Incorporating Sorption Into Organic Wastewater Contaminant Fate and Transport Modeling for Soil Systems

Conventional onsite wastewater treatment system designs rely on a soil treatment unit (STU) for further treatment of effluent and integration of final effluent in to the environment. Organic water contaminants (OWCs), chemicals found in pharmaceutical drugs, detergents, surfactants, and other personal care cleaning products, have been observed in STU influent and the environment. The compounds can have a wide range of ecotoxicological effects. To maximize treatment of these compounds in an STU, fate and transport behaviors must be adequately characterized. Sorption of OWC mass to soil particles is a key mechanism in the removal and retardation of many of these compounds in effluent as it travels through an STU. Sorption models utilized in peer-reviewed publications assume that the soil-water partitioning coefficients (Kd) for these compounds are linearly related to the organic carbon content (foc) of the soil material using an organic carbon partitioning coefficient (Koc). Literature reported Koc values range over orders of magnitudes for a single compound. This suggests that the Koc-foc method of estimating sorption may not be valid. The purpose of this study was to investigate the relationship between the foc of a soil and the equilibrium sorption of a selected group of relevant and diverse OWCs and to evaluate current methods of modeling the sorption of selected OWCs in soil. Five point Freundlich isotherms were constructed from equilibrium sorption batch tests for target OWCs with five different soils. Results from the sorption isotherm batch tests indicated that Kd may be confined to a range that is realistic for modeling upper and lower treatment boundaries for a variety of soil using information from an empirical relationship based on the fraction of organic carbon at the site and previously observed Kd values.