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23 smaller land areas to hard reuse as these treatments simply tend to cost too much for soft end uses. There are many drivers for soft end uses of contaminated land. The site in question may simply not have a feasible alternative use because of size, location, geotechnical or topographical reasons, or levels of economic activity, as a result of global shifts in land use and industrial change (Menger et al, 2012). There may be important urban renewal arguments for developing amenity land, particularly in areas of urban deprivation (Handley, 1995, and National Urban Forestry Unit, 2001). In addition, there may also be opportunities for generating renewed economic activity, for example, through biomass production. The use of GROs can be highly compatible with biomass end use (eg Bardos et al, 2008, Puschenreiter et al, 2009, Bardos et al, 2011b, Van Slycken et al, 2013), and this creates an important and expanding niche for GROs. Figure 1 Contaminant linkage and risk management option (from Defra, 2012) GROs may offer a cost-effective treatment alternative for managing risks for soft end uses, rather than simply containing or transferring contamination. GROs appear to be attractive alternatives to conventional clean-up methods in these situations owing to their relatively low capital costs and the inherently aesthetic nature of planted or ‘green’ sites (ITRC, 2009). In addition, ‘greening’ of contaminated or marginal land may have additional wider benefits in terms of educational value, urban regeneration, CO2 sequestration, resource deployment (as a compost reuse) and providing a range of ecosystem services (eg Bardos et al, 2011a, and Witters et al, 2012). However, the application of GROs as practical site solutions is still in its relative infancy, despite substantial research. The barriers to wider adoption, particularly in Europe, arise both from the inherent characteristics of GROs as site management technologies, and market perceptions of uncertainties over whether these methods can achieve effective risk management in the long-term. The majority of remediation work in Europe has been carried out because of regulatory demand for critical risks and/or to stimulate the reuse of brownfield land. Most funded remediation and brownfield regeneration projects, so are in or around urban environments, and brownfields reuse is strongly driven by economic factors. These projects are often constrained by pressure on timescale and relatively limited site areas. Both of these factors have tended to exclude consideration of GROs that are perceived as slow and more suited to large area problems. The time taken before prescribed ‘total’ concentration-based targets such as soil quality thresholds are reached is also seen as a limitation for GROs. This has led to intensive discussions in particular about phytoextraction, which is perhaps the most well-known GRO, and has been widely tested at demonstration scale (eg Vangronsveld et al, 2009, and Mench et al, 2010). Phytoextraction has often been seen as a source management activity that seeks to gradually remove trace elements from soil over time into biomass. Phytoextraction has poor acceptance as a functional source management tool because contaminant removal may take decades and there is some concern over the fate of and contaminant concentration in harvested biomass (eg Van Slycken et al, 2013). Acceptance of other GROs related to phytostabilisation and in situ immobilisation is limited because source removal does not take place, and there is a perception that stabilisation or immobilisation is potentially reversible over time (eg Onwubuya et al, 2009). The constraints on acceptability of GROs seem inevitable when remediation success is judged solely using generic soil concentration targets. While this targetled approach can be attractive because of its simplicity, its built-in conservatism may lead to over-designed risk management solutions, which are costly and may not be sustainable. A site specific approach that properly considers source and pathway interventions in a comprehensive risk management strategy allows a more targeted and sustainable risk management solution (NICOLE, 2012, and Bardos et al, 2011a). This also creates a better rationale for the deployment of plant- and microorganism-based GROs. GROs may then facilitate land regeneration in circumstances where the case for intervention is economically marginal by virtue of their lower cost and also, potentially, by their linkage to other project services such as biomass, public green space, recovery of land values etc. GRO approaches can be tailored along pollutant linkages, for example: source: gradual removal or immobilisation of source term pathway: rapid reduction in flux of contaminants to receptors at significant risk receptor: using vegetation to manage receptor access to the subsurface. Examples of circumstances that do not favour existing treatment-based remediation solutions, but may be highly amenable to this broader GRO-based risk management approach, include: large treatment areas, particularly where contamination may be causing concern but is not at strongly elevated levels where biological functionality of the soil is required after site treatment where there are budgetary constraints where there are deployment constraints for land remediation process plant (eg as a function of area and location). Intelligently applied GROs can provide rapid risk management via pathway control, through containment and stabilisation, coupled with a longer term removal or immobilisation of the contaminant source term. These can be durable solutions as long as land use and land management practice does not undergo substantive change causing shifts in pH, Eh, plant cover etc suggesting that some form of institutional or planning control may be required. However, the use of institutional controls over land use is part-and-parcel of urban remediation using conventional technologies (eg limitation of use for food production), so any requirement for institutional control and management with GROs continues a long established precedent. The GREENLAND project (Gentle Remediation of Trace Element Contaminated Land) involves 18 European research teams and examines the practical application of GRO to remove or stabilise heavy metals in soils present at former industrial sites. For more information about the GREENLAND project go to: www.greenland-project.eu/


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