Sustainable ecological restoration of brownfield sites through engineering or managed natural attenuation? A case study from Northwest England

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This work is based on a recent UK£1.25M restoration project of a contaminated canal and adjacent brownfield land. The canal sediment contained extremely high concentrations of heavy metals and arsenic whilst adjacent brownfield land presented localised hotspots. Potential receptor risks had been identified in the context of trace element mobility to groundwater and entrainment of dust particulates; the engineering work was initiated due to concerns that the sediment was drying and was likely to release contaminants through leaching as anoxic conditions were removed. Extensive engineering of the canal site involved its narrowing through dredging, relocation of the sediment to the edges but within the canal basin, cement stabilisation, retainment by geotextile-lined gabions, then coverage with geotextile and capping with recycled soil-forming materials including green waste compost (GWC). The adjacent brownfield land received import of GWC as a soil improver to enhance biological activity, although subsequent studies indicated contaminants were already stabilised in the soil and biological activity on the brownfield site was not suppressed. The data suggest that GWC should be applied with caution due to its influence on trace element mobilisation. In this paper we question whether real ecological improvements have been achieved and have justified the high cost of the restoration. We conclude that this case study does not provide a realistic template for best restoration practice of brownfield land. A better alternative to the whole site may have been a less-engineered, more-ecological approach through enhancing natural processes of regeneration, contaminant stabilisation and attenuation. Management of the canal site to maintain a wet anoxic sediment, rather than abandonment and dereliction would provide a more sensitive and less costly solution. Vegetation cover alone may be sufficient to mitigate risk on urban brownfield land.