EAR-PF Understanding the Effects of Incomplete Mixing on Mixing Corrosion: Pore-scale Visualization and Upscaling

Dr. Michael A. Chen has been granted an NSF EAR Postdoctoral fellowship to study mixing induced calcite dissolution and develop outreach materials at the University of Minnesota – Twin Cities campus with Professor Peter K.

Kang and Dr. Diana Dalbotten. Carbonate minerals, such as calcite, make up many natural rocks and soils, including many karst systems, and are also an important sink for carbon in the carbon cycle.

Removal of these minerals by dissolution drives the evolution of subsurface landscapes and results in carbon release back into the carbon cycle, and vigorous dissolution can be spurred by even the simple mixing of waters saturated with different sources of calcite. This specific process, called mixing corrosion, is fundamentally a soil pore-scale process and known as a key mechanism for karst formation, but we still do not understand how this small-scale process can lead to the larger scale patterns of dissolution in time and space.

Aquifer scale models used to study mixing corrosion typically assume that solutions are well mixed as soon as they encounter each other, however, this is not necessarily true at the smaller scale where incomplete mixing may leave solutions segregated, thus reducing the total calcite dissolution rate. Thus, the primary goal of this fellowship will be to experimentally study mixing corrosion of calcite at the microscale, and develop a framework for understanding how microscale flow, mixing, and chemical reaction affect calcite dissolution at larger scales.

This framework and the fundamental knowledge gained through these studies can be extended to understand how other microscale processes influence larger scale processes, which can be used to improve predictions of landscape evolution, develop predictive models of carbon sequestration, and enhance efforts to remediate groundwater contaminants. This project will also support minority involvement in Earth Sciences through recruiting and mentoring of interns from underrepresented groups in the Earth Sciences, as well as through outreach with the Science Museum of Minnesota to improve public understanding of flow and chemistry of rocks and soils using new demonstration experiments.

There is a continuing need to better understand how the coupled processes of flow and geochemistry alter the dissolution or precipitation of calcite for its relevance in carbon cycling and the development of natural geologic formations, particularly karst. This is best exemplified in the process of mixing corrosion, where two solutions that are equilibrated with a mineral (i.e. calcite), but differing amounts of mineral constituents (i.e.

CO2 and Ca), are mixed, resulting in an undersaturated solution that will dissolve calcite. While this process has been studied extensively with Darcy-scale models that assumes well-mixed conditions at pore scale, there are no reported observations of mixing corrosion at the pore scale, where incomplete mixing can strongly influence net dissolution by mixing corrosion.

There is also increasing recognition that flow and transport at the field or aquifer scale is impacted by pore scale flow and reaction, thus there is need for a framework that can integrate pore-scale processes into the larger scale models. Key among these processes is how incomplete mixing of solutions drives variation in geochemical reaction rates and transport of dissolved minerals.

Innovative microfluidic experiments and benchtop reactors using calcite minerals are used to study how parameters of flow and geochemical reaction influence mixing and subsequent dissolution of calcite. The experimental results will then be synthesized into a framework which is able to relate overall dissolution rate to the key parameters of flow, pore geometry, and geochemical conditions.

This framework lays the groundwork to then understand how coupled flow and geochemistry at the pore scale influences geochemical reaction at larger scales, which will be broadly relevant to many environmental processes.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.