Passiv poreluftscreening med Gore-Sober^® screening survey

Summary and conclusions

When soil and groundwater pollution is to be investigated, it is not always possible to measure the soil vapour in the traditional way. In certain cases a new method based on so-called passive measurement of the soil vapour can be applied. This is evident from the present project carried out in the framework of the Technology Programme for soil and groundwater pollution of the Danish Environmental Protection Agency. The method is called Gore-SorberÒ Screening Survey and has been tested at a site in the County of Frederiksborg, a former site of a cleaning establishment. A pollution of tetrachloroethylene in the groundwater was delimited at this site.

Background and purpose

Today soil and groundwater is polluted at many places in Denmark. In order to solve this problem, authorities and private enterprises spend considerable resources on investigating properties for possible pollution.

Investigations can be approached in many ways and new methods for pollution investigation are developed continuously. The newest method is the so-called passive measurement of soil vapour.

By this method the polluting compounds are collected at a given material by letting the air move naturally in the soil strata. The Gore-Sorber Screening SurveyÒ is a new product, which is based on the described method.

Testing the new method

In 1997, heavy pollution of tetrachloroethylene was found in the ground under a former cleaning establishment. The substance had been used as cleaning fluid and the pollution had spread over a large area through sandy parts in the soil strata.

At this site, Gore-Sorber was used to delimit the pollution. The Gore-Sorber module consists of a small thin "sock" made of e-PFTE, known as GoreTexÒ . The material is water-repellent, but can be penetrated by vapour. The module further contains a material which can absorb a large number of frequently occurring polluting substances.

During the investigation 45 of these modules were installed at measuring points with an interval of five times five metres over the entire site. The modules were placed at a depth of one metre.

After 14 days the Gore-Sorber modules were collected and analysed in a laboratory for a number of chlorinated compounds, including tetrachloroethylene and breakdown products of this substance.

Many advantages of the method

The Gore-Sorber method has proved a suitable method for investigating pollution in moraine clay, where it is not possible to measure soil vapour in the traditional way. The Gore-Sorber method can be applied even if the groundwater level of the particular site is high.

The material in the Gore-Sorber module can absorb a number of different substances, including e.g. oil products, solvents, chlorinated compounds and tar. Therefore only one module is necessary for absorption of the soil vapour, irrespective of which polluting compounds one wish to analyse for. Installation of the Gore-Sorber module is also fairly simple and does not require special equipment.

The most essential drawback of the method is that the absolute concentration of polluting compounds in the soil vapour is not determined. Only the amount absorbed by the module is measured. Furthermore, the method is more expensive than the traditional method for measuring soil vapour.

Results can be confirmed

The following substances were found in the investigation: Tetrachloroethylene, trichloroethylene as well as cis- and trans-1.2-dichloroethylene. The pollution mainly consists of tetrachloroethylene of which considerable amounts were found on the Gore-Sorber modules. The other substances found are breakdown products of tetrachloroethylene.

The results of the measurements are stated in µg substance per module. That means that the result figures express the total amount of the substance in question which has been absorbed by the Gore-Sorber module. This amount cannot be immediately compared with the concentration of the substance found in soil vapour, soil or groundwater. However, the amount of polluting compounds found on the module corresponds very well with what has previously been found at the site.

In the project report the results are presented graphically in colour. The presentation gives a fine survey of the results and of the dispersion of the pollution at the site.

When the colour presentation of the Gore-Sorber investigation is compared with a record of the groundwater pollution, the results correspond well.