Summary and conclusions

Variationer i poreluftens forureningsindhold. - Spor 1. Opstilling af modelscenarier, udvælgelse af modeller og modelberegninger.

The main objective of this soil gas project is to develop a method for evaluating the variation in soil gas measurements, as earlier projects have shown large variations in soil gas concentrations with regard to both time and space. The project is divided into two phases, and the objective of this first phase is to improve existing knowledge about which factors have the most significant impact on the variation in soil gas concentrations.

The results and conclusions in this report are solely based on model simulations. The simulations are produced by a 3-dimensional numerical model developed by Abreu and Johnson that includes the most significant processes for soil gas transport. The Abreu and Johnson model was chosen on the basis of a literature study carried out to find models for the simulation of soil gas transport. However, as with all numerical models, this model does not describe the full complexity of real life conditions, and therefore it is not possible to transfer the results directly to an actual pollution project. Instead, the results can be used to evaluate which factors are most significant for soil gas variation and can, in that way, be used as a tool in the planning and execution of actual soil gas investigations.

In order to establish the model scenarios, it was necessary to limit the number of parameters, as well as the range of values modelled. Within each of the 4 groups of parameters – (1) contaminant conditions, (2) geological and hydrogeological conditions, (3) meteorological conditions, and (4) construction conditions – limitations have been made both with respect to which of the selected parameters are varied and which parameter values are applied.

On the basis of the model scenarios, it is concluded that the largest variations in the soil gas concentrations with regard to time are seen near fractures, where advective airflow is most dominant. Generally, there are smaller variations in the unsaturated zone, where diffusion is the dominant transport process. Furthermore, it is concluded that the variation in the soil gas concentration in the unsaturated zone is less than 82.5% above or below the average value – which corresponds to a factor of less than 10 between the highest and the lowest concentrations.

It is also concluded that the variation in the indoor air concentration in the building with regard to time is generally larger than for the soil gas concentrations in the soil, and that this variation is controlled by factors including the pressure within the building, the air change in the building, the size of the fracture and the permeability of the soil.

Furthermore, it can also be concluded that a building with a basement will have a certain influence on both the horizontal and the vertical concentration distribution in the immediate vicinity and beneath the building. Finally, it is concluded that, in particular, the location of the source in the unsaturated zone and the stratification (geological heterogeneity), and aerobic degradation may have a major impact on both the horizontal and the vertical concentration distribution in the immediate vicinity and beneath the building. In some cases, both the horizontal and the vertical variation may thus vary by one order of magnitude over a distance of 10 cm, while in other situations this variation may be seen over a distance of 10 m.