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Måling af indtrængningen af gasformige forbindelser fra forurenet jord til
indeluften: Foliemetoden Del 1. Laboratorieundersøgelse
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 during a project supported by the Danish Environmental Protection Agency.
The flux of VOC’s through concrete floors can be estimated through model
calculations of the diffusive and convective transport. In Denmark, model calculations
should be performed according to guidelines from the Danish EPA /4/,
/5/. Calculation models are based on empirical formulas, and the
contribution of the flux of VOC’s to the indoor air can be strongly over- or
underestimated, depending on the level to which the site can be described. Often, the
quality of input data is questionable for parameters such as the properties of the
concrete, the magnitude and distribution of the soil pollution, the description of cracks
in the concrete etc. Therefore there is a need to validate model calculations with direct
measurement methods for field determination of the flux of gaseous compounds from polluted
soil to the indoor air.
It has been the objective of Danish EPA to develop a measurement method to be used as a
supplement to the presently used model calculations.
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 from the flow of air supplied to the enclosure and from the concentration of
pollutants in the outlet air. If the pressure under the film is significantly different
from the pressure in the indoor air, and when measurement is performed on the flux from a
porous material such as concrete, there is a risk that the flux into the enclosure will be
over- or underestimated. By using a soft film with a low weight, the pressure difference
between the indoor air and the air under the film can be minimised.
It is expected that the described "film enclosure method" can also be used
for measurement of the emission from "hot spots" that can contribute
significantly to the transport of e.g. VOC’s from soil pollution. Hot spots can arise
because of gaps in constructions such as pipe penetrations in basement floors. It is
expected that such hot spots can be enclosed by film, and that measurement can be
performed according to the same principle as for floor areas. A test of the suitability of
the method for use at hot spots was performed in connection with a second phase of the
project, where a field evaluation of the method was be performed. The field evaluation is
described in a separate report /1/.
To evaluate the method proposed for measurement of the flux of VOC’s through
concrete floors, a test facility was build in the laboratory. The test facility consisted
of a 1m x 1m x 0.05m concrete slab, placed in a closed stainless steel chamber. The
concrete slab was placed in horizontal position, and it divided the chamber into two
chambers: a top chamber and a lower chamber. The lower chamber was a closed chamber to
which polluted air could be added in a known concentration and with a known flow. Clean,
filtered air was added to the top chamber, and the diffusion through the concrete slab due
to the concentration gradient could be determined by measurement of the emission via the
outlet air from the top chamber. The top chamber had a removable lid, allowing for change
of films.
The test facility was found suitable for test of the film enclosure method. This was
concluded from the results of a trace gas experiment, where SF6 was injected
into the lower chamber. When the lower chamber was closed and pressurised, the flux of SF6
to the top chamber was found to be constant and independent of pressure. From this
experiment, it is concluded that the flux from the lower chamber to the top chamber is
solely controlled by diffusive transport.
The influence on the measured flux from the use of TEDLAR or Nalophan film was found
not to be statistically significant. The area specific flow was tested in the range 0,07 m3/h/m2
– 0,28 m3/h/m2. The test showed no significant effect of
variations of the area specific flow.
From the tests performed, only minor losses of TCE and toluene to the walls of the
TEDLAR film were found. The loss of TCE and toluene through sorption to the tape was also
very small.
It is concluded that TEDLAR as well as Nalophan film is suitable for use in connection
with the film enclosure method.
An assessment of the suitability of the method will depend on the requirements for the
precision and accuracy of the method. From the performed laboratory test it is estimated
that the method is applicable for measurements of the flux from concrete floors with a
precision of ± 50%. This should be compared with the
variations of the flux found under actual field conditions. The flux can under field
conditions vary from place to place on the same floor with a much larger variation than ± 50%.
From mass balance calculations, a considerable loss of TCE and toluene to the concrete
slab was found. The calculated loss was about 20% - 30%, and this loss was found even
after several months of constant mass stream of TCE and toluene to the lower chamber. The
ability of the concrete to adsorb VOC is outside the scope of this project and further
investigation of the sorption capacity was not performed.
The effect of the temperature in the test facility on the measured flux to the top
chamber was investigated. In the temperature range of the tests performed (19°C –
23°C), no effect of the temperature was found.
Recent studies have proved that the ability of concrete to adsorb and desorb volatile
organic compounds can play an important role in the transport of volatile pollutants in
indoor climate. This is confirmed by the results of the laboratory tests performed in this
study. The results indicate that sorption capacity of VOC’s such as TCE and toluene
in concrete is very high. Concrete can adsorb considerable amounts of VOC’s,
depending on temperature, humidity and concentration gradients. If the influence of
sorption to and from concrete is as strong as indicated by the test results, measures must
be taken to avoid misinterpretation of results from model calculations and monitoring
programmes.
The measurement results from the individual test series have been compared to the
theoretical flux, found through calculation of the diffusive transport through a concrete
floor according to the guidelines given by the Danish EPA. The value of the constant N for
diffusive transport through concrete was 0,002, corresponding to N for a concrete without
reinforcing iron and with a strength of 15MPa. In most of the performed test series, and
for both TCE and toluene, the difference between the measured and the calculated flux was
less than ± 50% of the measured flux. The measured flux in the
first test series was 2-3 times higher than the theoretical, calculated flux. The reason
for this has not been found. The charge of TCE and toluene to the lower chamber was
constant in all test series, and a reduction in the flux can not be explained by
variations in temperature either. A possible explanation can be variations in the relative
humidity. However, it has not been possible from the test results to find a relation
between the relative humidity and the flux.
More detailed procedures for the performance of the "film enclosure method"
will be laid down during the field evaluation programme planned to be performed as a
second phase of this project.
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