8 REMEDIATION OPTIONS - Castings Technology International

8. Remediation Options
8 REMEDIATION OPTIONS
The remediation options available to manage unacceptable risks are to:
• manage (remove, destroy, modify or immobilise) the source,
• interrupt the pathway or
• modify the receptor or the behaviour of the receptor.
In the context of hazard management, such as under the PPC regime, the options are restricted to
source management.
Source management may involve the removal, destruction, stabilisation or transformation of the
source. Pathway interruption may involve either the blocking of the pathway or the destruction or
removal of contaminants moving along a pathway.
It will by now be clear that a sound understanding of the site and pollutant linkage components,
encapsulated in a conceptual model, is essential to the identification of an appropriate and cost
effective remediation option.
Remediation may be carried out either in situ (in the ground) or ex situ (after removing the
contaminated soil, fluid or vapour from the ground). Ex situ remediation may take place either on site
or at an off site treatment or disposal facility.
Remediation technologies may involve conventional civil engineering techniques such as excavation
and off site disposal or the installation of barriers. They could also use chemical, physical, biological
or thermal processes to manage the source or interrupt the pathway.
The following discussion is presented in terms of the component of the pollutant linkage being dealt
with rather than the class of remediation technology being used. This is not meant to be either a
comprehensive coverage of all remediation technologies nor to be a substitute for specific expertise in
the selection, implementation and verification of remediation.
Further descriptions of remediation technology are summarised by Nathanail et al. (2002) and the EA
(2002c). A series of biological case studies from the UK are described by Finnamore et al. (2000).
Barr et al. (2003) describe a series of non-biological case studies from the UK. Additional information
is available from CL:AIRE (www.claire.co.uk) - a publicly funded body aiming to disseminate
information about practice and research in contaminated land in the UK.
Page 91

8. Remediation Options
An overview of remediation options that have been applied in the UK is provided below.
Table 32. A Classification of Some Remediation Options Applied in the UK
Technology Class Application Medium
Linkage Element
Affected
Excavation and
off site disposal
Civil Engineering
Ex situ
Soil, Free product,
water, vapour
Source management
Pump and treat Physical Ex situ Free product, Water Source management
Air sparging Physical In situ Water Source management
Soil washing Physical Ex situ Soil Source management
Soil vapour
extraction
Physical In situ Soil – vadose zone Source management
Windrows Biological Ex situ Soil Source management
Biopiles Biological Ex situ Soil Source management
Thermal
desorption
Thermal Ex situ Soil Source management
In situ vitrification Thermal In situ Soil Source management
Incineration Thermal Ex situ Soil, free product Source management
Capping Civil Engineering In situ Soil Pathway interruption
Vertical barriers Civil Engineering In situ
Free product, water,
vapour
Pathway interruption
Bioslurping Biological In situ Water Pathway interruption
Biosparging Biological In situ Water Pathway interruption
In situ oxidation Chemical In situ Soil Pathway interruption
In situ reduction Chemical In situ Soil Pathway interruption
Monitored Natural
Attenuation
Biological,
chemical &
physical
In situ natural
processes
Water
Pathway interruption
Permeable reactive barriers (PRB) are a means of bringing some of the advantages of ex situ
technologies to the in situ domain. They comprise an impermeable barrier (the ‘funnel’) with an
opening (the ‘gate’) that allows contaminated water or vapour to flow through. PRBs operate by
allowing or funnelling contaminated water or vapours to reach the gate and then treating the
contamination while allowing the carrier water or soil air to pass freely through the barrier.
Page 92

8. Remediation Options
8.1 Source management
Remediation targeted at the source of contamination can seek to remove the contaminants, destroy
them, change their chemical form or immobilise them.
8.1.1 Soil
Both heavy metals and organic contaminants can adhere to the soil particles, especially clay and
vegetation components. In situ source management options for contamination in soil include:
• solidification and stabilisation,
• vitrification.
Ex situ methods involve excavation of the contaminated soil or pumping contaminated water or
vapour out of the ground followed by disposal or treatment. Technologies include:
• off site disposal to licensed landfill site,
• encapsulation in an on site engineered cell,
• soil washing and then treatment or disposal of the contaminated residue.
8.1.2 Free product and non aqueous phase liquids
Leaking tanks or spills can result in free product migrating down through the ground. Some products
will dissolve readily in groundwater. Others are not soluble or only sparingly soluble in water – they
are called non-aqueous phase liquids (NAPL). NAPL such as petrol or diesel are lighter than water and
will float on the water table (LNAPL). Chlorinated solvents such as trichloroethene (TCE) are denser
than water and will sink through the ground water to the bottom of an aquifer (DNAPL).
In situ remediation options include monitored natural attenuation (MNA). Natural attenuation has
been observed at the edges of many occurrences of free product. However degradation of the core of
a source area is only very rarely possible. MNA is particularly effective for dissolved phase
hydrocarbons and phenol (in low to moderate concentrations, but ineffective for dissolved chlorinated
solvents. Once contamination has entered the fissures of bedrock it is almost impossible to remove it
all. This makes it difficult to develop ex situ techniques for fractured bedrock.
Ex situ methods include pumping free product and destroying, treating or disposing the recovered
fluids. This can reduce the amount of free product remaining in the ground, but is a slow and
expensive process with variable results.
8.1.3 Vapour/ gas
In situ remediation options are limited, but include soil vapour extraction. Preventing infiltrating water
from reaching waste can slow down the rate of methane production.
Ex situ methods involve recovering the vapour or ground gas followed by capture or destruction.
Technologies include:
• capture via an activated carbon filter,
• thermal desorption from excavated soil,
• destruction by an incinerator.
Page 93

