Gasoline multiphase and multicomponent partitioning in the vadose zone: Dynamics and risk longevity

© Soil Science Society of America 5585 Guilford Rd., Madison, WI 53711 USA. All rights reserved. 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 Darcy-scale, 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.