Coupled and partially coupled Eulerian‐Lagrangian Model of freshwater‐seawater mixing

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Abstract
The problem of density‐dependent transport of salt in unconfined coastal aquifers is solved numerically by means of an implicit Eulerian‐Lagrangian finite element formulation. Such a formulation leads to symmetric positive definite finite element matrices which are ideally suited for efficient solution by preconditioned conjugate gradient methods. Additional known advantages of the formulation are unconditional stability, reduced numerical dispersion and suitability for parallel computation. The method has been used to study the effect of dewatering on seawater intrusion within a vertical cross section through an aquifer in southern Italy, related to the construction of a thermoelectric power plant. To investigate the extent to which the dependence of fluid density on salt concentration affects the numerical solution, the flow and advection‐dispersion equations were solved in coupled (iterative), partially coupled (noniterative) and completely decoupled modes. Partial coupling was found to yield results very close to those obtained by full coupling but at great savings in computer time; the less rigorous decoupled approach led to results substantially different from those obtained through coupling and partial coupling. Effects of aquifer heterogeneity and the construction of a cutoff wall on seawater intrusion are discussed.