Technical Abstract: Leachate derived from coal ash disposal facilities is a potential anthropogenic source of arsenic and selenium to the environment. To establish a practical framework for predicting attenuation and transport of As and Se in ash leachates, the adsorption of As(III), As(V), Se(IV), and Se(VI) had been characterized in prior studies (Burns et al., 2006; Hyun et al., 2006) for 18 soils obtained down-gradient from ash landfill sites and representing a wide range of soil properties. The constant capacitance model was able to describe adsorption of these ions on all soils as a function of solution ion concentration by optimizing only one adjustable parameter, the anion surface complexation constant. This chemical model represents an advancement over adsorption isotherm equation approaches which contain two empirical adjustable parameters. Incorporation of these anion surface complexation constants into chemical speciation transport models will allow simulation of soil solution anion concentrations under diverse environmental and agricultural conditions.