Gasoline Multiphase and Multicomponent Partitioning in the Vadose Zone: Dynamics and Risk Longevity

Abstract

The multiphase and multicomponent dynamics of the release of light nonaqueous-phase liquid (LNAPL) petroleum hydrocarbons into the subsurface determines the longevity of health and environmental risks. Gasoline is of particular concern, with a wide range of volatilities and solubilities. A Darcyscale, three-dimensional, multiphase and multicomponent approach simulated the effects of the release depth and duration (20-500 d) on the distribution, partitioning, and fate of gasoline components, highlighting major changes in composition and mass during the initial release period. The simulated release occurred at either the ground surface (shallow) or immediately above the water table (deep). The LNAPL mass losses were directly related to the duration of the release. As much as 20% of the initial LNAPL mass was lost from shallow releases mainly as a result of ongoing volatilization of C4-C6 alkanes in the vadose zone over the release period. This was up to 59% higher than the deep releases, mostly resulting from the greater penetration of the deep release below the water table. Over the longer term, the mole fraction of the components within the LNAPL plume from the shallow releases asymptoted to values observed for a weathered gasoline sampled from the field. The mole fraction of toluene increased from 13 to 17% and short-chain alkanes decreased from 49 to 19%. Interestingly, the particular balance of partitioning processes left the benzene mole fraction approximately constant over the time of release and for an appreciable period beyond. This has important implications for long-term risk in the vapor and water phases.

A Critical Review of the Modelling Tools for the Reactive Transport of Organic Contaminants

“…The activity coefficient and molar fraction undergo variation as the dissolution process unfolds, influenced by changes in the composition of the remaining NAPL and the solubility of individual components within the mixture [126,127]. Approximations for the activity coefficients can be made using the universal quasi-chemical functional group activity coefficient (UNIFAC) method, which has demonstrated strong agreement with experimentally derived ternary phase diagrams [128,129].…”

Numerical investigation on light non–aqueous phase liquid flow in the vadose zone considering porosity effects on soil hydraulic properties

“…(2016). In addition, some research revealed that LNAPL partitioning (volatilization into gaseous phase and dissolution into aqueous phase) could alter fluid properties such as the surface tension and affect LNAPL flow, particularly in the long‐term condition (Lari, Johnston, et al., 2016; Lekmine et al., 2017).…”

Evaluation of LNAPL Behavior in Water Table Inter-Fluctuate Zone under Groundwater Drawdown Condition

“…The works involved entrapment of LNAPL, the effect of water table fluctuations, assessing LNAPL recoverability, transient effects, and other aspects. In addition, investigators conducted multidimensional, multiphase simulations considering free, residual, and entrapped LNAPL [28,30,[41][42][43] to predict subsurface LNAPL behavior.…”