The active concentration of a pesticide declines with time due to volatilization, absorption and translocation, microbial degradation, adsorption on soil particles and plant parts, chemical reactions, ...

Diffusion is the movement of particles from a region where they are in high concentration to a region where they are in low concentration, and is one of the ways substances can move across the cell ...

Diffusion is the net movement of molecules from an area where they are at a higher concentration to areas where they are at a lower concentration. This is due to the random movement of the molecules.