In some cases, remediation is not feasible due to multiple causes, from the physical characteristics of the site, for example, instability of the terrain, proximity to active volcanoes, high risk of earthquakes, risk of flooding, etc., to the vast extent of the area affected by pollution or the existence of multiple contaminants that require the combination of several techniques, making remediation a very costly and inefficient. There is also the circumstance where the polluted area has high natural (ecological) or scientific value that would be irrevocably destroyed using certain remediation technologies. For these cases, there are a number of management options to mitigate the risk to public health and the environment. It is also possible to adapt the use of the site such that it still provides an amenity value.
If the site still has a viable soil biota and a natural plant colonization, some natural attenuation of the pollution without any external intervention can occur. Such solutions could be suitable for large areas that have been subject to diffuse pollution. However, the remediation process is very slow and consequently the effectiveness can be limited. Careful assessments need to be undertaken to ensure that the risks of exposure to the contaminants are minimized. This could include allowing the land to remain fallow and to minimize any activities that could allow the contaminants to become available. Working the soil should be avoided as should livestock grazing. The site will require monitoring to check that pollution levels are reducing and to ensure that the risk assessment remains valid (Mulligan and Yong, 2004).
In areas with high levels of pollution where there is a high potential for exposure, and there are insufficient funds available to remediate the site, exclusion of all access to the site may be the only solution. If there is a local human or animal population that are vulnerable to the pollution, it is important that they are excluded. It is important that there is some institutional control over the area with the authority and resources to impose and enforce the restrictions. The exclusion can be in the form of fencing or other physical barriers and warning signs. This should be supported by communications programmes to sensitize the population of the dangers to them and their animals of entering the site. The case study below illustrates exclusion as part of a comprehensive risk reduction strategy.
Suzak A is hazardous waste storage site (“polygon”) constructed during the Soviet era, which contains about 3 000 tonnes of waste with a high concentration of DDT (Toichuev et al., 2017). The waste had been buried and covered in topsoil. Since 1989 the site has had minimal security and was being “mined” for the DDT, which was being sold in local markets as a pesticide. The scavenging operations caused the waste to be exposed and dispersed by wind and rain. There were records of the death of livestock and health impacts in the local population from eating animal products from livestock that had grazed at the site. While efforts were being made to find funding for the removal and destruction of the DDT, a network of local and international NGOs worked with the regional authorities to implement a risk reduction project. The project had a total budget of USD 100 000 which was funded by the GEF Small Grants Programme with matched funds from Green Cross Switzerland. The project components were:
Since the project completed in 2014 there have been no further reports of human or animal poisoning, however there were some lapses in security. Following such interventions, it is important that the responsibility for the long-term management of the site is appropriately assigned and resourced. In 2018 the GEF approved a full-sized project for the destruction of the DDT waste using non-thermal techniques.
If the future activities on the site are unlikely to disturb the underlying soil and the hydrogeology of the site allows it, it could be an option leave the polluted soil in situ and to seal the area of pollution with an impermeable layer. This will minimize the effects of wind and water erosion and prevent human or animal contact with the polluted soil. Capping is a temporary solution and should only be considered in cases where there are constraints that prevent best practice solutions from being deployed.
The capping could be clean soil, in which case it would be sensible to install an impermeable membrane such as geotextile and high-density polyethylene liner, or a layer or tier system with materials of low permeability such as clay. Alternatively, a concrete or asphalt slab can be installed over the site. As well as minimizing water ingress and therefore the potential for leaching, the membrane serves to avoid the clean capping materials becoming polluted. This is important for any subsequent intervention to remediate the site. It will be necessary institute a mechanism for the long-term management and monitoring of the site to ensure that the cap continues to prevent access to or escape of the polluted soil and that the contaminants are not migrating in groundwater. To protect the integrity of the impermeable layer plants grown in the clean soil should be limited to an herbaceous cover and shrubs with a shallow root system.
Capping is not considered one of best technologies available. A key point associated to unsustainability is the provenience of the clean soil since it can be a natural soil extracted from other local areas or even the use of compost as clean soil.
