Longitudinal Dispersion and Tracer Migration in a Radial Flow Field

Abstract

Hydrodynamic dispersion is an important factor controlling contaminant migration in the subsurface environment. However, few comprehensive data sets exist for evaluating the impact of travel distance and site heterogeneity on solute dispersion under non-uniform flow conditions. In addition, anionic tracers are often used to estimate physical transport parameters based on an erroneous assumption of conservative (i.e., non-reactive) behavior. Therefore, a series of field experiments using tritiated (³H) water and other commonly used hydrologic tracers, bromide (Br) and fluorinate benzoic acid (FBA), were conducted to evaluate solute transport processes in a diverging axisymmetric flow field. Tracer migration was monitored using a set of six, multilevel sampling wells concentrically spaced at distances from 2.0 to 4.5 meters around the injection well. Soil hydraulic parameters for aquifer matrix were inversely optimized and an average longitudinal dispersivity for ³H breakthrough was estimated using numerical finite-element code (i.e., HYDRUS-2D) capable of describing an axisymmetric diverging flow. Tremendous variation in tracer arrival times between similar sampling locations and multiple arrival peaks observed for some sampling locations were observed for both, ³H and Br. Migration of the Br was retarded, when compared to that of the ³H, and analysis of the anion data assuming conservative behavior would thus yield higher dispersivity values than conservative tracer.