Summary and conclusions, Miljøstyrelsen

Måling af indtrængningen af gasformige forbindelser fra forurenet jord til indeluften: Foliemetoden Del 2. Felttest

Summary and conclusions

This report describes a new method for determination of the emission of gaseous compounds from polluted soil to indoor air in buildings. The method has been developed and evaluated through a project supported by the Danish Environmental Protection Agency.

The measurement method is based on the enclosure with an inert film of an area of the floor over the polluted soil. The film is attached to the floor by means of an adhesive tape, and clean, filtered air is pumped into the enclosure. The compounds that are transported from the area covered by the film will be mixed with the air pumped into the enclosure, and the compounds are removed by an outlet flow. The emission from the area can be determined by measurement of the emission of pollutants through the outlet air.

The method was developed and tested under laboratory conditions in the first phase of this project. The results of the laboratory test has been described in a separate report /1/. The present report describes an evaluation of the method under field conditions. Furthermore, a comparison of the measured and the calculated flux of gaseous pollution to the indoor climate from soil pollution was done. The calculation was done by means of the JAGG-model according to guidelines from the Danish EPA /2/, /3/.

The field test was performed on a contaminated site in Copenhagen. The contamination consisted of perchloroethylene (PCE) in the soil under a 5 storeys building, where a former laundry had been situated in the 1^st floor. The field test was performed in the basement of the building. With the film enclosure method, emission flux measurements were performed on the concrete floor and, supplementary, emission flux measurements were performed on "hot spots", i.e. around the penetration of a sewage pipe, where cracks were found in the concrete floor. Furthermore, the test included measurement of indoor and ambient concentrations, air exchange rate, soil air measurements, and secondary parameters such as differential pressures and temperature. A series of measurements were also performed at a non-contaminated "reference-site".

On the basis of the field test, the repeatability of the method is estimated to be ± 20% - ± 30%. Under field conditions, the found variation in the repeatability will be quite small compared to the spatial variations in the flux through a floor placed on contaminated soil.

Out of the 4 measurement points, where emission flux measurements were performed, the results from 2 points showed a good agreement with the flux calculated according to the JAGG-model. However, there is a large horizontal variation in the concentration of PCE in the soil air under the floor, and this makes it difficult to determine the flux of PCE by calculation. The variation in soil air concentrations is probably due to the high content of clay in the soil.

The measurement of emissions of PCE from cracks in the concrete around the sewage pipe showed that this hot spot was a very significant source of PCE. From on-line measurements performed over a period of one week, the difference in temperature inside/outside the building was found influence the measured flux from concrete cracks in the floor. This is due to the buoyancy of the air in the building, creating a flow of air from the basement through the backstairs and upward, and causing a vacuum in the basement, especially during periods with low outdoor temperatures. During the measurement period, variations in barometric pressures were found to have a minor influence on the flux through cracks as compared to the difference in temperature inside/outside the building.

It is concluded that the cracks in the concrete floor around the sewage pipe penetration is the predominant source to PCE in the indoor air in the basement. There is a risk that a similar situation can be found at other contaminated sites, especially where pipes penetrate the floor at polluted zones. It is generally difficult to estimate the flux of VOC from hot spots such as larger cracks and fissures are present in the floor by calculation as suggested by the Danish EPA. This is due to the difficulties in the identification and measurement of cracks that are open throughout the concrete floor. The soil air concentration directly under the spots can also be difficult to measure due to practical hindrances, especially when hot spots are located close to sewage pipes etc. This increases the uncertainty of the calculation of contributions from hot spots.

Through this project, a method has been developed for determination of the flux of volatile compounds from polluted soil to indoor air. A procedure for the performance of measurements on concrete floors has been described. Tentative experiments with the film enclosure method used on hot spots has shown that the method has a high potential as a tool for tracking and quantifying major sources to harmful pollutants in indoor air in buildings at contaminated sites.