Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:1850:p45
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 1850 (pt 45/117)
Character Range: 472642–475650

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 development of the CSM. If the presence of DNAPL is suspected, care should be taken to avoid drilling through the DNAPL and dragging or spreading pooled DNAPL beyond the current location or creating a pathway for the DNAPL.
It is now commonly accepted that direct visual observation of DNAPL does not occur at most DNAPL sites and instead, the presence of DNAPL is usually inferred from converging lines of evidence. Site-specific considerations will dictate which lines of evidence (see below) should be pursued. Care, however, should be taken to ensure that a negative response to one or more lines of evidence is not simply attributable to inadequate characterisation and an insufficient amount of data.

The site investigation methods and related interpretation techniques (lines of evidence) which can be useful for characterising DNAPL source zones include:
    * visual observation in groundwater samples or drill core
    * chemical saturations in soil above threshold DNAPL saturation
    * chemical concentrations in soil above equilibrium partitioning threshold
    * mapping of a vapour-phase plume (based on shallow soil vapour measurements) if present
    * hydrophobic dye testing of DNAPL in soil or water samples or using a down-hole ribbon sampler impregnated with dye
    * interpretation of groundwater concentration data from locations immediately downgradient of the suspected source zone and trends with depth and over time.
As a general 'rule of thumb', groundwater concentrations in excess of 1% effective solubility may indicate that the groundwater has come into contact with DNAPL. Values of 1% solubility concentration for various chlorinated solvents can be found in Appendix B of US EPA (2009).

Further information may be found in Keuper and Davis (2009) and references therein. A tabulation of parameters and other information that may be needed at various stages of site investigation, risk assessment and selection of management options can be found in EA (2003).

    8.3.4          Attenuation of groundwater contaminants
Source: EA (2000b)
Assessors should be aware that dissolved contaminants may move at different rates not only as a result of physical processes, but also because of chemical interactions with soil and aquifer components. Attenuation processes include advection, dilution, dispersion, diffusion, sorption, degradation (biotic and abiotic) and volatilisation.

Consideration should be given to the fate of the contaminant(s) as it moves along the migration pathways. This requires that chemical, physical and biological interactions between sources and sub-surface materials are taken into account. The CSM should describe the processes that control the movement of contaminants in soil and the unsaturated and