Laboratory study of non-aqueous phase liquid and water co-boiling during thermal treatment.

In situ thermal treatment technologies, such as electrical resistance heating and thermal conductive heating, use subsurface temperature measurements in addition to the analysis of soil and groundwater samples to monitor remediation performance. One potential indication of non-aqueous phase liquid (NAPL) removal is an increase in temperature following observations of a co-boiling plateau, during which subsurface temperatures remain constant as NAPL and water co-boil. However, observed co-boiling temperatures can be affected by the composition of the NAPL and the proximity of the NAPL to the temperature measurement location. Results of laboratory heating experiments using single-component and multi-component NAPLs showed that local-scale temperature measurements can be mistakenly interpreted as an indication of the end of NAPL-water co-boiling, and that significant NAPL saturations (1% to 9%) remain despite observed increases in temperature. Furthermore, co-boiling of multi-component NAPL results in gradually increasing temperature, rather than a co-boiling plateau. Measurements of gas production can serve as a complementary metric for assessing NAPL removal by providing a larger-scale measurement integrated over multiple smaller-scale NAPL locations. Measurements of the composition of the NAPL condensate can provide ISTT operators with information regarding the progress of NAPL removal for multi-component sources.

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. Temperature monitoring displayed the existence of a TCE-water co-boiling plateau at 73.4°C, followed by continued heating to 100°C. A 5cm 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. The theoretical maximum TCE soil concentration within the region of gas accumulation, following gas collapse, was estimated to be 888mg/kg. Post-heating soil sampling revealed TCE concentrations in this region ranging from 27mg/kg to 96.7mg/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 (6.11mg/kg to 136mg/kg) are calculated to range from 19.8mg/l to 440mg/l. The results of this experiment illustrate the importance of providing gas phase venting during the application of ERH in heterogeneous porous media.

Relative velocities of DNAPL and aqueous phase plume migration.

Numerical simulation is used to examine the relative velocities of DNAPL and aqueous phase plumes in sandy aquifers where lateral spreading of DNAPL has occurred at the base of the aquifer. The scenario being modeled is one where a permeable aquifer is underlain by a sloping aquitard, which results in lateral migration of the DNAPL down the slope, in addition to lateral migration of an aqueous phase plume subject to a specified hydraulic gradient. A sensitivity analysis is presented to the impacts of both DNAPL properties and geologic properties. The most important chemical properties governing the relative velocities of the DNAPL and the shallow aqueous phase plume are the DNAPL viscosity and the aqueous component soil-water partition coefficient (Kd). The dip of the underlying aquitard was found to be relatively unimportant, at least for the range of values studied. The scenario under consideration can be important in conceptual model development and remedial design, as in certain cases DNAPL could be migrating in areas without the evidence of a well-developed aqueous phase plume. The implication of this work is that the absence of a shallow aqueous phase plume directly downgradient of a DNAPL source zone does not rule out the possibility of deep occurrences of DNAPL beyond the shallow monitoring well network. A further finding of this study is that the occurrence of a highly sorbing compound in groundwater at virtually any concentration may indicate the immediate upgradient presence of residual or pooled DNAPL.

Removal of PCB-DNAPL from a rough-walled fracture using alcohol/polymer flooding.

