Hyperfiltration of sodium chloride through kaolinite membranes under relatively low-heads – Implications for groundwater assessment

Three relatively low-head hyperfiltration experiments were conducted to investigate clay membrane behavior at lower hydraulic heads. Dilute Cl− solutions (187 and 336 mg/L) were forced through thin layers of kaolinite (0.67–1.62 mm) under heads ranging between 1.45 and 1.89 m. At the end of each experiment, there was a Cl− concentration increase (15–29%) within the cell. The concentration increase is attributable to solute-sieving by the kaolinite. Calculated final concentration increases at the membrane face ranged between 1.51 and 1.65 times the initial concentration and the calculated values of the reflection coefficient ranged between 0.35 and 0.40. These experiments show that clays are capable of significant hyperfiltration effects at lower pressure heads more typical of natural aquifers. Consequently, hyperfiltration effects may need to be considered in a broad spectrum of shallow subsurface processes in which these effects have been regarded as inconsequential; particularly in perched aquifers.