ADVECTION AND DIFFUSION IN A VARIABLE-SALINITY CONFINING LAYER

A numerical model that simulates groundwater flow and solute transport for cases in which fluid properties are variable was applied in one d imension (vertical) to the shallow, low-permeability, clayey, confinin g layer in Donana National Park in southwestern Spain. The salinity in the 80-m-thick confining layer decreases from a brine near the land s urface to fresh water near its base. Results of model simulations indi cate that the system could be in or close to a steady state condition. The model calibration was very sensitive to small variations in indiv idual model parameters and was nonunique in the sense that equally goo d calibrations could be achieved by compensatory joint perturbations i n the permeability, diffusion coefficient, and overall governing hydra ulic gradient. At present, there is probably an upward flow of the ord er of 1 mm yr-1 to 1 cm yr-1 and a balance in the solute flux between upward advection and downward diffusion. The time scale of calculated responses to changes in boundary conditions in this low-permeability s ystem ranges from thousands to hundreds of thousands of years when con sidering extremes within the range of uncertainty of values of the eva luated parameters.