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

摘要

This report is concerned with transport and attenuation of dissolved-phase volatile organicudcompound (VOC) plumes in the unsaturated zone. The report reviews the international literatureudin this subject area, but emphasis is placed upon UK context and relevance. The review makesudreference to, but does not critically evaluate how current understanding of VOC migration in theudunsaturated zone is translated into practical risk-based assessment frameworks. It does, however,udmake observations as to the critical migration processes that should be considered in suchudframeworks to allow more effective knowledge-based decision making for the development ofudconceptual site models and the implementation of remedial actions appropriate to contaminatedudsites.udThe key drivers for this review are the cost-effective management of VOC sources andudassociated contaminant plumes and the protection of groundwater. This requires anudunderstanding and quantification of the processes by which the unsaturated zone offersudprotection to underlying groundwater resources from chemical releases at ground surface. Timesudtaken for water to infiltrate and recharge the underlying water table may be long, possiblyudseveral decades where zones are thick. Furthermore, dissolved-phase chemical contaminantsudwithin infiltrating water may undergo attenuation and potentially be prevented from reaching theudwater table or delayed in arrival with much reduced concentrations present. A combination ofudprocesses including dispersion, sorption, volatilisation, chemical reaction and biodegradationudmay contribute to contaminant attenuation and protection of underlying aquifers.udThe review is structured in eight sections, which comprise:ud Section 1, where the context, aims and objectives of the review study are described. Inudassessing VOC plumes in the unsaturated zone leached from shallow source zones it isudimportant that: (i) understanding of transport and attenuation of chemical contaminants inudthe unsaturated zone is sufficient to both recognise and quantify the critical controllingudprocesses; and, (ii) that understanding is translated into practical risk-based assessmentudframeworks to allow contaminated land problem holders, investigators and regulators alikeudto make effective knowledge-based decisions on remedial actions appropriate toudcontaminated sites.