Additional challenges exist for the long-term use of PRB in areas of freezing ground. The catchment, plume and the barrier may melt out at different rates in different locations, leading to local, temporary mounds in soil water tables (see Daniel and Staricka 2000) which create changes to aquifer permeability or the direction of the hydraulic gradient or permeability, potentially bypassing barriers in areas of low gradient.
The hydraulic conductivity (K) of the barrier and upstream area is critical for remediation success, for if the hydraulic conductivity of the aquifer is greater than the barrier, then water will pond against or flow around the barrier (Benner et al. 2001) leading to contaminants bypassing the treatment. To avoid this, barriers are designed to have, at installation, a greater hydraulic conductivity than the contaminated zone upstream. The maintenance of this relative hydraulic conductivity is essential, and contaminant retention or treatment may be adversely affected by any process which increases K of the contaminated upstream zone, decreases barrier K or causes barrier K to become heterogeneous, with the development of preferential flowpaths.
Disturbance by fauna is an additional hazard. In the Arctic, three barriers installed at Resolution Island, Nunavut, to treat PCB contamination have culverts placed over them to guard against disturbance by polar bears (Ursus maritimus; Poland et al. 2001). Interference by fauna in Antarctica is less likely, but at coastal Davis Station for example, Southern elephant seals (Mirounga leonina) occasionally crush piping and service lines, and any barriers installed there or at similar locations will need to be protected from disturbance.
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