English Summary

Sporgasundersøgelser til fastlæggelse af aktive transportveje til indeklimaet

Background and study purpose
The study
Main results

Background and study purpose

At sites where underlying soil or groundwater contamination leads to an unacceptable vapour intrusion (VI), a thorough understanding of the VI pathways can enable a better choice of remediation strategy.

Normally, VI pathways are investigated by performing a thorough building inspection followed by chemical analyses of vapours at identified potential pathways.

This project investigates and presents an active tracer gas method for tracking VI pathways. The presented method is an alternative to other investigation methods. An English version of the presentation can be found in /5/.

The primary purpose of the study is to describe a best practice for applying the method, whereas the secondary purpose is to seek a better understanding of the potential of the method, e.g. strengths and weaknesses.

Finally, attenuation factors across concrete floors were investigated at two of the sites where the prerequisites for applying the Danish EPA model JAGG are not met.

The study

Throughout the project, DMR A/S performed tracer tests at six sites at which the Region of Northern Jutland were performing other investigations. Four of the sites were former dry cleaning sites (PCE) and two of the sites had benzene as the contaminant of concern.

The tracer test is performed by: (i) injecting tracer gas into a suspected source area for vapour intrusion, (ii) performing control measurements to ensure that the tracer gas is distributed as required, and (iii) taking a semi-quantitative measurement of tracer gas intrusion by a hand held instrument.

Main results

By performing tracer gas tests at the six sites included in the study, a best practice has been established in which a small number of potential shortcomings of the method are countered by applying a systematic approach. The method and the equipment needed is described in detail so that other parties can easily adapt them to suit their particular needs.

Strengths of the active tracer gas test

The equipment needed for the test is inexpensive (tracer gas, leak detector and standard soil vapour investigation equipment).

The results are very “visual”, whereby the investigator is granted a very good direct sense of the advective vapour intrusion pathways at the site. The test is very adaptive and different intrusion hypotheses can be tested “on the go”. The combination of a very buoyant tracer gas (5% hydrogen) and a robust and fast-reacting gas leak detector contributes significantly to the strength of the method.

Weaknesses of the active tracer gas test

The method is not suitable for investigating diffusive vapour intrusion pathways and there is a risk of false negative results if the tracer gas saturation on the source side of the construction is not as assumed. Hence, it is important to follow the established procedure, which takes this into account.

The leak detector used can yield false positive results in very wet environments or where household chemicals are kept. The test procedure takes this into account.

Duration and cost

All of the tests performed were conducted within one day by two persons. In our experience, tests at a small site can be carried out and documented within a budget of roughly 15,000 Danish krone, whereas larger sites (within one field day) will require about 20-30,000 Danish krone.

Of course, actual costs will vary depending on the hourly rate of the consultants involved, driving time and distance and the level of documentation required.