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ARSENIC AND A NEW TEST

GEOENVIRONMENTAL ENGINEERING

A Cornish development involved the use of site won material despite the recording of high arsenic levels. Rod Smith reports.

Redmoor Parc is a residential development of 21 houses at Callington, Cornwall, where building work is nearing completion. Originally part of the same development site, Redmoor Parc is adjacent to Westmoor Parc, which has 30 more properties.

AN SI showed that both sites were reasonably free of mining activity and that Redmoor Parc had made ground. But although the ground beneath the Westmoor Parc site comprised a consistently undisturbed natural soil profile, contamination testing indicated that the soil contained naturally occurring high levels of arsenic. Arsenic levels in the made ground beneath the Redmoor Parc portion of the site were even higher.

At the levels found, typical remedial works would have included the capping of the soft landscaping with a minimum 700mm thick topsoil.

But in south west England, this can be very diffi cult due to the levels of arsenic in the soil. It might also involve importing soil from long distances at a time when clean soils are at a premium.

Under the current land contamination regime, soils are considered potentially contaminated if levels exceed SGVs. There are SGVs for ten contaminants, including arsenic.

The SGV for arsenic is 20mg/kg.

But soil arsenic concentrations in Cornwall often exceed 250mg/kg.

In calculating the SGV for arsenic it is assumed that all of the arsenic in the soil is bioaccessible (it passes into the body when ingested). Also, that all arsenic is in an inorganic form and that plants uptake the arsenic from the soil to a specifi c extent. It can then be ingested by residents from vegetables grown in their garden.

Consultant John Grimes Partnership decided to carry out a detailed quantitative risk assessment (QRA) and designed a remedial scheme where excess topsoil was used as a capping material for soils within the Redmoor Parc site. This meant that topsoil did not have to be brought in from an outside source.

The assumptions made in deriving the SGV were tested for the site using a range of standard and non-standard test methods not applied to a QRA before.

The amount of soil arsenic that was bioaccessible was measured using the standard physiologically based extraction test (PBET) method. This technique has been accepted by local authorities in the south west for a number of years and the Environment Agency recently published a number of guidance notes on its use.

Although vegetables were not grown on the site, the QRA used the results of an in-house developed chemical test to compare the potential for plant uptake from the site soil with other soils in the south west where plant studies have been done. This is a sequential extraction method where a number of increasingly strong chemical solutions progressively extract arsenic from the soil.

These studies have indicated that the plant uptake assumed in the SGV errs considerably on the side of caution. As far as John Grimes is aware, this method has not been applied in a QRA before. The method shows promise and has the advantage that it can enable estimates of plant uptake to be carried out on the same timescale as PBET testing.

Finally, the QRA measured the amount of inorganic and organic arsenic present within the bioaccessible fraction. As organic arsenic is less toxic than inorganic arsenic, the assumption that all arsenic is inorganic in the SGV could be overly cautious.

Using another method not commonly applied to soil extracts, a signifi cant amount of the bioaccessible arsenic was shown to be organic.

This is known as speciation, and has been applied to a number of other contaminants in the past like chromium, mercury and cyanide.

The technique is a solvent sequential extraction where the different forms of inorganic arsenic can be separated because they have different solubility in an organic solvent.

Once isolated, these can be analysed independently. The remaining inorganic and organic arsenic can be segregated by chemically altering both forms sequentially so that they are soluble in the organic solvent. This will help indicate how much arsenic in the soil is organic and how much is inorganic.

John Grimes has developed these two methods over the last three years using a number of reference soils. The test methods include a Tessier type sequential extraction and solvent sequential extraction for arsenic speciation.

While the local authority, Caradon District Council, supported the remedial scheme, it insisted the assessment and the methodologies behind the risk assessment be peer reviewed.

The 18-month peer review was completed at the end of last year.

Because the reviewer decided that the methodologies underpinning the QRA complied with current guidance, the local authority has accepted them and is awaiting validation of the completed remedial scheme.

John Grimes is currently doing its validation testing.

As a result, the developer has been able to develop the site using site won material without the need for the importation of expensive topsoil, and the development is more sustainable overall.

Rod Smith is associate director with John Grimes Partnership.

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