The objectives of this project described are to investigate how various elements which contribute to the heterodispersity of GAC systems impact the adsorption of DOM and the subsequent adsorption of target contaminants. These elements include the chemical and physical polydispersity of the DOM, the chemical and physical heterogeneity of the adsorbent materials, and solution chemistry. Understanding how system heterogeneity affects preloading and the subsequent adsorption of target contaminants will form the basis for: the rational selection of pretreatment technologies; the specification of optimal adsorbent properties; and the selection of appropriate modeling approaches. The research reported here will focus on two elements of system heterogeneity: the impacts of macromolecule size and solution chemistry; in particular, the effects of ionic strength and calcium.