Practical contaminated land management: methods of remediation

By Edie Newsroom

1st July 1997

Introduction

In the first contaminated land module we looked at the regulatory context of contaminated land and the current philosophies for its management. We then considered the factors involved in the investigation of land suspected of being contaminated, the differences in various exploratory techniques for investigating soils and groundwaters, and we discussed basic requirements for sample storage, transport and dentification. In the second contaminated land module of the series, we considered the meaning of analytical data and how they can be used to interpret specific site situations. In this module we consider the methods for remediating contaminated land appropriately and what can be achieved in order to convert a contaminated site into a valuable resource.

Categories of land remediation

Fitness for use is the key factor in land remediation. This is achieved when the potential for significant environmental harm is reduced to an acceptable level for the proposed land use. There are three main categories of contaminated land remediation:

Removal of contaminated material

Site engineering

In situ treatment

In the first and second contaminated land modules we used the source – pathway – receptor philosophy to assist in addressing the investigation a nd assessment of contaminated land, and in this module we shall again use it to help us consider the options for remediation.

Historically in the UK and many other developed countries, the traditional approach to land remediation has been to dig up the contaminated material and deposit it in a landfill. In extreme cases the contaminated soil has been incinerated. This is an effective way of removing the source of contamination at the site and, if carried out properly, can result in a site acceptable for redevelopment after the import of clean material to replace that which has been removed. Unfortunately, a great deal of valuable landfill space has been taken up by this type of activity, and it has become recognised as the mere transport of the problem elsewhere as opposed to sustainable remediation. Recent planning and waste management policies, not least of which is the introduction of the Landfill Tax in theUK, discourage the practise of transporting contaminated materials away from sites and promote the development of on-site remedial techniques, many of which have been available for several years – especially in the USA.

Simple engineering techniques have been used to eliminate the pathways for the passage of contaminants through the ground. These are predominantly impermeable vertical and horizontal barriers in the ground, which may be designed either to encapsulate the contaminated material or to prevent its ovement towards sensitive receptors. The advantage of these techniques that, because they prevent the movement of water which is the main vehicl for contaminant migration, they can be used for a very wide range of contaminants. An example of an impermeable barrier is the capping of a contaminated site with clay.

In-situ methods for contaminated land treatment involve methods of immobilising, stabilising, washing, transforming or separating contaminants in the soil. By utilising these methods we are able to change the nature of the contaminant source in order to reduce the risk to the surrounding environment.

Site remediation strategies

After a site has been assessed as contaminated and requiring attention, further study is necessary to investigate the feasibility and likely success of appropriate remedial techniques. The types of contaminants which present a significant environmental risk to either the environs or the proposed development should have been identified in the site investigation stage, but more detailed and extensive sampling is required

to address specific remedial needs.

A focused study to delineate the areas and volumes of significantly contaminated ground is the first stage of the remedial strategy. This will build on the original site investigation data and provide further information on the degree of clean-up required and help to define the remedial objectives that can be realistically achieved.

The feasibility study should centre on discussions with relevant regulators to provide an assurance that the potential for significant environmental harm will be effectively reduced to acceptable levels. Furthermore, regulators, financial institutions, insurers and site purchasers will also need assurances that the level of clean-up taking place is defensibly measurable, so an independent monitoring regime should also be proposed and approved. Based on the level of contaminant source reduction required to achieve the set goals an array of remedial options can then be considered for their suitability and regulator approval. This work usually involves the use of bench-scale simulations and a review of factors which could affect the process viability physically such as odour release, space restrictions or ground stability. Once the suitable technique is agreed between the parties involved, detailed methodologies are set out and the appropriate funding, project management and sub-contract parties are identified and selected.

The actual site remediation process then comprises three further stages:

Implementation of the works on site to reduce contaminant concentrations

Verification of results

Site aftercare

The implementation of works on site requires permanent technical staff to assess, monitor and verify the actions of the various contractors. Also public and operator health and safety issues need to be addressed, and will have formed part of the approval by the relevant regulator. At various stages and locations of the remedial process, analysis of soils, effluents and groundwater must be used to verify both the removal of the targeted contaminants from the soil and their disposal or treatment routes. Once the remedial work on site has been performed to the satisfaction of the regulator, samples are taken from soil and groundwater once again as part of an aftercare program to monitor the continued absence or reduction of the selected contaminants.

Common remedial methods

Vertical Impermeable Barriers

This technique is commonly used to attenuate the lateral movement of a contaminant through the ground i.e. the pathway for movement of the contaminant is blocked in order to prevent the undesirable presence of the contaminant elsewhere. These are used extensively to eliminate the lateral migration of landfill gas from waste deposits through permeable strata, preventing the potential for accumulation of gases in confined spaces.

These barriers only function fully where impermeable horizontal strata exist below the site, into which the artificial barrier can be keyed tocontain the contaminant. These barriers can also be used to prevent the lateral migration of liquids and dissolved contaminants, and are usually manufactured from geosynthetic materials which are water (or gas) impermeable and resistant to tearing and puncture. Nevertheless these membranes are normally installed with protective layers of sand. At present most vertical barriers are prescribed with a secondary layer of impermeable material to back up the primary protective barrier, typically

bentonite clay.