The population of Dong Mai village suffered from chronic lead poisoning from decades of recycling used lead acid batteries. In 2008, the recycling activities were relocated to an industrial area away from the town, however the concentration of lead in the surface soil of the villagers’ plots remained high. Following consultation with the villagers and authorities, it was decided that capping with clean soil represented the most effective method to protect the villagers’ health. The polluted soil of 39 plots was first covered with a membrane and then a 30 cm layer of clean soil (Figure 38). The membrane minimized the risk of the lead migrating to the clean soil layer. Health monitoring of children aged 0 to 5 years before and after the intervention showed that blood lead levels decreased 72 percent from a geometric mean of 390 μg/l to 110 μg/l (Pure Earth, 2017).
Where the pollution could migrate through the soil, for example by liquid contaminants directly or by ground water transporting contaminants, and threaten sensitive environmental resources such as a well, the construction of an impermeable barrier could provide protection. It is important to understand the hydrogeology of the site, the conceptual site model of the pollution and all the potential pathways for its migration in order to determine the likely effectiveness and design for such an approach. The range of options for installing a barrier include, sheet metal piles, soil and bentonite slurry walls, soil and cement walls, and injected grout barriers. For a barrier wall to be effective, it should be deep enough to key into the impervious bedrock below the ground water (Pearlman, 1999).
The combination of phytoremediation and crop production, with or without the application of amendments, is known as phytomanagement. Phytomanagement is a site management strategy which, alongside risk management, places realization of wider (including economic) benefits as the basis of its design. It uses less invasive remediation options as part of integrated site management strategies rather than applying plant monocultures over extensive areas to gradually extract or immobilize the bioavailable contaminant pool. The main aim of phytomanagement is to exploit the socio-economic value of plants in polluted areas. For example, plants can be used in the manufacture of bioplastics, biofuels, biochar, paper and furniture. Such uses potentially enhance local economies. Phytomanagement approaches allow the use of plant-based systems as a “holding strategy” prior to development of favourable economic conditions for redevelopment or other site regeneration. Phytomanagement reduces the risks of exposure to contaminants on vacant sites, while providing other benefits such as biomass generation, amenity and leisure, improving the aesthetics of the surroundings, urban climate management and providing ecosystem services. Potential plant products are non-food products such as biofuel, fibre, wood or, depending on the contamination level and contaminants concentration in shoots, animal feed (Robinson et al., 2009).
Change of land-use can help to maintain the beneficial use and value of the contaminated lands.
For healthy soil biota and nutritious crops, the presence of essential trace elements, such as copper, iron and zinc, is important for metabolic processes. At elevated levels, however, they can cause adverse effects. Non-essential trace elements such as arsenic, cadmium, lead and mercury are toxic even in very low concentrations (Rai et al., 2019). Where soil has become polluted with marginally elevated levels of trace elements, by changing agricultural practices it may still be possible to reduce the risks of them entering the food chain. It is also possible to reduce the plant uptake of trace elements by adding organic and inorganic amendments such as manures, biosolids, lime, zeolites, biochar or iron oxide to the soil (https://doi.org/10.1080/09593338309384197). The propensity to accumulate each trace element varies from crop to crop (Kloke, Sauerbeck and Vetter, 1984). Risk is reduced by changing from the cultivation of crops with higher accumulation rates such as leafy vegetables like spinach and lettuce to less sensitive ones such as pulses, tubers and certain grains. Crop rotation can also influence the rate of uptake. Rhizosphere effects of some plants may affect the bioavailability of a trace element to the following cropping cycle. For example, citric acid released by lupins can increase the uptake of cadmium in wheat planted in the following rotation (Rai et al., 2019).
In cases where agricultural land has become more heavily polluted and it is no longer safe to grow crops for human or animal consumption, it may be possible to continue agriculture with the cultivation of non-edible crops for fibre (e.g., cotton or flax), trees for timber, or biomass for renewable energy generation. Consideration, however, should be given to the future use of the products and the potential impact of accumulated trace elements. For example biomass power generation process could use pyrolysis or gasification processes to avoid the dispersal of the accumulated trace elements (Ghosh, 2005). The case study of the Berg Aukas mine site in Namibia combines the relocation of youth training facilities with sensitising the local population to modify their agricultural practice to minimize the risk of ingestion of trace element contaminants.
