ABSTRACT
Anywhere that natural water comes into contact with solid surfaces, some of the dissolved ions adhere, or adsorb, to the solid. In soil or sediment, in which the small particles provide a particularly large surface area, the amount of adsorbed ions per unit of volume greatly exceeds the amount in solution. This is illustrated in Figure 13.1, which shows a cube of soil water 10 nm across containing four small idealized clay crystals. The 33 000 water molecules are not shown, but cations (mainly Ca2+) are shown both in the water and adsorbed to the surface of the crystals. The drawing is to scale, and is accurate for the adsorbed ions. However, it exaggerates the dissolved concentration of cations: four are shown, while a typical soil water of this volume would only have a 0.25 probability of containing a single ion at any given time.
