Aggregation versus Dispersion: Opposing Effects of NaCl and NaHCO₃ Salinity on the Hydraulic Conductivity of Porous Media.

The long-term effects of saline water intrusion on the hydraulic conductivity of aquifers are a critical concern in hydrogeology. However, these effects are frequently generalized, the precise role of salt chemistry as it evolves over time remains insufficiently understood. This study investigates these effects through a 28-day laboratory experiment, where sand specimens were saturated with solutions of Sodium Chloride and Sodium Bicarbonate at concentrations from 2000 to 10,000 ppm. Hydraulic conductivity was measured weekly using a constant head permeameter and compared against uncontaminated control.The results showed that the behaviors were very different depending on the type of salt. NaCl, a neutral salt, causes clay aggregation, resulting in a "rise and fall" in permeability. At 21 days, hydraulic conductivity (K), which represents the rate of water flow through soil under a hydraulic gradient, reaches a peak corresponding to the salt content. NaHCO₃, an alkaline salt, increased clay dispersion due to its higher pH. At Day 7, small particles were washed away, resulting in a significant increase in k. However, internal blockage led to a gradual decrease. High NaHCO₃ concentrations (10,000 ppm) had a negative long-term effect, causing k values to fall below that of the control.This study shows that salt chemistry is the primary factor governing long-term hydrogeological behavior, shifting soils from structural strengthening through aggregation (NaCl) to structural weakening through dispersion (NaHCO₃). The findings highlight the limitations of short-term permeability tests and the need to consider specific ionic composition—not just overall salinity—when evaluating saline-affected aquifers.