Monitoring and Decommissioning

Reactive barriers are installed to prevent the spread of groundwater contamination. During operation, effluent from an operating barrier should be sampled routinely to ensure that barrier failure and contaminant breakthrough has not occurred. Barriers can fail for a range of reasons, including the development of preferential flowpaths with resultant reduced residence time, freezing damage to the media (either grain shattering or the development of macropores), media saturation, exhaustion, gas clogging and biofouling. For compounds which are reductively degraded by zero valent iron (e.g., trichloroethene), degradation products such as vinyl chloride should also be monitored. Routine water chemistry (pH, Eh, dissolved oxygen, water temperature) provides indications of barrier operation, and may provide early warnings of improper barrier operation. Datalogging is particularly important for barriers in remote areas where visits are infrequent or seasonal. Tracer tests are useful to assess the flux of water and contaminants through the PRB, as well as to measure the residence time of water within the reactive zone.

Decommissioning barriers may be necessary once they are no longer needed, or when the media needs replacement. Some barrier types can be left in the ground, where there is no environmental harm in doing so. For example, adsorbents used to trap petroleum hydrocarbons and allow biodegradation may be left intact unless the barrier has accumulated recalcitrant compounds which have resisted biodegradation. Alternatively, barriers which trap and accumulate contaminants such as metals need to be removed, to prevent later desorption and remobilisation of the contaminant. Where there is reason to remove a barrier from areas of permafrost soils, there must be a mechanism for separating the base of the barrier from the frozen ground. This may take the form of the reactive media being held within a rigid cage with lifting hooks allowing extraction, a cage placed on a sacrificial layer, or a heat trace to melt the base of the barrier out. Alternatively, the barrier media may need to be removed using an excavator with a frost claw. Consideration of the decommissioning of a PRB in permafrost soils needs to be incorporated into the design stage.

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