Users` manual for LEHGC: A Lagrangian-Eulerian finite-element model of HydroGeoChemical transport through saturated-unsaturated media -- Version 1.0. The computer program LEHGC is a Hybrid Lagrangian-Eulerian Finite-Element Model of HydroGeoChemical (LEHGC) Transport Through Saturated-Unsaturated Media. LEHGC iteratively solves two-dimensional transport and geochemical equilibrium equations and is a descendant of HYDROGEOCHEM, a strictly Eulerian finite-element reactive transport code. The hybrid Lagrangian-Eulerian scheme improves on the Eulerian scheme by allowing larger time steps to be used in the advection-dominant transport calculations, causing less numerical dispersion and alleviating the problem of negative concentration, and it also is more computationally efficient. LEHGC is designed for generic application to reactive transport problems associated with such systems as contaminant transport in subsurface media. Input to the program includes the geometry of the system, the spatial distribution of finite elements and nodes, the properties of the media, the potential chemical reactions, and the initial and boundary conditions. Output includes the spatial distribution of chemical element concentrations as a function of time and space and the chemical speciation at user-specified nodes.

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