Sponsors: This research is supported by the Department of Energy, Office of Science, Biological and Environmental Research as part of the Subsurface Biogeochemical Research (SBR) Program through PNNL SBR Scientific Focus Area Research Project.

Research Area: Subsurface Science

User Facility: EMSL

Research Team: Chongxuan Liu, Yuanyuan Liu, Sebastien Kerisit, and John Zachara, PNNL

Reference: Liu C, Y Liu, S Kerisit, and J Zachara. 2015. "Pore-Scale Process Coupling and Effective Surface Reaction Rates in Heterogeneous Subsurface Materials." Reviews in Mineralogy & Geochemistry 80:191-216. DOI: 10.2138/rmg.2015.80.06