The thin layer of soil on the Earth’s surface has major environmental and socioeconomic impacts. The soils sustain our life and society, and do not act only as a growth medium for food, feed and fuel. Soil is an invaluable resource and the processes within it are responsible for waste disposal, purification and recharge of groundwater aquifers. Many environmental contaminants are broken down through their passage through the soil and slow percolation through the soil cleanses rainwater before it reaches the groundwater. Strongly sorbing environmental contaminants cannot be assumed to be immobile, but via colloid-facilitated transport they may bypass the purification stage and be leached to the groundwater reservoir and via tile drains to surface waters.

The main focus of the work presented in this thesis as part of the SOIL-IT-IS project was to increase knowledge about the processes controlling mobilization and transport of colloids. This was done using a series of state-of-the-art methods, including end-over-end dispersion measurements of soil aggregates and undisturbed soil cores, and irrigation experiments using undisturbed soil cores. In addition, a novel laser diffraction method was used to measure soil dispersibility.

Soils for this project were sampled from a wide variety of fields from five countries across different moisture and temperature regimes, with different contents of clay and organic carbon, and subjected to different management practices.

The use of laser diffraction in combination with a wet-dispersion unit proved to be an accurate and highly efficient method for measuring colloid dispersion from aggregates. The method gave highly detailed temporal data, enabling more detailed studies of dispersion kinetics. The contents of water-dispersible colloids measured with this method gave a remarkably good correlation to basic soil parameters across all the investigated sites. The results indicate that the soil water, clay, and organic carbon contents are by far the most important drivers of colloid mobilization. The effect of management practices and cropping rotation was mainly seen as an impact on soil organic carbon.

Results from the column leaching experiments from three sites likewise indicate that basic soil properties, such as the clay content, were the main drivers of colloid mobilization and transport. Effects of management and cropping system seemed secondary in this respect. A close correlation was observed between the amount of water-dispersible colloids – that is the potentially mobile fraction ‒ and the amount actually leached.

Visualization techniques using micro-CT scanning and confocal microscopy was applied to characterize internal and surface properties of soil aggregates, as well as to better characterize the water-dispersible part of the clay fraction. Prospectively, these methods can provide valuable information about soil structure formation and provide information that could lead to a more detailed risk assessment based on clay mineralogy.