Grouting

Traditional grouting can also be used to form impermeable vertical barriers in the ground to prevent contaminant migration. This has been widespread in the past, but is now becoming restricted to locations where othermethods are impractical. Also some grouts are now recognised as unsuitable due to their chemical effect on groundwater.

Capping

Capping sites has an effect on:

Vertical and lateral migration of dissolved contaminants

Upward and lateral movement of gases

The ingress of rain and surface waters is reduced or eliminated by the presence of an impermeable engineered cap on contaminated land. The result is the elimination of the media in which the vertical and lateral movements of contaminants take place. Therefore the pathway for contaminants is eliminated and the source is isolated from the receptor.

Capping can have both positive and negative effects on gas and vapour migration – preventing these from permeating through the surface and potentially accumulating in confined spaces, but encouraging lateral migration where they may previously have diffused harmlessly into the atmosphere in very low concentrations.

Capping is common practice on closed landfill sites and other contaminated sites where the main environmental concern is the ingress of water rather than the direct emission of undesirable substances. In the event of gaseous contaminants being present in a capped site, e.g. landfill gas, vents are required in order to avoid build up of gas, lateral migration and, depending on concentration, the gas may be subsequently released to atmosphere, burned off, or burned to produce energy.

Total Encapsulation

The above methods can be combined to form a total seal around contaminated soil. The combination of horizontal barriers above and below the contaminants with vertical barriers to the sides isolate the contaminants from their migration pathways and prevent the ingress of media such as water that may encourage movement from the source to a sensitive receptor 2E

This technique has been used extensively in the past to isolate soils with high concentrations of heavy metals from the surrounding environment.

Bioremediation

Bioremediation is a technique for degrading certain contaminants by the enhanced action of micro-organisms. During bioremediation, the contaminants are transformed in situ to other substances by the biochemical action of micro-organisms whereas in traditional techniques the contaminants are merely moved from one location or phase to another. This has been a major step forward in that the intermediates and products of bio-remediation (if correctly applied) do not require disposal and may even enrich the soil matrix.

In the early days of bioremediation, soils where often excavated and treated ex-situ in soil beds or mere heaps of soil. Similar methods are still necessary sometimes but generally more recent techniques utilise the natural micro-organisms in the vadose (unsaturated) zone in the presence of enhanced nutrient concentrations and oxygen where necessary. These techniques are particularly suited to contaminants such as oils, aliphatics, aromatics and some inorganic compounds such as some cyanides.

Typically, hydrocarbon concentrations in soils can be reduced to one fiftieth their original concentration with little physical disruption to the site. Bioremediation therefore affects the contaminant source by transforming it (or at least an appreciable proportion of it) to another substance which is more environmentally acceptable, and reduces the need to use external facilities such as waste disposal sites and their associated direct and indirect environmental impacts.

Unfortunately, bioremediation is a relatively slow process (remediation period often exceeds one year) and is restricted mainly to organic contaminant species. Therefore extensive pre-remediation study is required to assess the likely timescales and range of contaminants that can be effectively remediated in this way. As a result bioremediation has only been successful where:

A sufficiently long period of time is available for remediation

The contaminants are predominantly organic materials which can be degraded readily by natural or enhanced microbial metabolic activity

There is an opportunity to combine the techniques with other methods, such as barrier methods for non-biodegradable substances, on the site.

Vitrification

For soils which are virtually untreatable by other methods, such as those with high levels of heavy metals, in situ vitrification can provide a solution. A current is passed through the soil and this causes the temperature to rise very quickly to about 2000C. The physical nature of the soil is changed to form a vitrified mass which is virtually impermeable to water. The effect of this is to isolate the source of the contaminants from the pathway to sensitive receptors, thus reducing substantially the potential environmental harm that can occur.

Soil Washing

In Northern European States, soil washing has been extensive over the past fifteen years. Various detergents have been mixed with soils to facilitate the extraction of various soluble compounds into water which is pumped through the soil matrix. These techniques are effective when the contaminants present are both water soluble and do not adhere to the particles in soils such as certain clays. This is a source elimination technique which is effective on sites where soil movement is possible and where the treatment of the contaminated process water is cost-effective.

Contaminant Stripping

Contaminant stripping involves the forced evaporation of compounds present in the ground via extraction wells followed by their recovery or venting outside the soil matrix. Contaminants present in the ground and accessible from the vadose zone can be remediated by vapour stripping only if they are sufficiently volatile. Therefore this technique has been especially useful in the remediation of petroleum-based contamination and for other compounds which are volatile such as toluene. For compounds which are not water soluble, such as toluene, this technique can be used to remove layers of floating contamination on the groundwater table. This is an effective method of removing the source of volatile contamination which may migrate in vapour, aqueous or liquid phase to undesired locations.

Extensive feasibility study is required for this method because there is usually a need to recover the extracted vapours or to measure and control fugitive emissions from the process. The advantages of this technique are that it is relatively fast and the ground is only marginally disturbed by the extraction wells. The disadvantages are that only volatile substances can be extracted and that it is difficult to deal with contaminants below the groundwater table.