Thermal Considerations

The arrival of thaw in areas of frozen ground is a time when the PRB must be ready to accept surface and subsurface flow. In order to do so, the barrier media must be unfrozen and permeable in advance of the contaminated catchment. The barrier should be designed so that this thaw occurs. For example, if any metal used in the barrier is in contact with permafrost, then heat could be conducted downwards, chilling the barrier and delaying thaw of the media. Alternatively, thaw might be enhanced via passive solar heating of the barrier surface. Snow-lie is an important aspect of thermal management of the barrier. Modelling of the thermal characteristics of the barrier and reactive media should be part of barrier design prior to installation, and if required, heat trace and insulation of the barrier base may need to be installed to ensure barrier thaw prior to runoff generation from the aquifer. Basal insulation also helps keep the barrier keyed into the permafrost. Ideally, at the end of the melt season, the PRB should drain freely or be pumped dry, to help prevent segregation ice formation during the winter.

Temperature is an additional variable in the efficiency of contaminant capture by PRB media. Zeolites exhibit reduced exchange kinetics and capacities for metal ions at 2°C compared with 20°C (Woinarski et al. 2003, 2006), and similarly, activated carbon and surfactant modified zeolite were found to have reduced adsorption efficiency at 4°C compared with 20°C (Hornig et al. 2008). The difference in sorption behaviour with cold temperatures depends on the hydrocarbon (Hornig et al. 2008). These constraints imposed by low temperature need to be integrated into barrier design in order to treat contaminant plumes effectively. In particular, the distance of the barrier in a down-hydraulic gradient direction is critical for the duration of interaction of the contaminant plume with the media.

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