Vapor-phase transport of trichloroethene in an intermediate-scale vadose-zone system: Retention processes and tracer-based prediction

Abstract

Gas-phase miscible-displacement experiments were conducted using a large weighing lysimeter to evaluate retention processes for volatile organic compounds (VOCs) in water-unsaturated (vadoze-zone) systems, and to test the utility of gas-phase tracers for predicting VOC retardation. Trichloroethene (TCE) served as a model VOC, while trichlorofluoromethane (CFM) and heptane were used as partitioning tracers to independently characterize retention by water and the air-water interface, respectively. Retardation factors for TCE ranged between 1.9 and 3.5, depending on water content. The results indicate that dissolution into the bulk water was the primary retention mechanism for TCE under all conditions studied, contributing approximately two thirds of the total measured retention. Accumulation at the air-water interface comprised a significant fraction of the observed retention for all experiments, with an average contribution of approximately 24%. Sorption to the solid phase contributed approximately 10% to retention. Water contents and air-water interfacial areas estimated based on the CFM and heptane tracer data, respectively, were similar to independently measured values. Retardation factors for TCE predicted using the partitioning-tracer data were in reasonable agreement with the measured values. These results suggest that gas-phase tracer tests hold promise for characterizing the retention and transport of VOCs in the vadose-zone.

Experimental study on retardation of a heavy NAPL vapor in partially saturated porous media

“…3) suggested that the smooth-sphere assumption holds for glass beads but not necessarily for Geba fine sand; thus, the interfacial area for Geba fine sand might have been significantly underestimated. Interfacial areas measured with microtomography are smaller than those determined with vapor-phase tracer experiments (Brusseau et al, 2006;Costanza-Robinson et al, 2008). Furthermore, they reported that A IA from microtomography measurements approach a maximum value similar to the smooth-sphere assumption while measurements from vapor-phase experiments rather approach a maximum close to the specific surface area of the material (Brunauer-Emmett-Teller measurements) as water saturation approaches zero.…”

“…The air-water interfacial area can be estimated with a correlation found by Costanza-Robinson et al (2008) based on Xray microtomography measurements of glass beads and natural sands.…”

“…This correlation with log(K OW ) = 2.00 and the dimensionless Henry constant K H = 1.04 yielded the air-water partitioning coefficient of CS 2 K IA = 6.87 × 10 −6 cm. Costanza-Robinson et al (2008) found a correlation (Eq. 6) based on X-ray microtomography measurements of glass beads and natural sands to estimate the interfacial area.…”

“…Experimental studies (e.g., Brusseau et al, 1997;Kim et al, 1998;Costanza-Robinson et al, 2013) have been conducted to investigate retardation of the most common volatile organic carbon (VOC) in unsaturated porous media using short columns in the range of few decimeters and high flow velocities. Experimental results have been compared with standard as well as advanced advection-dispersion models (Popovičová and Brusseau, 1998;Toride et al, 2003).…”

“…For instance, noble gases show no retardation at all. In summary, retardation of a contaminant vapor is a combination of different partitioning processes; their relative contribution depends on the physicochemical properties of the component as well as the properties of the porous medium (Costanza-Robinson et al, 2013).…”