Mixing between fresh and salt waters at aquifer regional scale and identification of transverse dispersivity

In aquifers in which freshwater flows above saltwater, a mixing layer develops between the two water bodies. In a typical regional aquifer, this mixing layer is thin compared to the length scale of the aquifer. Its modeling by available numerical codes is impractical due to the needed fine discretization. Here, an approximate model of the mixing layer in steady state 3D flow is developed, based on the boundary layer approach. At first, mixing is neglected and a sharp interface solution is derived. Subsequently, the flow and mixing equations are rewritten in a curvilinear coordinates system, attached to the sharp interface solution. In line with the boundary layer approximation, only transverse dispersion is considered. A simplified solution for the mixing layer is obtained by assuming similarity and using von Karman integral method. The approach is demonstrated for Yarkon-Taninim basin (Israel), a Karstic aquifer extending over 6000 km². The main aim of the research was to identify the regional scale transverse dispersivity for the aquifer. The determined value was α_T ≃ 0.04 m. This is an important finding, as it is the first time the parameter is evaluated for an aquifer at regional scale.