Review of unsaturated-zone transport and attenuation of volatile organic compound (VOC) plumes leached from shallow source zones

• Published in: Journal of contaminant hydrology 2011-04, Vol.123 (3), p.130-156
• Main Authors: Rivett, Michael O., Wealthall, Gary P., Dearden, Rachel A., McAlary, Todd A.
• Format: Article
• Language: English
• Subjects: Attenuation; Biodegradation; Dissolution; Earth sciences; Earth, ocean, space; Exact sciences and technology; Hydrogeology; Hydrology. Hydrogeology; Leachable sources; Mathematical models; Mechanical Phenomena; Natural attenuation; Plumes; Risk Assessment; TCE; Transport; Trichloroethene; Trichloroethylene - analysis; Trichloroethylene - isolation & purification; Unsaturated; Unsaturated zone; Vadose zone; VOC; Volatile organic compound; Volatile organic compounds; Volatile Organic Compounds - analysis; Volatile Organic Compounds - isolation & purification; Water Pollutants, Chemical - analysis; Water Pollutants, Chemical - isolation & purification
• ISSN: 0169-7722; 1873-6009
• DOI: 10.1016/j.jconhyd.2010.12.013

Reliable prediction of the unsaturated zone transport and attenuation of dissolved-phase VOC (volatile organic compound) plumes leached from shallow source zones is a complex, multi-process, environmental problem. It is an important problem as sources, which include solid-waste landfills, aqueous-phase liquid discharge lagoons and NAPL releases partially penetrating the unsaturated zone, may persist for decades. Natural attenuation processes operating in the unsaturated zone that, uniquely for VOCs includes volatilisation, may, however, serve to protect underlying groundwater and potentially reduce the need for expensive remedial actions. Review of the literature indicates that only a few studies have focused upon the overall leached VOC source and plume scenario as a whole. These are mostly modelling studies that often involve high strength, non-aqueous phase liquid (NAPL) sources for which density-induced and diffusive vapour transport is significant. Occasional dissolved-phase aromatic hydrocarbon controlled infiltration field studies also exist. Despite this lack of focus on the overall problem, a wide range of process-based unsaturated zone — VOC research has been conducted that may be collated to build good conceptual model understanding of the scenario, particularly for the much studied aromatic hydrocarbons and chlorinated aliphatic hydrocarbons (CAHs). In general, the former group is likely to be attenuated in the unsaturated zone due to their ready aerobic biodegradation, albeit with rate variability across the literature, whereas the fate of the latter is far less likely to be dominated by a single mechanism and dependent upon the relative importance of the various attenuation processes within individual site — VOC scenarios. Analytical and numerical modelling tools permit effective process representation of the whole scenario, albeit with potential for inclusion of additional processes — e.g., multi-mechanistic sorption phase partitioning, and provide good opportunity for further sensitivity analysis and development to practitioner use. There remains a significant need to obtain intermediate laboratory-scale and particularly field-scale (actual site and controlled release) datasets that address the scenario as a whole and permit validation of the available models. Integrated assessment of the range of simultaneous processes that combine to influence leached plume generation, transport and attenuation in the unsaturated zone is required. Compone