The exchange of Sr2+, Mg2+, Na+, Li+, and NH4+ for Ca2+ on a sediment from the Vadose Zone Research Park at Idaho National Laboratory was characterized for use in modeling transport and remediation of the 90Sr-contaminated vadose zone at the nearby Idaho Nuclear Technology and Engineering Center. Cation exchange was measured at ionic strengths of 0.1 and 0.005 and a range of cation compositions. The Sr–Ca exchange on this sediment is nonpreferential (Vanselow selectivity coefficient KV = 1), whereas in NH4–Ca exchange NH4 is preferentially adsorbed (KV ≫ 1). The data were modeled using the Vanselow, Rothmund–Kornfeld, and Gapon (KG) exchange coefficients. The Gapon coefficient was the least dependent on exchanger-phase composition. The KG values were incorporated into a geochemical software program that includes aqueous- and solid-phase reactions. Modeled NH4+ and Ca2+ exchanger phase and solution composition were in good agreement with the experimental data at both ionic strengths.