A combined water infiltration and chemical fate and transport model was constructed to simulate the transport of a sorbed chemical present at a low concentration (mimicking the residual amount of chemical left in soil after clean-up) from soil to groundwater in the presence of competing solutes. A multisolute equilibrium model, the ideal adsorbed solution theory, was used to predict sorbed and aqueous concentrations for both the primary chemical and a secondary competitor. The objective was to analyze the extent to which including competition induced desorption in the transport model would change soil clean-up standards required to meet a particular water concentration goal. The presence of competitors caused a decrease in the allowable concentration of a chemical sorbed to soil, increasing the level of clean-up required. The magnitude of the competitive effect on the transport of a sorbed contaminant through the soil column depended on the sorption affinity of the sorbent, on the properties of the competitor such as octanol–water partition coefficient (KOW) and also on the concentration of the competitor, with the largest effects observed for competitors of similar sorption affinity and for the highest competitor concentrations.

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