Laboratory-scale experimental and modelling investigations of 222Rn profiles in chemically heterogeneous LNAPL contaminated vadose zones

Abstract

The potential of LNAPL delineation by 222 Rn soil-gas monitoring in a chemically heterogeneous vadose zone was investigated in this study based on laboratory (batch and columns) experiments and numerical modeling. An enhanced version of the MIN3P reactive transport code was used to simulate Rn transport in both uncontaminated and NAPL-contaminated vadose zones and results were validated against analytical solutions and laboratory experiments. Results show that 222 Rn activity profiles are mainly controlled by porous media 222 Rn production, vadose zone fluid saturations and especially the type and distribution of NAPL in contaminated areas. The results also show that decreases in 222 Rn activity and variations in activity gradients provide evidence for the presence and saturation of NAPL. This study demonstrates that LNAPL delineation via 222 Rn gas surveys at contaminated sites works best, if gas measurements extend as deep as possible and include regions where 222 Rn activity decreases due to elevated NAPL. In addition, collection and analysis of depth-discrete gas samples allows the characterization of vertical NAPL distribution based on the 222 Rn activity gradient. The determination of 222 Rn production in the unsaturated zone, as well as water capillary pressure curves are of key importance in enabling the discrimination of an uncontaminated from a NAPL-contaminated area.

Understanding of Co-boiling between Organic Contaminants and Water during Thermal Remediation: Effects of Nonequilibrium Heat and Mass Transport

“…NAPL can be classified into light nonaqueous phase liquids (LNAPL) and dense nonaqueous phase liquids (DNAPL) based on the relative magnitude to water density. ,− LNAPL include petroleum hydrocarbons, benzene, toluene, xylene, and other hydrocarbons. , DNAPL include PCE, TCE, carbon tetrachloride, coal tar, creosote, etc. ,− As depicted in Figure a, relative positions of NAPL to water can have a significant impact on the co-boiling and contaminant removal processes due to different removal paths of NAPL vapor and heat transfer. As shown in Figure b, we further subdivided DNAPL and LNAPL into four categories based on their boiling points relative to water because the co-boiling point relies on the combinations of boiling points of NAPL and water (detailed thermodynamic explanations seen in S1).…”

Assessing Light Non-Aqueous Phase Liquids in the Subsurface Using the Soil Gas Rn Deficit Technique: A Literature Overview of Field Studies

“…An overview of Rn applications as an environmental tracer in hydrogeological and geological investigations is provided by Sukanya et al [51]. One of the applications briefly discussed is the use of Rn as an environmental tracer for the assessment of NAPL contamination [22] in both the saturated [18,24,28,30,[52][53][54] and vadose zone [19,21,23,26,29,[55][56][57][58][59][60], due to its tendency to preferentially partition into organic phases. To implement and use Rn as a tracer for environmental applications, it is crucial to comprehend the mechanisms of its production and behavior in the subsurface.…”

Numerical simulation and experimental study of three-phase distribution characteristics of leaked light non-aqueous phase liquid from buried pipelines in soils containing groundwater and gas

“…However, due to the capillary pressures in soils, there was also a recorded distribution in the lateral direction, which was less than in the vertical direction. The ability of soil particles to adsorb diesel caused a reduction or blockage of the pore diameter of soils, resulting in changes in distribution direction (Cohen et al, 2019). Moreover, the soil bulk density increased gradually along the vertical direction downward, and the soil compactness increased accordingly due to the action of soil gravity (Abd Rashid, Alias, Ahmad, Sa'Ari, Tsai, et al, 2020;Chevalier & Petersen, 1999).…”