Source zone remediation by air sparging and soil vapour extraction

Anil Waduge, Cambridge University
Geotechnical Engineering Group


Non aqueous phase liquids (NAPLs), which exist as a separate phase in the subsurface, have become a major environmental threat. They are of particular interest because of their high toxicity and persistence. Therefore, attention has been directed at the remediation of non-aqueous phase liquids, which can constitute a source zone, in order to reduce risk to both the environment and humans.

The lack of knowledge in source zone remediation increases the need for the investigation of source zone behaviour with remediation techniques in order to design an effective treatment technology. Therefore, this dissertation performed physical and numerical modelling of source zone remediation by air sparging to investigate the interaction between air sparging and light non-aqueous phase liquids (LNAPL), entrapped in the subsurface. Toluene was used as the LNAPL in this research.

A series of 16 one-dimensional column experiments were performed to investigate the effects of flow interruption, flow rate and different entrapments on source zone mass removal. The column experiment results were used to understand the variation of mass removal efficiency with LNAPL saturation in the soil. Furthermore, the column experimental results were extended to develop a phenomenological model for the mass transfer coefficient from pooled LNAPL to the gaseous phase.

Seven well-controlled two-dimensional tank experiments were conducted to examine the effect of different entrapments, fluctuation of water table and subsurface heterogeneity on mass removal efficiency from a LNAPL source zone by air sparging. In this study, two scenarios were considered; (i) a coarse sand lens in a fine sand matrix and (ii) a fine sand lens in a coarse sand matrix. The tank experimental results reveal that the subsurface heterogeneity greatly controls mass removal from the source zone by air sparging. It governs the pattern of the air plume in the subsurface that determines the degree of air-NAPL contact area across which mass transfer occurs. Finally, a comparison between column and tank experimental results allows the issue of up scaling on physical modelling to be addressed.

The reduction of contaminant concentration and percentage of mass removal may not be good measurements for evaluating the effectiveness of a treatment method. The mass flux approach, which evaluates the post-remediation risk, can be used to address the removal efficiency instead. Therefore, variation of gaseous and aqueous concentrations in the soil model before and after remediation was monitored. Results clearly show that phase concentrations increase from the value at the end of the treatment, and the level of increase varies according to the LNAPL pool entrapments. Furthermore, a finite element code, FEMLAB, was used to model the mass partitioning before and after remediation, and hence typical mass partitioning coefficients were determined for different configurations of NAPL entrapments.

Finally, the prediction capability of a multiphase flow numerical code, TOUGH/T2VOC, was evaluated by simulating several tank experiments. Simulation results show that the predictions for drainage and LNAPL entrapments display a reasonable agreement with experimental observations. However, the numerical simulator which was developed on the classical multiphase transport and local equilibrium interface mass transfer theories overpredicted the actual mass removal during the experiments. Several modifications were proposed to improve predictions of mass removal by air sparging.