On the Dagan model of solute transport in groundwater: Application to the Borden site

In the first part of this study, Sposito and Barry (1987) derived an ensemble-mean convection-dispersion equation (CDE) for tracer solute transport subject to a random velocity field. It was shown that the model dispersion coefficients originally presented by Dagan (1984) could be derived from the general expression for the dispersion coefficients in this mean CDE. Under the assumption of ergodicity, the Dagan model is used in this paper to predict chloride and bromide concentrations in the well- documented Borden aquifer experiment reported by Roberts and Mackay (1986). Because of a possible influence on the solute from the upper aquifer boundary, it was appropriate to apply the two-dimensional form of the model. A number of steps was necessary to reduce the three-dimensional raw data to a two-dimensional form, the main ones being integration over the vertical axis and the use of a gridding algorithm to form a two-dimensional solute concentration surface. Incomplete sampling of the solute plume during the early sampling sessions, as well as the assumptions made with respect to the data analysis, produce a rather large degree of uncertainty in the specification of the initial solute plume. These factors hinder a thorough experimental evaluation of the Dagan model. Data from some of the later sampling sessions were more complete, however, and the model predictions appeared to agree well with the field concentration data, especially in the preasymptotic region for the longitudinal dispersion coefficient.