Understanding the effects of periodic freshening and salinization on coastal water quality and aquifer sediments through laboratory-based column experiments

The coastal aquifers experience periodic freshening and salinization events due to frequent changes in hydrological conditions. Such frequent changes have vital effects on the coastal groundwater quality and aquifer properties. This forms the basis of the present study. Laboratory-based column experiments have been conducted to investigate the long-term impact of such periodic events on coastal groundwater quality and aquifer sediment characteristics. Conditions similar to the natural coastal aquifer systems were generated in sediment-filled columns by injecting freshwater and seawater periodically. Changes in pore water chemistry and sediment properties were monitored over a period of one year. It has been observed that periodic injections of freshwater (freshening) and seawater (salinization) caused significant changes in the pore water chemistry, mainly due to sediment-water interactions. In addition to seawater-freshwater mixing, geochemical processes such as ion-exchange, mineral precipitation/dissolution, and adsorption/desorption of elements have been identified to control the pore water chemistry. Trace elements, including barium, boron, bromide, lithium, and strontium, have been identified as effective tracers for monitoring the saltwater intrusion. Further, the changes in pore water chemistry are found to be interlinked with the changes in aquifer properties. It is observed that with the increase in the number of salinization cycles, the effectiveness of the freshening process to bring back the original aquifer pore water chemistry diminishes, and the impact of salinization lasts longer. At the end of the experiment, changes in the porosity and permeability of the sediments have been observed. Such changes in the aquifer properties may affect the specific yield and storativity of the aquifer.