8. Remediation Options
8.2 Pathway Interruption
Once contamination has been released into the ground it can migrate vertically and laterally
contaminating ever-larger volumes of ground. Once contamination reaches the water table, it may
float on top of groundwater, sink through it or either readily or sparingly dissolve in the groundwater.
Preventing the spread of contamination may be an interim solution until the source can be dealt with
or it may be sufficient to render land fit for its current or intended use without removing the source.
Under the PPC regime pathway interruption is unlikely to be acceptable to regulators as a permanent
solution but may be needed to prevent further spread of pollution.
8.2.1 Soil
Receptors can be protected from exposure to contaminants in soil by means of barriers between
source and receptor. Options include:
• capping layer thick enough to stop people coming into contact with contamination,
• vertical barrier keyed into an underlying low permeability soil.
8.2.2 Water (dissolved phase)
In situ remediation options include:
• biostimulation, where natural microbiological processes are accelerated by adding something
for ‘bugs’ to eat and/or to breathe.
• permeable reactive barriers,
• air sparging.
Ex situ methods of treating dissolved phase contaminants are based on conventional waste water and
effluent treatment and include:
• filtration,
• biostimulation,
• pH adjustment.
8.2.3 Vapour/ gas
The distinction between source and pathway for vapour or gas contamination is determined by
concentration and is therefore a blurred one. Vapours may also form part of the pathway by which
volatile liquid contaminants can reach people. In situ remediation options include:
• permeable reactive barriers,
• venting trenches,
• capping,
• vertical barriers,
• gas proof membranes beneath buildings,
• soil vapour extraction.
Page 94

8. Remediation Options
8.3 Receptor Behaviour
The land use will influence who will be at greatest risk. By choosing a less sensitive land use,
contamination may be shown to pose an acceptable level of risk. Examples of restricting receptor
behaviour include prohibiting the growing of vegetables for home consumption.
Security measures such as fencing should not be relied up on to keep out members of the public.
They may however be used to protect an informed workforce.
8.4 Options appraisal
The overwhelming majority of remediation in the UK involves excavation and off site disposal to a
landfill or a surface cap (Nathanail C.P. et al. 2002; Rivett et al. 2002). However changes are arising
from the implementation of the Landfill Directive, the introduction of new technical ‘Model Procedures 1
by the Environment Agency (EA 2003g) and an increasing understanding of the need to protect
groundwater. These will almost certainly promote a shift to more treatment based technologies and a
requirement by regulators for a more transparent documentation of the process by which the
implemented remediation technology has been selected.
The Environment Agency, in their draft ‘Model Procedures’ (EA 2003g), suggest a two stage approach
to remediation options appraisal. The first involves a rapid screening to identify three to four feasible
strategies that are likely to achieve the essential or core objectives of managing the risk followed by
detailed evaluation of these short listed strategies. Finally a remediation strategy is developed that
will demonstrably address all relevant pollutant linkages.
The starting point for options appraisal is the conceptual model of pollutant linkages and a clear
statement of the remediation objectives. There are about 30 generic remediation technologies and
several hundred commercial variants of these. While not all have been applied in the UK, many are
commercially available and should be considered. The core remediation objectives coupled with any
project or site specific constraints (such as space, time available, depth to contamination) should be
used to identify which of these technologies are not appropriate on the site. Each option should be
‘mapped’ onto the conceptual model showing which linkages it is breaking. Only once all the linkages
have been addressed does the technology(ies) constitute a valid remediation option. Generic
information on remediation technologies that may be used to assist in options appraisal is available
from the EA (2002c), US EPA (http://www.clu-in.org/remed1.cfm) and in Nathanail et al. 2002.
8.4.1 Breaking a pollutant linkage
To determine feasible remediation options, you should refer to the conceptual model (see Figure 11 in
Section 5 or Table 27 in Section 7). From that you should establish what the contaminant is (e.g.
arsenic, benzene or trichloroethene), what form it is in (e.g. elemental mercury, hexavalent
chromium, free product, dissolved phase), what its fate and transport properties are (e.g. solubility,
volatility, durability, bioaccumulation potential) and what medium it is present in (e.g. soil, soil air,
groundwater).
Then remediation options can be selected that will deal with the specific form of the contaminant in
the actual medium it is present in. For example soil vapour extraction will remove hydrocarbon
vapours from the unsaturated zone in a sand and gravel soil. However air sparging will remove the
hydrocarbon dissolved in the groundwater deeper in the same sand and gravel. If groundwater is
moving laterally, a permeable reactive barrier can intersect the flow and remove or destroy the
hydrocarbon. Alternatively if there is sufficient travel time before an abstraction borehole is impacted,
monitored natural attenuation may be invoked.
1 The Model Procedures are written from a risk management perspective and relate to breaking the entire
pollutant linkage. This is appropriate to remediation being carried out under the planning system or for Part IIA
purposes. For PPC purposes, the source of any new spills or releases must be dealt with.
Page 95

8. Remediation Options
8.4.2 Breaking several pollutant linkages
Where a conceptual model involves several pollutant linkages, then either a remediation technique
that deals with all of the linkages or a combination of techniques may be needed. If the linkages on a
site are spatially separated and involve different sources and pathways, then separate solutions may
be more likely than where the same source or pathway appears in several linkages.
In any event the aim is to be able to carry out remediation that demonstrably breaks all the pollutant
linkages in the site conceptual model.
Finally it should be borne in mind that the selection and design of remediation options is as much an
art as a science and there is no necessarily optimal solution. However, there will be many sub optimal
or inappropriate options. Selection of a solution without adequate evaluation could result in residual
contamination that still poses an unacceptable risk and future liability for problems in the future.
Page 96