The remediation strategy will aim to render the site appropriate for recreational purposes. This will be achieved by finding the most appropriate method do breaking the significant pollution linkage (figure 1).
This may be done by removing the contaminant or pathway. Since the proposed land use is already determined the receptor cannot be removed. In this instance the most sensitive receptor is likely to be children since it is proposed to redevelop the site for recreational purposes.
Leachability tests show that a high proportion of the cyanide and polycyclic aromatic hydrocarbon compounds present in the samples taken are adversely affecting passing waters. This is one of the pathways of the pollutant to the receptor.
Another pathway is through ingestion, especially since children are likely to be present on the site and may consume some of the soil.
Other pathways likely to affect less sensitive receptors include skin contact and inhalation of small particles of the contaminant suspended in the air.
Significance to Human Health and the Environment
Heavy metals, such as lead and zinc as found on the site, when ingested in large enough concentrations can cause respiratory cancers, kidney dysfunction, birth defects and emphysema and other lung disorders. Since small children are likely to be on the site due to its proposed use as a recreational area, this is a significant pathway. Also, if the metals become airborne and are inhaled they have the potential to cause respiratory cancers and other lung disorders. This is another significant pathway since the high leachability of the soil suggests it is comprised of fairly fine particles. Zinc and copper can also stunt plant growth and cause discolouration.
Skin contact with phenols, again, a likely pathway, can cause burning and may be carcinogenic and teratogenic (cause birth defects).
Some polycyclic aromatic hydrocarbons are more harmful than others; several types have a carcinogenic effect. On the site, exposure is likely to be as a result of inhaling suspended particles in the air, although they can also be detrimental to human health when ingested, as the soil on the site may be by young children, and as a result of skin contact with high concentrations.
Dust containing cyanide particles may be inhaled, especially since the soil on the site is suggested to be comprised of fine particles. Exposure to relatively low levels may cause shortness of breath, convulsions and loss of consciousness. Long term exposure, such as may be experienced by someone working on the proposed recreational area, can cause hearing and vision problems, loss of muscle coordination and may affect the thyroid gland.
All of the pollutants present in the groundwater in sands and gravels and the Bunter Sandstone are classified as list I substances by the Discharge of dangerous substances EC Directive 76/464 EEC. The substances included in this list are selected regarding their high toxicity, persistence and bioaccumulation or their biologically harmful nature.
The cyanide contamination of the River Irk and reservoirs will have a detrimental impact on the aquatic life which are particularly sensitive to cyanide exposure. Concentrations as low as 5ï¿½g/l can inhibit the reproduction of may species of fish. Concentrations as high as in the upper reservoir adjacent to the tip (90ï¿½g/l) are toxic to many species of fish
Site Remediation Options
Removing the receptor is not an option in this instance since the future land use has already been determined.
Several methods of removing the pathway are available, such as cover systems. These are relatively low cost, allow rapid site remediation and are suitable for dealing with large volumes of material, however considering the sensitivity of the receptors involved in the future land use this is not appropriate.
The pollutant linkage must therefore be broken by removing the contaminants or using physical, chemical or biological techniques to reduce the toxicity or mobility.
One remediation method for removal of the contaminants in the ground is through excavation of the contaminated material and disposal through landfill. This is a very quick method, relatively low cost and is suitable for the removal of a wide range of contaminants. However, the variation and concentration of the contaminants requires much of the soil to be treated as hazardous waste. This is likely to restrict the landfills which will accept the waste, meaning it is likely to require transporting further. Considering the proximity principle, the cost of landfilling the contaminated soil depends somewhat upon the distance from the landfill and accessibility, for example, whether it could be transported my train. Also, the soil may require treatment before disposal in order to be accepted by a landfill, considerably contributing to costs.
Cyanides and polycyclic aromatic hydrocarbons can be reduced significantly through ex situ thermal treatment; technologies including thermal desorbtion, incineration and vitrification. Very high reduction rates of cyanide and polycyclic aromatic hydrocarbons are reported including where dealing with high concentrations and material variations, although thermal techniques are generally ineffective at dealing with metals and most inorganic contaminants. Thermal technologies are expensive due to high energy consumption, implying it would be too expensive to excavate, transport and treat the whole site in this way.
Several methods available for treating heavy metal contamination in the soil are appropriate for treatment of the site, some appropriate for the remediation of other types of contamination too.
Soil washing is a technique suitable for removing heavy metals including zinc and lead from the ground. Soil washing is generally only viable on sites with relatively permeable ground, otherwise implying unacceptably high tipping charges due to the quantity of highly contaminated sludge, which must be disposed of as hazardous waste. The results of the leachability test imply the ground is sufficiently permeable to allow for in situ soil washing. A variation of this technique using froth flotation is effective at removing low concentration cyanide and polycyclic aromatic hydrocarbons. However this can be an expensive process when dealing with a large volume of material.
Chemical stabilization is suitable for significantly reducing the mobility of heavy metals as well as hydrocarbons. This can be a quick method of immobilizing the contaminants and significantly reducing their leachability.
Phytoremediation is a method appropriate for breaking down heavy metal and organic contaminants in the soil. This is cheap and effective, however takes longer than other methods.
The most significant contaminant affecting the water is cyanide, present in the surface and ground water.
I would consider ex-situ thermal treatment, possibly vitrification, the best available technique for remediation of cyanide and polycyclic aromatic hydrocarbons in the soil. This is, however, an expensive method for use on the entire site. Therefore it may just be used for the soil with the highest concentrations of cyanide and polycyclic aromatic hydrocarbons. I would recommend excavation and thermal treatment of the most contaminated soil, which is isolated to the first 1m of fill and a borehole 4m below ground, respectively.
The highest concentrations of zinc and copper are situated on and around the main tip area. I would recommend soil washing for this area of heavy metal contamination. In situ treatment should be appropriate since the leachability of the soil suggests the soil is sufficiently permeable.
The main issue relating to the low level of contaminants which affect the whole site; heavy metals, polycyclic aromatic hydrocarbons and phenol, is the extent of the land which is contaminated. This implies high costs when considering most remediation strategies. Three methods are appropriate, the most so depending on several factors. If time is not a significant issue, phytoremediation is the cheapest technique available. Alternative methods are soil washing with froth flotation or disposal and landfill. These are more expensive techniques, however much of the cost relate to the proximity of the nearest soil washing plant or landfill which will accept the waste. The best available technique is therefore determined by whether time or cost is more of an issue for the site remediation and the proximity of the required facilities.
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Pratt, M, Remedial Processes for Contaminated Land, Institution of Chemical Engineers.
Syms, P, Contaminated Land: The Practice and Economics of Redevelopment.