title: LNAPL source zone delineation using soil gases in a heterogeneous silty-sand aquifer.
abstract: Source delineation of hydrocarbon contaminated sites is of high importance for remediation work. However, traditional methods like soil core extraction and analysis or recent Membrane Interface Probe methods are time consuming and costly. Therefore, the development of an in situ method based on soil gas analysis can be interesting. This includes the direct measurement of volatile organic compounds (VOCs) in soil gas taken from gas probes using a PID (Photo Ionization Detector) and the analysis of other soil gases related to VOC degradation distribution (CH4, O2, CO2) or related to presence of Light Non-Aqueous Phase Liquid (LNAPL) as (222)Rn. However, in widespread heterogeneous formations, delineation by gas measurements becomes more challenging. The objective of this study is twofold: (i) to analyse the potential of several in situ gas measurement techniques in comparison to soil coring for LNAPL source delineation at a heterogeneous contaminated site where the techniques might be limited by a low diffusion potential linked to the presence of fine sands and silts, and (ii) to analyse the effect of vertical sediment heterogeneities on the performance of these gas measurement methods. Thus, five types of gases were analysed: VOCs, their three related degradation products O2, CO2 and CH4 and (222)Rn. Gas measurements were compared to independent LNAPL analysis by coring. This work was conducted at an old industrial site frequently contaminated by a Diesel-Fuel mixture located in a heterogeneous fine-grained aquifer. Results show that in such heterogeneous media migration of reactive gases like VOCs occurs only across small distances and the VOC concentrations sampled with gas probes are mainly related to local conditions rather than the presence of LNAPL below the gas probe. (222)Rn is not well correlated with LNAPL because of sediment heterogeneity. Oxygen, CO2, and especially CH4, have larger lengths of diffusion and give the clearest picture for LNAPL presence at this site even when the gas probe is somewhat distant.
authors.affiliation: Bordeaux INP - EA 4592 - ENSEGID, 1 allée F. Daguin, 33607 Pessac, France. Electronic address: gregory.cohen@ipb.fr.
authors.forename: Grégory J V
authors.initials: GJV
authors.lastname: Cohen
authors.affiliation: Bordeaux INP - EA 4592 - ENSEGID, 1 allée F. Daguin, 33607 Pessac, France.
authors.forename: Florie
authors.initials: F
authors.lastname: Jousse
authors.affiliation: Bordeaux INP - EA 4592 - ENSEGID, 1 allée F. Daguin, 33607 Pessac, France.
authors.forename: Nicolas
authors.initials: N
authors.lastname: Luze
authors.affiliation: Aix-Marseille Université - CNRS, Laboratoire Chimie Environnement UMR 7376, 3 place Victor Hugo - Case 29, 13331 Marseille, France.
authors.forename: Patrick
authors.initials: P
authors.lastname: Höhener
authors.affiliation: Bordeaux INP - EA 4592 - ENSEGID, 1 allée F. Daguin, 33607 Pessac, France.
authors.forename: Olivier
authors.initials: O
authors.lastname: Atteia
mesh_terms: D001673:Biodegradation, Environmental; D004058:Diffusion; D004784:Environmental Monitoring / Q000379:methods; D005602:France; D005740:Gases / Q000032:analysis / Q000737:chemistry; D060587:Groundwater / Q000737:chemistry*; D006838:Hydrocarbons / Q000032:analysis / Q000737:chemistry; D008697:Methane / Q000032:analysis / Q000378:metabolism; D011886:Radon / Q000032:analysis; D012822:Silicon Dioxide; D012987:Soil / Q000737:chemistry*; D012989:Soil Pollutants / Q000032:analysis* / Q000737:chemistry; D055549:Volatile Organic Compounds / Q000032:analysis*; D014874:Water Pollutants, Chemical / Q000032:analysis* / Q000737:chemistry
chemical_list: D005740:Gases; D006838:Hydrocarbons; D012987:Soil; D012989:Soil Pollutants; D055549:Volatile Organic Compounds; D014874:Water Pollutants, Chemical; D012822:Silicon Dioxide; D008697:Methane; D011886:Radon
keywords: Heterogeneous silty-sand aquifer; LNAPL source delineation; Soil gases; VOCs