Chlorinated organic solvents belong to a class of groundwater contaminants commonly referred to as dense non-aqueous phase liquids or DNAPLs. As the acronym implies these liquid are more dense than water and are immiscible in water. DNAPLs also have lower viscosities than water making them highly mobile in the subsurface. In a spill situation DNAPLs will migrate downwards through the watertable as a separate liquid phase. As it moves downward the DNAPL will leave a trail of residual concentration often exceeding 15 % of the pore volume (Feenstra and Cherry, 1988). When a low permeability zone is encountered the DNAPL will tend to pool, until it can build up enough pressure to breakthrough and continue its downward migration path. Pooling may cause considerable lateral spreading of the DNAPL (Keuper and Frind, 1988). Eventually the DNAPL will come to rest, pooling on a horizon which is impermeable to the DNAPL. There it will act as a long term contaminant source as it is slowly dissolved into the passing groundwater. Although DNAPLs are mechanically immiscible in water they will dissolve in concentrations orders of magnitude higher than acceptable drinking water limits.


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