OBSERVATION OF TRAPPED GAS DURING ELECTRICAL RESISTANCE HEATING OF TRICHLOROETHYLENE UNDER PASSIVE VENTING CONDITIONS

A two-dimensional experiment employing a heterogeneous sand pack incorporating two pools of trichloroethylene (TCE) was performed to assess the efficacy of electrical resistance heating (ERH) under passive venting conditions. The vaporization and disappearance of liquid phase TCE in the source zones, as well as the upward migration of the vapour produced was recorded using a digital camera. Temperature monitoring displayed the existence of a TCE-water co-boiling plateau at 73.4 ^oC, followed by continued heating to 100 ^oC. A 5 cm thick gas accumulation formed beneath a fine-grained capillary barrier during and after co-boiling. The capillary barrier did not desaturate during the course of the experiment; the only pathway for gas escape being through perforated wells traversing the barrier. The thickness of the accumulation was dictated by the entry pressure of the perforated well. Chemical samples were collected 48 hours after experiment termination and subsequently analyzed to evaluate efficacy of treatment in each case. The theoretical maximum TCE soil concentration within the region of gas accumulation, following gas collapse, was estimated to be 888 mg/kg. Post-heating soil sampling revealed TCE concentrations in this region ranging from 27 mg/kg to 96.7 mg/kg, indicating removal of aqueous and gas phase TCE following co-boiling as a result of subsequent boiling of water. The equilibrium concentrations of TCE in water corresponding to the range of post-treatment concentrations in soil (6.11 mg/kg to 136 mg/kg) are calculated to range from 19.8 mg/l to 440 mg/l. The results of this experiment illustrate the importance of providing gas phase venting during the application of ERH in heterogeneous porous media.