Influence of transport parameters and chemical properties of the sediment in experiments to measure reactive transport in seawater intrusion

A study of seawater intrusion under controlled laboratory conditions using multicomponent heterovalent chromatography is presented. The aim was to understand the influence of several variables on hydrogeochemical transport, especially on gypsum precipitation and cationic exchange. In addition, the study aimed to provide experimental data on how water composition changes during simulated seawater intrusion. The experimental results show that dispersion modifies the shape of the elution curves for the different solutes for processes such as cation exchange and precipitation–dissolution. The maxima and minima of these curves are very smooth, and entirely absent in several instances. Altering the cation exchange capacity of the sediment produced changes in the height of the calcium peak and in the maxima and minima of magnesium. In several experiments the high concentrations of calcium and sulfate during the first stages of the intrusion induced gypsum precipitation. The subsequent dissolution of the gypsum raised the concentration of sulfate higher than that in seawater. Saturation indices (SI) for gypsum in the samples collected were calculated with PHREEQC (version 2). Gypsum SI values are in agreement with experimental observations. Piper diagrams demonstrate that the experimental variables of transport parameters and cation exchange capacity (CEC) strongly influence the hydrochemistry of seawater intrusion. The experimental data deviate substantially from the theoretical freshwater–seawater mixing line, and the shape of the pathway between the end members depends on the experimental conditions. The experimental data obtained during column experiments and the physical and chemical parameters determined in each experiment can be used in the validation of multicomponent transport models. These hydrogeochemical models may aid in the interpretation of field data.