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Risk based remediation

Jan Hellings described the history of a decommissioned fuel storage and distribution terminal which occupied about 45ha, and had provided an annual throughput of more than 100M litres. It had predominantly been used for the handling of lighter (white) oils, and storage of heavy (black) oils. The landowner wanted the site rendered suitable for residential redevelopment. A phased soil and groundwater investigation indicated a number of isolated areas of soil contam- ination by petroleum hydrocarbons, and a groundwater plume of BTEX compounds originating in the white oil tank farm and extending to the site perimeter. The extent and carcinogenic nature of the contaminant required remediation of the site.

Mark Mackintosh described the approach adopted. The objective was to render the site suitable for residential redevelopment. The remedial approach involved development of risk based clean-up levels (RBCLs) which were used to produce a site specific quantitative risk assessment (QRA). Risk assessment considered contaminant source pathways and receptors in terms of safe risk levels only in association with development for housing. Negotiations were undertaken with relevant authorities to ensure the proposed clean-up criteria met all their requirements.

The three main pathways identified for inclusion in the risk assessment were ingestion, groundwater and the rivers. RBCLs were calculated for the top 1m of the soil column where a risk of ingestion to residents was identified. Leaching to groundwater required less stringent clean-up levels, which meant soil excavation to 1.5 and 2m depth was only required for grossly contaminated areas to remove potential sources of groundwater contamination. Groundwater contamination detected did not exceed RBCLs and was considered to create no significant risk to either the residents or nearby rivers.

Before remediation, the entire facility had to be demolished. Demolition was linked with shallow soil removal as one contract. As part of this, waste oils were removed, and contaminated bitumen bases material was taken to a licensed landfill. All the tanks and pipework were cleaned and scrap metal recycled off site. Inert tank base sand and clean concrete were crushed on site and stockpiled for backfilling.

The remedial approach adopted incorporated site conditions, contaminants, migration potential and risk based logic. Excavation of the shallow soil with disposal to licensed landfill was chosen on the basis of cost, target levels, and the short time scale the client had requested for completion of the works. If more time had been available land farming would have been a preferred option since remediation can be achieved at a lower cost than by excavation.

Dames & Moore supervised the extent of excavation and off site removal of contaminated soils. Use of tracked excavators, which backfilled along the way, minimised the extent of open excavation (Fig- ures 6 and 7). Only soil with concentrations exceeding the risk based threshold levels was excavated. The extent of excavation was determined by inspection of excavated surfaces and an analytical assessment of the soil by field and laboratory testing. When extended excavation was necessary, the removal of further materials was followed by revalidation to provide the purchaser with a thorough assurance that remedial goals had been achieved. A sampling grid methodology was used for validation testing, defined on a balance of analysis costs and disposal costs. Field screening, used to reduce lab costs and to give a 'real time' method for determining the extent of excavation, comprised the use of a PID to analyse soil vapours, and test kits which used an enzyme approach to determine TPH concentrations. Analysis was undertaken by a laboratory approved according to Dames & Moore's laboratory QA/QC policy. Feedback data obtained during the works was used to further refine the risk analysis, and provided an increased confidence in the cleanliness of areas peripheral to contaminated ones. A combination of imported materials and crushed concrete produced during demolition was used for backfilling. This material was tested to ensure that no contamination was being replaced into the ground.

Mackintosh concluded that the use of a risk-based remedial approach and integrated demolition/remediation process resulted in savings to the client. The works cost about £900,000 and the site was subsequently valued at several times this sum. The remedial project allowed the developer to fit to the local plan for expansion of a home counties town without affecting the surrounding greenfields.

Discussion

Asked why growth of the contamination could not have been prevented in the first place at the Northern Telecom site, Jefferis explained that the spill had occurred 20 years previously, and said no further spillage was taking place.

The rate of replacement of iron in the reactive treatment zone was queried. Jefferis replied that the 15t of iron used was expected to have a working life of 50 years. He said the technology was new and the actual requirement for material replacement would be identified in the future.

In response to a question why iron used in the reactor was imported from the US, he explained that this iron had been used in the laboratory trials and he wanted to use a matching iron in the field. In the future it would not be necessary to import iron. The only factors influencing the choice of iron were surface area and cleanliness.

Asked how the reaction could take place without acid, Jefferis explained that the iron was involved but not stoichiometrically, ie there may not be a direct relation between the quantities of iron used and solvent dechlorinated; rusting of the iron was necessary to provide a reducing environment. Complete understanding of the reaction taking place had not yet been achieved.

The fuel storage and distribution terminal project presenters were asked why no actual barrier was installed to protect the site. Mackintosh explained that there was little potential for capillary rise and noted that the groundwater plume was naturally degrading with time.

The choice of risk levels at the fuel storage terminal was queried. Mackintosh said the risk assessment took into consideration the possible contaminant source/pathway/receptors, and determined the levels of risk to cause harm using toxicological effects.

Asked how landfill tax affected the project, Mackintosh replied that it was undertaken before the enforcement of the tax - however current remedial projects were not exempt from landfill tax. Hellings commented that as landfill became more expensive, new technology would become more economic.

Questioned whether other methods of clean-up such as bioremediation could have been used, Mackintosh replied that several different scenarios were presented to the client before the final approach was chosen. Because of the clay base it was considered feasible to farm the area, feeding with nutrients and aerate; but the 12 months or so this would require was longer than the client's specified time scale.

Mackintosh said a landfill with a special waste licence was used, and all of the contaminated soil was transported by a licensed waste carrier who provided the documentation to meet current guidelines.

On the attitude of lenders towards remediation, Mackintosh explained that a close out report was supplied to the developer. The value of the land depended upon clean-up level chosen for the end user.

Asked whether methane generation beneath the site had been addressed, Mackintosh said gas monitoring on site showed only small amounts of methane in the centre of the plant. The risk assessment evaluated the levels to be below the target values for remediation.

In response to a question on Dutch clean-up criteria, Mackintosh replied that Dutch regulations in this case were considerably stricter, as these were focused on Dutch soils with a variation for clay content. A site specific risk based approach produced more relevant clean-up levels.

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