Studying the movement of fluid through soils

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Developing novel imaging capacities to study the movement of fluids through soils

The movement of fluids, including water, gases, and nutrients, through soils is a critical process that governs soil health, plant growth, and ecosystem function. Understanding fluid dynamics in soils is essential for improving land management, water conservation, and predicting the impacts of climate change. Advances in imaging techniques have significantly enhanced our ability to visualize and quantify fluid movement through soils at various scales. This overview explores the pioneering imaging technologies that are being developed to study fluid transport in soils, focusing on their applications at different spatial and temporal scales and how they are improving our understanding of soil-water interactions.

Fluid movement in soils: an overview

Fluid movement through soils occurs through a combination of processes, including infiltration, percolation, capillary rise, and drainage. These processes are influenced by the physical properties of the soil, such as porosity, texture, and structure, as well as external factors like soil moisture, temperature, and the presence of vegetation. Understanding how fluids move through the soil is essential for fields ranging from agriculture (to optimize irrigation strategies) to environmental science (to predict contaminant spread and groundwater recharge). Fluid dynamics in soils are complex due to the heterogeneity of the soil matrix and the variability in pore structure, which can lead to preferential flow paths and significant spatial and temporal variability in water distribution.

Imaging technologies for studying fluid transport in soils

Recent advances in imaging technologies have made it possible to directly observe and quantify fluid movement through soils in ways that were previously not possible. These techniques enable the visualization of fluid flow at fine spatial and temporal resolutions, offering new insights into fluid dynamics and soil behaviour.

  • X-ray Computed Tomography (CT) Scanning: X-ray CT has become a key technique for imaging soil microstructure and visualizing the movement of fluids in 3D at high spatial resolutions. By scanning soil samples before and after fluid application, researchers can create detailed images of the internal structure of soils, including pore size distribution, connectivity, and the dynamics of fluid infiltration and redistribution (Liu et al., 2017). X-ray CT can also be used to track the movement of fluids such as water or solutes through the soil matrix in real-time, enabling the study of preferential flow, drainage, and capillary rise. This technique is particularly useful for studying the impact of soil structure and texture on fluid movement at the pore scale.
  • Magnetic Resonance Imaging (MRI): MRI is another powerful technique used to study fluid transport in soils. Unlike X-ray CT, MRI can non-invasively visualize the movement of liquids (e.g., water, oil) and gases (e.g., methane, carbon dioxide) through soil samples in real time. MRI is particularly advantageous for studying dynamic processes like water flow and gas diffusion, as it allows for continuous monitoring of fluid movement without disturbing the sample (Song et al., 2021). Additionally, MRI can capture soil-water interactions and quantify the moisture distribution within the soil, which is crucial for understanding the impacts of soil moisture variability on fluid transport.
  • Neutron Radiography: Neutron radiography is a technique that uses neutron beams to visualize the movement of fluids through porous materials, including soils. It is particularly useful for tracking water and other hydrogen-containing fluids in soils because neutrons interact strongly with hydrogen atoms. Neutron radiography can provide real-time, high-resolution images of water infiltration, redistribution, and drainage processes (Knudsen et al., 2016). This technique is non-destructive and can be used to study fluid flow in large, undisturbed soil columns, making it valuable for field-based applications and for understanding water movement in soils under natural conditions.
  • Synchrotron Radiation-Based Imaging: Synchrotron radiation, produced by particle accelerators, provides highly intense X-rays that can be used for high-resolution imaging of soil microstructures and fluid movement. Synchrotron-based techniques, such as X-ray microtomography (μCT), enable visualization of the pore structure in soils at micrometer or even nanometer scales. This high level of detail is crucial for understanding the interplay between fluid flow and soil architecture, especially in heterogeneous soils. Additionally, synchrotron imaging can be combined with other techniques, such as infrared thermography, to study the transport of heat and fluids simultaneously (De Roo et al., 2019).

Spatial and temporal scales of fluid movement studies

The movement of fluids through soils occurs across a wide range of spatial and temporal scales, and imaging technologies are evolving to capture these dynamics at different levels.

