- internal chemical and physical reactions
- external stresses from the surrounding environment
- physical degradation of the solid matrix due to erosion or cracking, and
- loss of matrix constituents due to the leaching process itself.
The process of leaching includes the partitioning of contaminants between a solid and liquid phase (e.g., assuming local equilibrium) coupled with the mass transport of aqueous or dissolved constituents. Mass transport is the summation of diffusion, hindered diffusion, tortuosity effects, and effective surface area effects through the pore structure of the material to the environment. Important chemical factors, those that influence the liquid-solid partitioning (LSP) of a constituent, include solution pH, redox, the presence of dissolved organic matter, and biological activity. Physical factors, such as relative hydraulic conductivity, porosity and fill geometry, play an important role in determining the rate at which constituents transport through a solid into a passing liquid phase.
The process itself is universal, as any material exposed to contact with water will leach components from its surface or its interior depending on the porosity of the material considered.