On-Site Remediation of Hydrocarbon Impacted Soils on Petrol Stations
As a direct result of the landfill directive (our newsletter June 2004), developers and owners of contaminated land are now faced with the task of identifying possible technologies and commercial partners that can undertake on-site remediation of hydrocarbon impacted soils.  The objective being to reduce the volume of contaminated soil that is required to be removed from site and taken to landfill.
Any possible solution needs to be cost effective, sufficiently quick to complete, relatively simple to implement, reliable, present few risks (commercial or technical) and sufficiently effective to secure the clean-up objectives agreed with the client and regulator. Additionally it must satisfy any regulatory or licensing requirements.

REVIEW CRITERIA
Most technologies will be assessed against the following criteria:

• Cost
• Ease of Implementation
• Effectiveness and Reliability
• Timescale
• Regulatory Requirements of Method
• Track Record of Method and Organisation
• Risk

GENERAL OVERVIEW
Most underground fuel tanks and lines have leaked at some stage, resulting in a degree of hydrocarbon contamination in the surrounding soils.

It is usually necessary for this contamination to be remediated prior to the redevelopment of the site. The scope of the remediation required depends on the levels of contamination in the soil, the size of the area affected, the sensitivity of the proposed development, proximity of any sensitive environmental features and the geology and hydrogeology of the site.
All of these factors are considered by the appropriate regulators; the Local Authority, the Environment Agency and NHBC / Building Control. Planning permission will often only be granted subject to condition requiring assessment and clean-up of the site before/during redevelopment.
Remediation can either take place at the time of the decommissioning work or at a later date. To avoid the double handling of soils, minimise mobilisation costs and reduce the timescales involved the preferred option is usually to remediate the site at the same time as the decommissioning and tank removal operations.
A site investigation is usually carried out prior to decommissioning commencing to obtain information on the degree of contamination, its physical extent, the groundwater conditions at the site etc. Sufficient information needs to be obtained to design the subsequent remediation.
The remediation process usually involves on-site determination of impacted soils, by visual and olfactory means and testing using portable equipment. Soils considered contaminated are dispatched for disposal to landfill. Excavation is continued until soils considered ‘clean’ are encountered, which are then sampled. The objective is that the results of this validation testing are below the target concentrations agreed with the regulators, if so the site is generally considered remediated.

ALTERNATIVES TO LANDFILL
Removing excavated material off-site for disposal to landfill is a straightforward and sure way of remediating the site, it presents only limited residual risk and is straightforward to arrange. It is however increasingly expensive and following the introduction of the requirement for pre-treatment of all waste going to landfill, as part of the Landfill Directive, has become even more so.

Two broad alternatives to disposal to landfill exist:

1. Treating the contamination in the ground, thus avoiding excavation and surface handling (in-situ).
2. Excavating the material, treating it and then putting it back in the ground (ex-situ). This treatment may either be carried out on the site being remediated or elsewhere.

It is important to understand that remediation of the site needn’t mean the removal of all ‘contaminated material’. Use of biological or physical processes to break-down contaminants into less harmful forms, are techniques available now, that will render contaminants immobile and prevent an exposure pathway, and are legitimate ways of rendering a site ‘safe’ prior to redevelopment.

IN-SITU TECHNIQUES
Most in-situ technologies involve drilling boreholes and subsequent extraction of contaminated groundwater or vapour. The required plant (pumps, fan, filters etc) have to be housed on site and coupled to power and water services as necessary.
This process is expensive and can take many, many months to carry out.
In-situ solutions are most useful where redevelopment is proposed and large-scale excavation of soil cannot be undertaken. In-situ techniques are also the only real way of treating large volumes of contaminated groundwater.
There is a fair amount of risk involved in implementing an in-situ solution. The success of the scheme is very sensitive to the design, which in turn is based on the findings of the site investigation. A comprehensive investigation is essential to obtain sufficient information on soil type, groundwater regime, contaminant levels and delineation, ground permeability etc. and provide sufficient confidence that the remediation will work.
Errors or omissions in the investigation are inevitable and therefore an optimisation period is usually required during which pumping rates and sequences, treatment processes etc may be modified. Once ongoing monitoring indicates the target levels have been achieved then the system is shut down for a period before final monitoring takes place. This shutting down allows for an assessment of the extent of  ‘rebound’, which often occurs following shut down. If ‘rebound’ occurs and levels rise again, then the system has to be run for a further  period.
All of the variables mean that it is often difficult to accurately predict timescales and costs. To some extent it is possible to trade time against cost.
In-situ systems are considered to be a waste treatment process and require waste management licensing. This is currently done through ‘Mobile Plant Licensing’, where the process/plant is itself licensed (held by the operator) and a ‘Site Specific Modification’ is obtained for each use of the system, taking account of particular circumstances on each site.
Licensing is carried out by the Environment Agency. ‘Site Specific Modifications’ have to be applied for in advance of remediation commencing and this generally takes 6 to 8 weeks.
Reagent Injection is slightly different. No extraction wells are required instead a substance is ‘injected’ into the ground to encourage the degradation of hydrocarbon contamination. Typically this is an oxygen release compound (ORC) and/or nutrient mixture, which produce a favourable aerobic environment for bacteria, which digest hydrocarbons, producing carbon dioxide and water.
Post remedial monitoring is still required following reagent injection, but it may be possible to do this from the boreholes installed during the site investigation. Significant risks remain however, the success of the technique depends on the local geology and hydrogeology.
Environmental Land Solutions supply the micro organisms to site as a powder, which we hydrate on site. We use our own equipment to ‘inject’ it in a tight grid across the contaminated area. Results can usually be achieved in a period of only weeks.

This may well be a very useful ‘tool in the armoury’ for sites where some degree of groundwater impaction has taken place (as it will treat contaminated groundwater in addition to soil), or where existing buildings are not to be demolished and excavations are therefore limited.

We offer a ‘pain/gain share’ agreement as the best contractual arrangement for using this technique, to reduce risks to an acceptable level.