SOME CALCULATIONS ON MULTICOMPONENT TRANSPORT WITH CATION-EXCHANGE IN AQUIFERS

Multicomponent transport with cation exchange induces a separation of cations during displacement of waters in aquifers. The spatial separation is due to different retardations of the various cations which originate from differences in selectivity of the exchange complex. The composition patterns are complex, and calculated mostly with numerical models. However, a basic notion of features such as salinity fronts, chromatographic sequence, and front type is useful for general understanding of the processes. These aspects have not obtained the level of generality in hydrochemical studies that they deserve, and are introduced and applied in the paper. Demonstrated are: (1) the calculation of a multicomponent exchange complex from solution composition with the law of mass action; (2) the front type (broadening, indifferent, sharpening) that develops in relation to the slope of the sorption isotherm; (3) the calculation of a chromatographic pattern for the case of sharp fronts; (4) the effects of salinity changes on solution composition. All these features are illustrated with examples and then applied to calculate and compare water compositions in an observation well from two field studies, one of a salt-water injection which displays the salinity effects very clearly in accordance with theory, and one of a fresh-water injection for which the complete sequence of three ions Na+, Mg2+, and Ca2+ can be calculated simply with a quadratic equation.