A LABORATORY STUDY ON THE EFFECTS OF DISSOLVED GAS EXSOLUTION ON THE PERFORMANCE OF IN-SITU THERMAL TREATMENT TECHNOLOGIES

Abstract

In-situ thermal treatment (ISTT) technologies are a set of remediation methods for treating chlorinated volatile organic compound (CVOC) dense non-aqueous liquid (DNAPL) source zones by increasing the temperature of the subsurface. However, the efficiency and effectiveness of the treatment can be greatly reduced by high groundwater flow as a result of convective heat losses. Dissolved gas exsolution can generate gas below the boiling point of water and may potentially reduce the aqueous relative permeability to mitigate cooling from groundwater flow. The effects of using dissolved gas exsolution in ISTT applications were quantified in laboratory scale experiments. Laboratory-scale heating experiments were conducted at temperatures above and below the co-boiling temperature using sand-water-NAPL mixtures with and without dissolved carbon dioxide-saturated water. Results indicated extraction of NAPL was possible at temperatures below the co-boiling point. However, limited volumes were extracted (1 to 2 mL from 27 mL emplaced initially) using one pore volume of carbon dioxide-saturated water. Estimates indicate more than 30 pore volumes of solution are required to extract the emplaced volume at a temperature near the co-boiling point. Pore volumes required for treatment increased when temperatures decreased. Additional electrical resistance heating (ERH) experiments were conducted in a 2D flow cell supplied by water with and without dissolved carbon dioxide to determine the effects on groundwater flow rate. Results indicated a reduction of flow by three to four orders of magnitude after exsolution. Results also indicated a change in flow regime from advection dominated flow to natural convection dominated flow. Overall, the two studies indicate that dissolved carbon dioxide exsolution has a potential to be used in practice for groundwater flow reduction, but may only see limited effectiveness for removing NAPLs at lower temperatures. Further work is required to determine the effects in field applications.