Abstract

The remediation of non-aqueous phase liquid (NAPL) using conventional aquifer treatment technologies is limited by the low solubility of NAPLs. Surfactants can promote the enhanced removal of NAPL through mobilisation, a mechanism that relies on the reduction of interfacial tension (IFT) at the flushing solution/NAPL interface. The conditions governing mobilisation can be represented by the total trapping number (N T ), a dimensionless quantity relating viscous and buoyancy forces to the capillary forces trapping the NAPL residual. A highly reliable experimental apparatus and procedure were developed to study the effect of flow rate and surfactant concentration on the removal of a residual NAPL. Column studies were conducted to investigate the relative flushing efficiencies of very dilute Triton X-100 solutions delivered through Ottawa sand spiked with light white mineral oil. At the higher flow rate, the surfactant solutions yielded N T values greater than the critical N T , promoting greater NAPL recovery as IFT dropped. While NAPL removal at the lower flow rate should not be enhanced because the critical N T was not surpassed, variations in mineral oil recovery during flushing clearly indicate a surfactant effect. At the lower flow rate, the surfactant-induced enhancement and retardation of NAPI, removal both highlight the limitations of the N T approach. For more realistic systems where free product NAPL is present, the total trapping number approach requires further study to define its applicability as an indicator for NAPL mobilisation