Experimental study on the effect of ion exchange on solute transport in a sandy tank

Objective To study the transport of pollutants in typical hydraulic sedimentary units such as river terraces or alluvial fans, solute transport experiments were carried out in an indoor seepage tank. Methods NaNO3 solution were introduced into the tank to simulate the point-pollution in hydraulic sediments. By measuring the concentration of the main ion components at different positions over time, the migration law of pollutants and the ion exchange process are analysed. Results The results show that NO32- is a conservative ion, and its breakthrough curve (BTC) is sharp and thin. The transport behaviour of Na+ is significantly affected by cation exchange, its concentration rising sharply and decreasing slowly. Cation exchange reduces the dispersion of Na+, and the effect becomes more obvious as the distance increases. In the early stage, the high concentration of Na+ can exchange Ca2+, Mg2+, and K+ in the sand layer. Cation exchange reduces the Na+ dispersion concentration. Due to the adsorption by sediment, the concentrations of Ca2+, Mg2+, and K+ will be lower in the later stage. The change in the reaction direction of cation exchange makes the BTCs of Na+ wider and gentler under the action of advective dispersion, and the phenomenon of "tailing" is more obvious. The water chemistry types in different areas in the seepage sank have different properties in space. Conclusion The research results have guiding significance for preventing and controlling groundwater pollution in hydraulic sedimentary units.