Phase behaviour experiments employing PCB (Aroclor 1242)/alcohol/water systems were conducted with ethanol (EtOH) and n-propanol (nPA). Both exhibited an affinity for the aqueous phase within the entire two-phase region. As much as 88% by volume (88% vol.) EtOH and 80% vol. nPA were necessary to achieve full miscibility of the PCB in the aqueous phase. DNAPL-water interfacial tension (IFT) was reduced from 38.9 dyn/cm to 4.7 dyn/cm and 2.4 dyn/cm with 80% vol. EtOH and 76% vol. nPA. The addition of alcohol brought about 41% and 54% reductions in DNAPL viscosity at maximal concentrations of EtOH and nPA. Density of the PCB-DNAPL was relatively unaffected by the presence of alcohol. A series of seven experiments were conducted where successive slugs of nPA and xanthan gum polymer solutions were injected into a fractured shale sample. A 30% vol. nPA solution injected under a hydraulic gradient of 0.36 allowed enhanced PCB removal primarily through reduction of IFT and resulted in 72% DNAPL recovery. Several pore volumes of alcohol solution were necessary to displace all the potentially mobile non-wetting phase since the high-viscosity DNAPL was mobilized at a lower flow rate than the overall fluid velocity, illustrating non-piston displacement. The injection of a 95% vol. nPA alcohol solution, theoretically at a sufficient concentration to produce fully miscible displacement of the residual DNAPL at equilibrium, resulted in non-equilibrium partitioning of the PCB into the flushing solution, likely due to the high fluid velocities in the fracture. The injection of 200 pore volumes of 95% vol. nPA solution resulted in 94% DNAPL recovery. Alcohol floods operated below the miscibility envelope appear to be a valuable source zone remedial alternative where the objective is to reduce DNAPL mobility to zero, but it should be noted that DNAPL mobility is increased during the application of the technology and steps may need to be taken to prevent unwanted vertical mobilization.

Numerical examination of the factors controlling DNAPL migration through a single fracture.

The migration of five dense nonaqueous phase liquids (DNAPLs) through a single fracture in a clay aquitard was numerically simulated with the use of a compositional simulator. The effects of fracture aperture, fracture dip, matrix porosity, and matrix organic carbon content on the migration of chlorobenzene, 1,2-dichloroethylene, trichloroethylene, tetra-chloroethylene, and 1,2-dibromoethane were examined. Boundary conditions were chosen such that DNAPL entry into the system was allowed to vary according to the stresses applied. The aperture is the most important factor of those studied controlling the migration rate of DNAPL through a single fracture embedded in a clay matrix. Loss of mass to the matrix through diffusion does not significantly retard the migration rate of the DNAPL, particularly in larger aperture fractures (e.g., 50 microm). With time, the ratio of diffusive loss to the matrix to DNAPL flux into the fracture approaches an asymptotic value lower than unity. The implication is that matrix diffusion cannot arrest the migration of DNAPL in a single fracture. The complex relationships between density, viscosity, and solubility that, to some extent, govern the migration of DNAPL through these systems prevent accurate predictions without the use of numerical models. The contamination potential of the migrating DNAPL is significantly increased through the transfer of mass to the matrix. The occurrence of opposite concentration gradients within the matrix can cause dissolved phase contamination to exist in the system for more than 1000 years after the DNAPL has been completely removed from the fracture.

Multiphase flow and transport in fractured clay/sand sequences.

A numerical model (Queen's University Multi-Phase Flow Simulator, QUMPFS) was used to assess the rate of trichloroethylene (TCE) dense, non-aqueous phase liquid (DNAPL) migration through fractured clay, with special attention focused on the influence of interbedded sand lenses. The presence of these sand lenses was found to increase the time required for the non-wetting phase to migrate through the full 30 m vertical extent of the clay sequence from a few days to several years. Applied vertical hydraulic gradients were found to be moderately influential in systems consisting solely of fractured clays, yet one of the dominant factors controlling speed of vertical migration when sand lenses were present. Larger displacement pressure of the sands relative to that of the fractures leads to slower DNAPL migration rates, due to the delays that occur during build-up of capillary pressures. Dissolution of DNAPL and subsequent matrix diffusion of the aqueous phase has little effect on the rate of DNAPL migration through systems consisting of fractured clay only, yet slows the rate of migration in systems containing sand lenses. In all cases examined, the rate of DNAPL loading to the lower aquifer far exceeded the rate of aqueous phase mass loading. It was also found that DNAPL reaches the lower aquifer at approximately the same time as the aqueous phase plumes even for systems experiencing downward groundwater flow due to the attenuation of the aqueous phase through matrix diffusion.