Which phenomena combine in hydrodynamic dispersion?

Hydrodynamic dispersion is a process that describes how contaminants or particles spread in a fluid, such as water, due to the combined effects of different mechanisms. The correct answer, which includes all the listed phenomena, highlights the comprehensive nature of dispersion in fluid dynamics.

Molecular diffusion refers to the process where molecules move from areas of higher concentration to areas of lower concentration, driven by random thermal motion. This results in a gradual mixing of substances within the fluid.

Advection is the transport of particles or solutes by the bulk motion of the fluid. It is driven by the fluid's velocity and plays a critical role in moving contaminants downstream in a water body or through soils.

Mechanical dispersion occurs due to variations in flow velocity and direction within the fluid. These variations cause differences in travel times for particles, resulting in spreading. This phenomenon often arises from the heterogeneity in porous media, causing some particles to move faster or slower than others.

The combination of these three processes—molecular diffusion, advection, and mechanical dispersion—creates a complex picture of how substances are transported in water. Recognizing that all three phenomena contribute to hydrodynamic dispersion is essential for understanding and modeling contaminant transport in environmental engineering.