In nature, matter is transported exclusively by means of a moving fluid. Thus, in geophysical systems pollutants are transported either by moving water or moving air. Without those fluids, any pollution source would remain local. The transport of matter is achieved in two ways:

1. In solution, when the matter is dissolved in the fluid. In this case, the molecules of the matter are mixed with the molecules of the fluid and retained in that condition, due to electrochemical forces acting at molecular scale. Any difference of density between the matter and the carrying fluid is not important for concentrations less than 1% (weight or volume of dissolved matter per unit volume of the solution).