Influence of Grain-scale Geometry on Colloid Transport

Steven L. Bryant

Funding source: U.S. Department of Agriculture

Funding amount: $246,000 for the period of Sep. 2004 - Aug. 2007

Water resources and environmental quality are important issues for agriculture. A key component of water quality is the concentration of very small (colloidal) particles suspended in the water. Example particles are bacteria, viruses and inorganic materials on which contaminants have adsorbed.

 Influence of Grain-scale Geometry on Colloid TransportColloids can be trapped or retained by soil grains as water carries them through the soil, with profound implications for water quality. Historically this retention is attributed to two mechanisms, filtration and straining. But many researchers have reported behavior that does not fit this classification: colloids theoretically too small to be strained are nevertheless retained, even when filtration is negligible. This project seeks to explain these observations.

The hypothesis underlying this research is that colloid straining occurs not just in pore throats but also in small gaps between pairs of grains. To test the hypothesis, this project will develop a quantitative analysis of the geometry of these gaps. The approach is predictive, relying on a novel class of model soils that are geometrically determinate and physically representative. We will adapt techniques for computing flow through the pore space of the model soils to determine fluxes through individual gaps. Incorporating the local distribution of flow through gaps into an existing theory of straining will enable testing the model against experiments reported in the literature. Because the approach involves no adjustable parameters, the results will contribute to the scientific basis for water resources stewardship.