Environmental Engineering Reference
In-Depth Information
16.6.2.
Compatibility
As chemicals migrate through clay soils that contain an appreciable amount of
clay minerals, such as CCLs and GCLs, interaction between the chemicals in the
pore water of the soil and clay particles can result in significant changes (increase)
in the hydraulic conductivity of the mineral barrier [RAH 01, SHA 94a, SHA 99,
SHA 00]. While the potential effects of these chemical - soil interactions are well
recognized and may be significant such interactions are rarely included in the
modeling of contaminant transport through waste containment liners. This is mainly
because of the lack of standardized procedures and reliable parameters.
Nevertheless, many studies and research programs have been performed on
chemical compatibility [MAN 00]. These studies have shown that chemical
compatibility can be evaluated indirectly. The approach mainly consists of assessing
the influence of solutes on Atterberg limits, sedimentation, swelling, cracking
pattern, setting, exposure or immersion and permeation. This type of approach is
popular, but needs to be used cautiously.
It needs to be pointed out that there is a lack of quantitative and practical
indications to define conditions that can be critical for compatibility problems. A
procedure for addressing these aspects has been attempted by Manassero and
Shackelford [MAN 94a]. This procedure, valid for organic contaminants, allows us
to take the activity of the compacted soil, dielectric constant of the pollutant
solution, contaminant concentration and density into account. Further research and
validations are needed before the proposed procedure can be used reliably. A
possible extension to inorganic pollutants can also be addressed. For more details
see [SHA 94a].
Another interesting study is reported by Kaczmarek
et al
. [KAC 97]. Their
results indicate that the potential effects of compatibility problems are controlled by
the magnitude of the concentration at the source boundary of the clay barrier. For
low to moderate contaminant concentration at the source boundary, the diffusive
component controls the transport so no significant changes of hydraulic conductivity
occur in the clay barrier. Significant changes (increases) in hydraulic conductivity
are only likely to occur where an extremely high concentration is imposed at the
source boundary. In this case, the hydraulic conductivity of the clay barrier may
increase to the extent that the advective component of the transport eventually
dominates the diffusive component. Thus, some consideration should be given to the
potential influence of contaminant concentration in the leachate on the long-term
performance of the barrier.
