Migration of contaminants in groundwater at a landfill: A case study: 5. Cation migration in the dispersion test

Abstract

A shallow natural-gradient tracer test was conducted in an unconfined sandy aquifer at the Borden landfill site in June 1978. The tracer injected was a chloride solution containing approximately equimolar concentrations of the major cations at levels significantly above those in the background groundwater. Samples collected during monitoring of the tracer were analysed for calcium, magnesium, sodium and potassium. The results showed an order of attenuation for major cations of K > Mg > Na. Ca, which was the dominant species on the exchange sites before injection of the tracer solution, was liberated during flow of the solution. This resulted in the development of a calcium halo at the leading edge of the tracer solution during flow. A one-dimensional mixing-cell model which includes a chemical equilibria subroutine to appraise the effects of major-ion complexing, calcite dissolution and precipitation, and cation exchange was used to simulate the results of the injection experiment. Both the field and model results show that the chemical composition of the background groundwater is changed due to exchange reactions as it flows in the path of the injection solution. These influences continue until all the exchange sites along the flow path re-equilibrate to their former pre-injection values by the passage of background groundwater. The mixing-cell model was calibrated to the dispersive characteristics of the aquifer by varying average groundwater velocity, mixing-cell size and dispersivity to match the chloride distribution. Model results provide simulated cation distributions which are in close agreement with the field results including a “Ca-halo” produced due to the liberation of Ca 2+ from exchange sites. The migration of Na + in the flow system is observed to closely parallel the movement of Cl − although the maximum concentrations are decreased over the concentration at injection. Mg 2+ migrates downflow at a rate slower than Cl −, and at a much reduced concentration reflecting a strong tendency for sorption onto exchange sites. Both model and field results show pronounced K + attenuation during flow of the injection solution. Concentrations throughout the flow system during the monitoring program were always very close to background values. The bulk of the injected potassium was sorbed very close to the injection wells and is slowly liberated by interaction of background groundwater following the passage of the injection solution.