Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:1850:p44
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 1850 (pt 44/117)
Character Range: 469917–472881

of chlorinated solvents such as trichloroethene (TCE) and tetrachloroethene/perchloroethylene (PCE). DNAPLs have the potential to migrate to significant depth below the water table through unconsolidated and consolidated materials and fractured bedrock, where they slowly dissolve into flowing groundwater and give rise to aqueous phase plumes.

Due to their physicochemical properties, DNAPLs migrate through the sub-surface in a very selective and tortuous manner and as a result can be challenging to investigate with traditional drilling techniques. Upon release, DNAPL will move and distribute itself into disconnected blobs and ganglia of liquid (residual DNAPL) and in connected distributions (pooled DNAPL). Residual DNAPL is found both above and below the water table within the migration pathways and typically occupies between 5% and 30% of pore space in porous media and rock fractures. Residual DNAPL is trapped by capillary forces and typically will not enter an adjacent monitoring well, even under the influence of aggressive pumping.

Pooling of DNAPL occurs above capillary barriers, typically layers, and lenses of slightly less permeable materials. Penetration through silts and clays may occur if windows are present within the layers or if the layers are penetrated by preferential pathways, for example, tree roots. The presence of dipping fractures, bedding planes, joints and faults may enable a DNAPL to continue to migrate downwards. Downward migration of chlorinated solvents may cease within a few months to a few years of release in relatively permeable media, compared with many decades for high viscosity DNAPLs such as creosote and coal tar to cease migration.

The DNAPL source zone comprises the overall rock volume of the sub-surface containing residual and/or pooled DNAPL. In addition to the DNAPL, there may be significant amounts of contaminant mass that has diffused into low permeability zones. Back diffusion of sorbed contaminant mass from the aquifer to groundwater may sustain dissolved-phase plumes for significant periods of time (decades to hundreds of years).

Above the water table, volatile DNAPLs can vaporise into air-filled pore spaces and for DNAPLs with significant vapour pressure such as chlorinated solvents, this can lead to expanded vapour-phase plumes in the unsaturated zone. Passive soil vapour surveys may be useful for delineation of DNAPL source zones particularly in situations where groundwater has been impacted by VOCs but the source has not been identified. In warm dry conditions, the persistence of some DNAPLs such as chlorinated solvents can be relatively short (months to a few years) in unsaturated media. The absence of residual and pooled NAPL in the unsaturated zone may not, therefore, be sufficient evidence to conclude that DNAPL has not migrated below the water table at the site of interest.

Determining the presence or absence of a DNAPL is an important consideration for the