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9 . Ion Exchange Capacity <br /> Porous geological materials that are composed of an <br /> appreciable percentage of colloidal-sized particles have the <br /> capability to exchange ionic constituents adsorbed on the <br /> particle surfaces . The tendency for adsorption of cations or <br /> anions is dependent on the pH of the solution present in pore <br /> spaces . The most important cation exchange reactions in <br /> groundwater systems are those involving monovalent and <br /> divalent cations . The Na+-Caz+ exchange reaction is of special <br /> importance when it occurs in montmorillonitic clays (smectite) <br /> because it can cause large changes in permeability. <br /> Therefore, ion exchange capacity, and its relationship to <br /> anticipated pore fluids, must be investigated for clays and <br /> soils proposed to be used as impervious liners . <br /> D-5 <br />