title: Evaluating the reliability of equilibrium dissolution assumption from residual gasoline in contact with water saturated sands.
abstract: Understanding dissolution dynamics of hazardous compounds from complex gasoline mixtures is a key to long-term predictions of groundwater risks. The aim of this study was to investigate if the local equilibrium assumption for BTEX and TMBs (trimethylbenzenes) dissolution was valid under variable saturation in two dimensional flow conditions and evaluate the impact of local heterogeneities when equilibrium is verified at the scale of investigation. An initial residual gasoline saturation was established over the upper two-thirds of a water saturated sand pack. A constant horizontal pore velocity was maintained and water samples were recovered across 38 sampling ports over 141days. Inside the residual NAPL zone, BTEX and TMBs dissolution curves were in agreement with the TMVOC model based on the local equilibrium assumption. Results compared to previous numerical studies suggest the presence of small scale dissolution fingering created perpendicular to the horizontal dissolution front, mainly triggered by heterogeneities in the medium structure and the local NAPL residual saturation. In the transition zone, TMVOC was able to represent a range of behaviours exhibited by the data, confirming equilibrium or near-equilibrium dissolution at the scale of investigation. The model locally showed discrepancies with the most soluble compounds, i.e. benzene and toluene, due to local heterogeneities exhibiting that at lower scale flow bypassing and channelling may have occurred. In these conditions mass transfer rates were still high enough to fall under the equilibrium assumption in TMVOC at the scale of investigation. Comparisons with other models involving upscaled mass transfer rates demonstrated that such approximations with TMVOC could lead to overestimate BTEX dissolution rates and underestimate the total remediation time.
authors.affiliation: CSIRO Land and Water, Private Bag No. 5, Wembley, WA 6913, Australia. Electronic address: gregory.lekmine@csiro.au.
authors.forename: Greg
authors.initials: G
authors.lastname: Lekmine
authors.affiliation: CSIRO Land and Water, Private Bag No. 5, Wembley, WA 6913, Australia; School of Engineering, Edith Cowan University, 270 Joondalup Drive, Joondalup, WA 6027, Australia.
authors.forename: Kaveh
authors.initials: K
authors.lastname: Sookhak Lari
authors.affiliation: CSIRO Land and Water, Private Bag No. 5, Wembley, WA 6913, Australia; School of Earth and Environment, University of Western Australia, Nedlands, WA 6009, Australia.
authors.forename: Colin D
authors.initials: CD
authors.lastname: Johnston
authors.affiliation: CSIRO Land and Water, Private Bag No. 5, Wembley, WA 6913, Australia.
authors.forename: Trevor P
authors.initials: TP
authors.lastname: Bastow
authors.affiliation: CSIRO Land and Water, Private Bag No. 5, Wembley, WA 6913, Australia.
authors.forename: John L
authors.initials: JL
authors.lastname: Rayner
authors.affiliation: CSIRO Land and Water, Private Bag No. 5, Wembley, WA 6913, Australia; School of Earth and Environment, University of Western Australia, Nedlands, WA 6009, Australia.
authors.forename: Greg B
authors.initials: GB
authors.lastname: Davis
mesh_terms: D001554:Benzene / Q000032:analysis / Q000737:chemistry; D005742:Gasoline / Q000032:analysis*; D060587:Groundwater / Q000737:chemistry*; D015386:Hazardous Substances / Q000032:analysis* / Q000737:chemistry; D007700:Kinetics; D008962:Models, Theoretical; D059392:Petroleum Pollution / Q000032:analysis; D012822:Silicon Dioxide / Q000737:chemistry*; D012995:Solubility; D014050:Toluene / Q000032:analysis / Q000737:chemistry; D014874:Water Pollutants, Chemical / Q000032:analysis* / Q000737:chemistry
chemical_list: D005742:Gasoline; D015386:Hazardous Substances; D014874:Water Pollutants, Chemical; D014050:Toluene; D012822:Silicon Dioxide; D001554:Benzene
keywords: BTEX; Equilibrium dissolution; LNAPL; Multiphase flow and transport; Petroleum hydrocarbons; Residual gasoline