Ion chromatography of fresh- and salt-water displacement: Laboratory experiments and multicomponent transport modelling

Abstract

Column experiments with fresh- and salt-water displacements were performed to obtain information on multicomponent ion exchange at varying concentration levels as well as to validate a multicomponent transport model. During salt-water displacement by fresh groundwater, a three times larger dispersivity was observed in the interface than for the reverse situation. Experiments with synthetic solutions with varying Ca/Na ratios indicate that solution density differences and chemical interaction between displacing solution and exchange complex determine the change in dispersivity. Using calculated selectivity coefficients, covering a broad composition range, and dispersivity values as input, a geochemical mixing cell model was applied to simulate the displacement experiments. The results show that fresh-water displacement by once diluted seawater can only be modelled when the Gapon selectivity of Ca over Mg decreases by a factor of two during the breakthrough.