Permeability changes during remediation of an aquifer affected by sea-water intrusion: A laboratory column study

During flushing of a seawater-affected aquifer with freshwater, fine particles can be mobilized and can subsequently clog up pores, thereby causing a reduction of permeability. Here, we used packed columns (25 cm length) to experimentally investigate the changes in permeability during such a remediation scenario. We hypothesized that a gradual and abrupt change in salinity, as well as the pressure boundary conditions along the column, will affect the particle release and permeability changes. We collected sediments from a coastal aquifer in China, in which seawater had intruded. The sediments were packed into laboratory columns. The columns were oriented horizontally and instrumented with a series of piezometers to measure hydraulic heads along the column. Several sequences of displacements of seawater (electrical conductivity (EC) 48.5 mS/cm) with freshwater (EC 0.78 mS/cm) were passed through the columns. Permeability was determined from measured flow rates and hydraulic heads. Experiments were conducted both with constant head as well as with constant flux conditions. Column outflow was analyzed for suspended particles and electrical conductivity. The results indicate that the permeability was reduced by up to 70% when seawater was displaced with freshwater. Under constant head conditions, permeability reductions were more pronounced than under constant flux. The critical salinity (i.e., electrical conductivity associated with the critical salt concentration) for particle release in the sediments was 7 mS/cm. When salinity was changed abruptly, fine particles were eluted from the column; however, when the salinity change was gradual, no fines were eluted. In agreement with previous literature data, we observed a more pronounced reduction of permeability under an abrupt than under a gradual salinity change. Permeability reduction was not uniform along the column, with generally decreasing permeability with increasing distance from the column inlet.