Journal of Contaminant Hydrology, 1 (1986) 77--93 77 Elsevier Science Publishers B.V., Amsterdam --Printed in The Netherlands INTERPRETING ORGANIC SOLUTE TRANSPORT DATA FROM A FIELD EXPERIMENT USING PHYSICAL NONEQUILIBRIUM MODELS MARK N. GOLTZ and PAUL V. ROBERTS Department of Civil Engineering, Stanford University, Stanford, CA 94305 (U.S.A.) (Received December 20, 1984; revised and accepted September 13, 1985 ) ABSTRACT Goltz, M.N. and Roberts, P.V., 1986. Interpreting organic solute transport data :'-":,m a field experiment using physical nonequilibrium models. In: D. L. Macalady (Editor), Transport and Transformations of Organic Contaminants. J. Contain. Hydrol., 1: 77-- 93. In a field experiment, two inorganic tracers and five organic solutes were injected into an unconfined sand aquifer. Breakthrough response curves were obtained at several points downgradient of the injection zone. These response curves are analyzed using a model which assumes equilibrium sorption and two models which postulate physical nonequilibrium. The physical nonequilibrium models hypothesize the existence of zones of immobile water, which act as diffusion sources and sinks for the solutes. The physical nonequilibrium models better simulate the sharp breakthrough and extended tailing exhibited by the experimental responses than does the model assuming equilibrium sorption. The reasonableness of parameters obtained from curve-fitting the data is assessed. The two physical nonequilibrium models are compared. INTRODUCTION Transport of hydrophobic organic chemicals by groundwater has been traditionally described using the advective/dispersive transport equation with a term to account for sorption of the organic solute onto the soil matrix. This sorption term is often developed assuming local equilibrium and a linear, reversible relationship between the quantity of chemical in the adsorbed and dissolved phases. Several investigators have found, in laboratory column studies, that the nearly symmetric, sigmoid forms of breakthrough curves predicted using models making these simplifying assumptions regarding sorption, do not agree with experimental observations (Van Genuchten and Wierenga, 1976; Rao et al., 1979; De Smedt and Wierenga, 1979). Experimentally observed breakthrough responses often exhibit highly asymmetric or nonsigmoid profiles, which will be referred to as t~iling. Either physical or chemical phenomena may cause tailing. That is, for physical and/or chemical reasons, one or more of the linear, reversible, equilibrium sorption assumptions may not hold, resulting in the inappli- cability of the advection/dispersion model. 0169-7722/86/$03.50 © 1986 Elsevier Science Publishers B.V.