Miljøprojekt, 1170; Teknologiudviklingsprogrammet for jord- og grundvandsforurening – Afprøvning af immunoassaymetode til bestemmelse afPAH-indhold i jord

Afprøvning af immunoassaymetode til bestemmelse afPAH-indhold i jord

Summary and conclusions

An evaluation of a screening method to supplement and replace some of the accredited analyses for PAH compounds was carried out. The objective was to use a screening method in order to reduce the cost of the technical investigations during mapping of soil pollution at information level 2 and to enable investigations to be performed quickly.

A screening method must determine the following conditions:

whether the soil sample contains PAH (requires analysis for a specific group of pollutants)

whether the content is less than the Danish Environmental Protection Agency’s soil quality criterion of 1.5 mg/kg DW (NB: later changed to 4 mg/kg TS) for the sum of 7 MST PAH (requires a semi-quantitative method)
whether the content exceeds the Danish Environmental Protection Agency’s soil intervention criterion of 15 mg/kg DW (NB: later changed to 40 mg/kg TS) for the sum of 7 MST PAH (requires a semi-quantitative method)

The objective for the project is to determine if the immunoassay method is sufficiently reliable to enable its use for technical investigations of contaminated sites at information level 2.

For the analysis of soils contaminated with tars, four different screening methods are considered:

Colour determination – dichloromethane extraction
Immunoassay
UV-fluorescence – fluorimeter
Thin layer chromatography

Only the immunoassay method was assessed to have the necessary properties to be able to carry out screening for the content of tar in contaminated soils during technical investigations at information level 2. The immunoassay method was also assessed to be the most financially advantageous method at the commencement of the project in October 2000 because an additional 45% of the normal number of soil samples could be analysed within the same budget and this improves the chances of delineating the soil pollution.

Interference from other hydrocarbons such as BTEX, phenols, oil hydrocarbons etc. is minimal. Since the soil quality criterion for the sum of PAH is based on seven PAH with 4 to 6 rings, it is an advantage that the immunoassay test kit is most sensitive to the 4-to-6-ringed PAH.

The test kit is approved and validated by the US EPA as method 4035 and is described as a method to screen soil samples for content of PAH in excess of 1 mg total PAH/kg. The method’s characteristics and capability are documented and there is generally good agreement between test levels and laboratory results. Likewise, the probability of a false negative result is assessed to be low. It is however reported that soil conditions as well as the presence of complex mixtures of hydrocarbons and degraded pollutants can lead to discrepancies between the results obtained with the test kit as compared to laboratory results.

During the trial of the test kit in Skagen, two tests of pollutant concentration are applied which defined the following three classification levels:

<1.5 mg/kg DW
1.5 – 15 mg/kg DW
> 15 mg/kg DW

Apart from the trial of the screening method, the effectiveness and efficiency of the extraction method used in the immunoassay method is also evaluated.

Only about 25% of the content of PAH is extracted using methanol and shaking for one minute as compared with a 16-hour standard extraction with dichloromethane in the laboratory GC-FID method. Especially the heavier 5 and 6–ringed PAH (indeno(1,2,3-c,d)pyrene and dibenz(a,h)anthracene) are poorly extracted in methanol.

The practical trial of the immunoassay test kit based on 164 soil samples indicated a clear tendency to underestimate the content of PAH as compared to the content determined by GC-FID and a suggestion to improve the extraction step in the immunoassay method was made.

The improved extraction method is based on warming the extract to 55 °C and increasing duration of extraction from one minute to one hour. This method is also evaluated for some soil samples and appears to give better agreement with the GC-FID analyses.

However with 2003-04 price levels, there isno longer a financial benefit to be achieved by using immunoassay instead of laboratory analyses. Since the immunoassay method is only a semi-quantitative screening method, it can therefore no longer be recommended in connection with mapping contaminated soils.

The immunoassay method is however still advantageous with respect to optimisation in connection with time restraints during remediation projects.