  • Microscale Imaging: At the microscale, imaging techniques like X-ray CT and MRI can be used to study fluid movement at the pore scale, where fluid flow patterns can be highly irregular due to variations in pore sizes, connectivity, and soil structure. This scale is critical for understanding how fluids interact with soil particles and how soil properties such as texture, structure, and organic matter content affect fluid dynamics. Recent developments in high-resolution X-ray CT imaging allow for the visualization of fluid flow through submillimeter pores, providing insights into pore-scale processes like water retention, preferential flow, and capillary pressure (Gao et al., 2018).
  • Field-Scale Imaging: At larger spatial scales, imaging techniques like neutron radiography and synchrotron-based imaging allow researchers to study fluid movement in large, undisturbed soil samples or in field conditions. These methods provide insights into the macroscopic patterns of fluid flow, such as water infiltration, lateral redistribution, and soil-water interactions over larger areas. Field-scale imaging is especially useful for understanding the effects of soil heterogeneity, land management practices, and environmental factors on fluid transport in real-world conditions.
  • Temporal Scales and Real-Time Monitoring: Temporal monitoring of fluid movement is essential to capture dynamic processes, such as transient flow events, diurnal changes in soil moisture, and the effects of rainfall or irrigation. Imaging techniques like MRI and X-ray CT are capable of capturing real-time or time-lapse images, providing valuable data on how fluid dynamics change over time. For example, MRI can be used to monitor the movement of water through soil over several days or weeks, capturing changes in moisture distribution, flow rates, and capillary rise (Song et al., 2021). Such temporal data is critical for understanding the impact of temporal fluctuations in water availability and for modeling soil-water interactions under varying climatic conditions.

Applications and implications

The ability to study fluid dynamics through soils at various scales has broad applications across multiple disciplines:

Agriculture

Understanding fluid movement is crucial for optimizing irrigation practices, especially in water-scarce regions. Imaging techniques can provide detailed insights into soil water retention, infiltration rates, and moisture distribution, helping to design efficient irrigation systems and manage water resources more effectively.

Environmental science

Imaging technologies play a key role in studying contaminant transport through soils. By tracking the movement of pollutants, researchers can better predict the spread of contaminants in groundwater and assess the effectiveness of soil remediation strategies.

Soil conservation

By understanding how fluids move through soils, researchers can identify soil management practices that reduce water runoff, prevent erosion, and enhance soil water retention, thereby improving soil health and promoting sustainable land use.

Climate change

Fluids, such as water and greenhouse gases (e.g., methane, carbon dioxide), play an important role in climate regulation. Imaging technologies can provide insights into the movement of gases through soils and their impact on soil carbon storage and greenhouse gas emissions, helping to predict and mitigate climate change.

Challenges and future directions

Despite the significant advances in imaging technologies, there are several challenges that remain:

  • Resolution and scale: While current imaging techniques provide high-resolution data, the resolution is often limited by the size and complexity of the soil sample or the imaging technique itself. The integration of high-resolution data from different scales remains a challenge in fluid transport studies.
  • Data interpretation: The large amounts of data generated by imaging techniques can be difficult to interpret, especially when fluid dynamics are influenced by complex soil properties and external environmental factors. Advanced data analysis techniques, such as machine learning, are needed to extract meaningful insights from these large datasets.
  • Cost and accessibility: High-end imaging techniques, such as synchrotron radiation or MRI, are expensive and often require specialized equipment. Making these technologies more accessible for field-based studies and lower-cost applications remains an important challenge.

Conclusion

The development of novel imaging technologies has revolutionized the study of fluid movement through soils, providing unprecedented insights into soil-water interactions at multiple spatial and temporal scales. Techniques such as X-ray CT, MRI, and neutron radiography enable researchers to visualize fluid flow at different scales, from the microscale to the field scale, and to study how soil properties influence fluid transport. These advancements have significant applications in agriculture, environmental science, and climate change research, and they hold promise for improving our understanding of soil processes and informing sustainable land management practices.

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