The Berg Aukas mine, which extends approximately 21 km2, produced lead, vanadium and zinc from 1920 until its closure in 1979 (Error! Reference source not found.). The main operations at the site were mining and roasting the ore. Since the 1990s, the land and buildings were used by the government for a youth vocational training centre, an agricultural vocational school, and the surrounding farmland for grazing and horticultural crops. The area included tailings piles and slag heaps that are subject to wind and water erosion. The slag was also being excavated to provide materials for road constructions. In 2005, the national government undertook an environmental survey that identified unsafe levels of trace elements (As, Cd, Cu, Pb, Hg and Zn) in the surface soils over a wide area of the site. The contaminants were associated with fine particles (<8 μm) that elevated the risk of exposure by inhalation through wind erosion. The contaminants were also present in water that was extracted from the mine for human and animal consumption, and for irrigation of horticultural crops (Mapani et al., 2010). The pollution was also being dispersed through the excavation, transportation and use of the slag for road construction. The Geological Survey of Namibia undertook a detailed survey of the site in 2013 and provided further recommendations for risk mitigation (Hijamutiti et al., 2014).
As the pollution was widespread, the government decided that remediation was not an immediate option, but that safeguarding human and animal health was a priority. The training centre and agricultural school were relocated to Rietfontein. The local farmers were sensitized to the location of the polluted areas and given advice on risk mitigation including avoiding crop production in the most polluted areas and changing the horticulture from root vegetables to less vulnerable crops such as maize, tomato and pepper in the less polluted areas (Hijamutiti et al., 2014).
Many national regulations set standards for maximum contaminant levels for different types of land use. Residential, parkland and agricultural land generally have more stringent limits than land designated for commercial or industrial use. The Canadian Council of Ministers of the Environment (CCME) provides useful guidance and soil quality standards for a range of contaminants (The Canadian Council of Ministers of the Environment, 2007, 2014). For sites where the levels of contaminants remain too high for agricultural use, it may be possible to change to a less sensitive use such as manufacture, industry or a civic amenity such as a car park. The construction of buildings and hard surfaces above stabilized and immobile polluted soils will prevent wind and water erosion and contaminant leaching, and provide a beneficial use of the land. Such solutions would require approval under the national regulations. It could be an appropriate approach for sites located close to urban environments. As the contaminants will remain in close proximity to the new construction, careful risk assessments should be undertaken to ensure that the potential for exposure is minimized. The site should be regularly monitored to check that the contaminants remain immobile and that the risks of exposure are acceptable.
Where sites are not suitable to be returned to agricultural, residential, commercial or industrial use, conversion for renewable energy generation is an option. Sites with residual contamination, or where the contaminants have been stabilized or sequestered, for example a capped landfill, abandoned mine, or a brownfield site are potentially suitable for the construction of photovoltaic cells or wind farms. These sites can have significant advantages over the use of undeveloped open space or green field sites. They are often close to users of power such as cities or industrial areas and can leverage existing infrastructure by linking to power lines. It is often more acceptable to local stakeholders to utilize an abandoned site rather than developing in an undeveloped landscape. The revenue from power generation can assist the funding of on-going maintenance and monitoring costs of the site. In cases where a remediation strategy will take a long time to implement, such as in natural attenuation or phytostabilization, it is possible to install renewable power generation during the implementation phase. The US EPA actively encourages the development of such renewable energy generation schemes as part of its Re-Powering programme which is illustrated in the following case study.
The US EPA has established the “RePowering” programme to encourage the use of polluted sites such as landfills and mine sites for renewable power generation. The programme’s website (US EPA, 2014) includes case studies and guidance for establishing such projects. The map in Figure 39 shows landfills, superfund sites, mine sites and brownfield sites in United States of America where renewable energy generation has been established.
In 2019 the programme included 327 sites with a total installed renewable energy generating capacity of 1 710 megawatts.