The development of dissolved-phase volatile organic compound (VOC) plumes in the unsaturated zone may result from a number of source zone scenarios, including disposal lagoons, landfill sites (lined or unlined) and point-source releases.
Elevated dissolved phase VOC concentrations may result from direct discharge of waste waters or from the dissolution of non-aqueous phase liquids (NAPL).
Small to medium releases of NAPL that fail to generate sufficient capillary pressure to substantially invade the unsaturated zone to depth (or where vertical migration is limited by the presence of low-permeability barriers) may remain as shallow sources that may be subsequently leached (dissolved) by infiltrating water. Where infiltration occurs, either directly through the unsaturated zone or through the waste materials in disposal facilities, a dissolved-phase VOC plume will evolve that could affect a significant thickness of the unsaturated zone.
Shallow leachable VOC sources are a relatively common phenomenon that may pose a substantial long-term environmental liability.
Research aims
The key drivers for understanding the effects of shallow, leachable NAPL source zones or aqueous discharge sources on the unsaturated zone are:
- reducing uncertainty in prediction of timeframes for migration through the unsaturated zone, which may range from hours to decades
- quantifying the significance of the various natural attenuation processes that may retard or remove dissolved phase VOCs and delay, or even prevent, arrival at the water table
- facilitating development of appropriate risk assessment methodologies and remediation strategies
Further reading
Wealthall et al. 2010. A review of transport and attenuation of dissolved-phase volatile organic compounds (VOCs) in the unsaturated zone. British Geological Survey Open Report, OR/10/06. 101pp. Available: http://nora.nerc.ac.uk/12787/
Contact
Please contact Dan Lapworth for further information.