Dispersivity estimates from a tracer experiment in a sandy aquifer

The success or failure of transport models in predicting the migration of a contaminant plume in ground mater depends to a large extent on the quality of flow and transport parameters used. in this study, we investigate the s patial variability in the tracer velocity and dispersivity in a shallow san dy aquifer in northern Belgium. Based on hydraulic conductivity measurement s on cores sampled along a vertical profile, the aquifer was found to be mi ldly heterogeneous, i,e,, with the variance of the log-transformed conducti vity K, sigma(lnK)(2), equal to 0.22. By means of a natural gradient tracer experiment, transport of a chloride tracer was investigated in a three-dim ensional network of multilevel point samplers (MLS). Least squares fitting of a two-dimensional transport model to the individual breakthrough curves resulted in an average Longitudinal dispersivity that was 10 times larger t han the transverse dispersivity. The results further showed the existence o f a dispersion-scale Effect whereby the depth-averaged longitudinal dispers ivity increases with increasing travel distance. The average longitudinal d ispersivity corresponding to a travel distance of 10 m was equal to 0.2 m, We finally show that theoretical expressions for the macroscopic dispersivi ty tensor, which require input on hydraulic conductivity heterogeneity, cou ld be used here to approximately the observed dispersive behavior. These co nceptually simple models are useful to estimate macroscopic dispersivities when no tracer data are available.