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Gargoyle is nothing to fear

A measured risk assessment approach made remediation of serious soil and groundwater contamination at a west London site considerably easier. Steve Wilson explains.

Developers traditionally perceive contaminated land as difficult and therefore something to be avoided. In fact, there are no insurmountable technical problems in dealing with brownfield sites, as long as sufficient information is available on the nature and severity of contamination and a considered view is taken on the risks associated with development.

Gargoyle Wharf in Wandsworth, a 5.2ha London riverside site, being redeveloped by Rialto and Somerset Holdings with Frogmore Estates, offered one such opportunity. The aim is to create a thriving community with a mix of commercial, residential accommodation and leisure facilities.

Consultant Card Geotechnics was appointed to solve and manage the treatment and removal of contamination identified from preliminary investigations. These showed significant hydrocarbon contamination in the soil and groundwater as well as some localised heavy metal contamination of the soil.

Ground conditions comprise up to 3.5m of fill overlying River Terrace Gravel. Below this, at 10m depth, is London Clay.Groundwater occurs at depths of 3m to 3.5m and varies by up to 1m according to the tides of the nearby River Thames.

After discussions with the environmental health department of the local authority, a pilot remediation trial was carried out to determine accurately the extent and nature of the hydrocarbon contamination. A series of trial pits was also dug to confirm whether any heavy metal contamination was present in areas not previously investigated.

These investigations revealed elevated concentrations of hydrocarbons in the soil and groundwater in two distinct areas of the site.At the northern end, motor or lubricating oil was detected, while at the southern end, diesel oil was identified.Free product was also present below the site.

Chemical testing indicated that the concentration of total petroleum hydrocarbons (TPH) exceeded the Dutch Intervention levels (levels above which contamination could seriously affect human, plant and animal life) next to the river wall. Soil contamination was present in a 1m thick layer, between 3m and 5m below ground level, which corresponds to the tidal variation in groundwater levels across the site.

TPH contamination in the groundwater exceeded intervention levels across the entire site, with the highest levels again recorded next to the river wall.The thickness of the free product varied up to a maximum of 750mm, depending on tidal variations. Fluoranthene and xylene were also found in slightly elevated concentrations in some localised areas.

It was considered that the lubricating oil hydrocarbons were unlikely to migrate off site as they were relatively viscous, insoluble in groundwater, generally immobile in the subsurface and tended to adhere to soil particles.

The diesel contained higher levels of light aromatics such as benzene, toluene, ethyl benzene and xylene.These have a relatively high solubility and, being poorly absorbed by the soil particles, were more likely to attempt to migrate from the site.

Results of the ground investigation and chemical testing did not identify widespread occurrence of significant levels of heavy metal contamination. Leaching tests showed that any contamination was fixed in the soil and should not migrate into the groundwater; it was considered to pose a low environmental risk.

A quantitative risk assessment was carried out to determine acceptable risk based screening levels (RBSL) and site specific target levels for remediation.On this basis the benzene, toluene and xylene in the soil exceeded acceptable levels for a residential development, as did the amount of xylene in the groundwater.

For commercial development the RBSLs were higher and only the xylene in the soil exceeded acceptable levels.

However, the presence of the free product did pose a potential risk to the river environment, and remediation was required.

This was was carried out in two phases. The first involved installation of abstraction wells and perforated manholes to remove the free product and groundwater in the northern area. At the same time, air sparging and vacuum extraction were used to remediate the groundwater, soil and free product contamination in the southern sector.

Some 175 abstraction, air sparging and vacuum wells were installed.

Remediation lasted just over a year by which time the hydrocarbon had been almost completely removed.

Regular tests were carried out to establish the progress of the cleaning and to assist the validation process at the end of the contract.

Much of the low-risk contamination will be removed during the excavation for a basement car park. Elsewhere, protection will be achieved either by incorporating an encapsulation layer in the landscaped areas or by the concrete slabs and road pavements that will cover most of the site.

Confirmation that remediation target values had been achieved was checked by validation testing. Monitoring was also carried out along the boundary to ensure that migration had not occurred, either on or off site, during remediation.

Following nominal completion of the work, and after the remediation system had been switched off, testing continued at fortnightly intervals for three cycles.Monitoring of wells and separately dug trial pits confirmed that target levels set for the contaminated soil had been achieved and that treatment had been completed to the specification agreed with the Environment Agency and the Environmental Health Office.

A film of hydrocarbon free product, less than 1mm thick was identified in groundwater in the validation pits.

Records show that 47,000 litres of oil were removed from the site.The site is now clear of hydrocarbons and is ready for the initial stages of construction.

Steve Wilson is associate director of contaminated land consultant Card Geotechnics

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