MODEL SIMULATIONS OF A FIELD EXPERIMENT ON CATION EXCHANGE-AFFECTED MULTICOMPONENT SOLUTE TRANSPORT IN A SANDY AQUIFER

A large-scale and long-term field experiment on cation exchange in a s andy aquifer has been modelled by a three-dimensional geochemical tran sport model. The geochemical model includes cation-exchange processes using a Gaines-Thomas expression, the closed carbonate system and the effects of ionic strength. Information on geology, hydrogeology and th e transient conservative solute transport behaviour was obtained from a dispersion study in the same aquifer. The geochemical input paramete rs were carefully examined. CEC and selectivity coefficients were dete rmined on the actual aquifer material by batch experiments and by the composition of the cations on the exchange complex. Potassium showed a non-ideal exchange behaviour with K-Ca selectivity coefficients indic ating dependency on equivalent fraction and K+ concentration in the aq ueous phase. The model simulations over a distance of 35 m and a perio d of 250 days described accurately the observed attenuation of Na and the expelled amounts of Ca and Mg. Also, model predictions of plateau zones, formed by interaction with the background groundwater, in gener al agreed satisfactorily with the observations. Transport of K was sim ulated over a period of 800 days due to a substantially attenuation in the aquifer. The observed and the predicted breakthrough curves showe d a reasonable accordance taking the duration of the experiment into a ccount. However, some discrepancies were observed probably caused by t he revealed non-ideal exchange behaviour of K+.