The effect of layering on the definition of aquifer dispersivity

When analyzing solute transport in groundwater, it is necessary to introduce a variable coefficient of dispersion. It has been established that dispersion is affected by the medium, filtration rate and the size of the area. It has also been determined that an increasing travel distance increases the value of the coefficient of dispersion in the model/calculation applied. Horizontal heterogeneity and vertical layering of an aquifer make the definition of dispersion more difficult. This is particularly true of alluvial sediments, where sedimentation occurs under varying conditions and results in layering within a single lithofacial unit. In the analysis of substance migration through a layered aquifer using the conventional dispersion equation, a model of the hydraulically equivalent homogeneous aquifer was applied, including the concepts of hydraulically equivalent aquifer and coefficient of equivalent geometric dispersivity. This procedure is often applied in practice. Calculation results indicate that if a real layered aquifer is substituted in this manner by an equivalent homogeneous aquifer, it is necessary to increase dispersivity with increasing model size, in order to achieve calibration. It was concluded that increasing transverse dispersion in a layered medium increases the possibility of an effective substitution of a real multi-layered aquifer by an equivalent homogeneous aquifer. The justifiability of this approach should be evaluated for every case study.