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Adsorption to pipe wall materials significantly affects the fate and transport of certain contaminants in water distribution systems. For example, arsenate has a strong affinity for iron oxide, a substance common in water distribution pipes. In this paper a mathematical model for arsenate adsorption to iron oxide pipe wall materials is developed. The effects of two common assumptions on modeled arsenate transport are explored: a theoretical smooth pipe mass transfer coefficient and an assumption of rapid equilibrium of adsorption at the pipe wall surface. The effects of these assumptions are explored in a single pipe sensitivity analysis and found to yield significantly different results than parameters estimated from experimental data. In simulations of a hypothetical arsenate contamination event in a model water distribution system, the two assumptions result in different predictions of system contamination and contaminant exposure to consumers. These results indicate that even though water quality modeling plays an essential role in planning for distribution system decontamination, modeling assumptions must be carefully chosen.