A review of transport and attenuation of dissolved-phase volatile organic compounds (VOCs) in the unsaturated zone

This report is concerned with transport and attenuation of dissolved-phase volatile organic compound (VOC) plumes in the unsaturated zone. The report reviews the international literature in this subject area, but emphasis is placed upon UK context and relevance. The review makes reference to, but does not critically evaluate how current understanding of VOC migration in the unsaturated zone is translated into practical risk-based assessment frameworks. It does, however, make observations as to the critical migration processes that should be considered in such frameworks to allow more effective knowledge-based decision making for the development of conceptual site models and the implementation of remedial actions appropriate to contaminated sites. The key drivers for this review are the cost-effective management of VOC sources and associated contaminant plumes and the protection of groundwater. This requires an understanding and quantification of the processes by which the unsaturated zone offers protection to underlying groundwater resources from chemical releases at ground surface. Times taken for water to infiltrate and recharge the underlying water table may be long, possibly several decades where zones are thick. Furthermore, dissolved-phase chemical contaminants within infiltrating water may undergo attenuation and potentially be prevented from reaching the water table or delayed in arrival with much reduced concentrations present. A combination of processes including dispersion, sorption, volatilisation, chemical reaction and biodegradation may contribute to contaminant attenuation and protection of underlying aquifers. The review is structured in eight sections, which comprise:  Section 1, where the context, aims and objectives of the review study are described. In assessing VOC plumes in the unsaturated zone leached from shallow source zones it is important that: (i) understanding of transport and attenuation of chemical contaminants in the unsaturated zone is sufficient to both recognise and quantify the critical controlling processes; and, (ii) that understanding is translated into practical risk-based assessment frameworks to allow contaminated land problem holders, investigators and regulators alike to make effective knowledge-based decisions on remedial actions appropriate to contaminated sites.