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Basically ground water tends to evolve towards the composition of seawater as it ages <br /> (Freeze and Cherry, 1979, pp. 241-242). Bromide is the fourth most common anion <br /> in seawater (Mason and Moore, 1982, p. 235). The prevalence of chloride and <br /> bromide in seawater is because almost all compounds of these ions are,soluble. The <br /> average residence time for bromide in the ocean is one hundred million years (Mason <br /> and Moore, 1982, p. 236). <br /> 8.0 LACK OF ADSORPTION OR ION EXCHANGE <br /> Davis et al. (1985, p. 87) state that anions are rarely affected by the aquifer medium. <br /> Neither adsorption nor ion exchange are expected to significantly decrease <br /> concentrations of bromide. Adsorption is any process by which molecules or ions <br /> adhere to the surface of a solid. Ion exchange occurs when ions in solution change <br /> places with ions at a solid surface. Ions which are adsorbed onto the surface are the <br /> most likely to be affected. <br /> Although certain natural geologic materials, especially colloidal-sized particles such as <br /> clays, may exchange ionic constituents, the likelihood of a particular constituent being <br /> exchanged depends upon such factors as pH and the valence of the involved ions <br /> (Freeze and Cherry, 1979). The charge on the surface of a colloidal particle may be <br /> negative, in which case only positive ions (cations) can be adsorbed, or the charge on <br /> the surface may be positive, in which case only negative ions (anions) can be <br /> adsorbed. For most colloidal particles in the geological environment, the surface will <br /> IAAD MIN\WP\DIAN E\BRO MIDE.RPT 6 <br /> Hal HYDRO-SEARCHAG A Tetra Tech Company <br />