How to model the situation of a basement in contact with groundwater?


In case you need to assess risks due to vapour intrusion in a building with a basement in contact with groundwater, careful evaluation of local site conditions is required:

  • If there is water in the basement or the basement walls or floors are moist, there is a high-risk situation for indoor air volatilisation: measuring indoor air concentrations is the preferred option.
  • If the basement is constructed to avoid entrance of moist, the floor and walls most probably restrict vapour intrusion. In case of modelling, you probably have to adapt the floor and wall quality parameter values in S-Risk. We do recommend however to verify with indoor air measurements.

In case of future projects, modelling vapour intrusion should be in line with construction requirements to avoid moist and vapour intrusion.

In some situations, the basement floor is situated below groundwater level (as shown in situation  of the scheme). Currently, S-Risk is lacking the models for calculations in this situation, and consequently does not allow for a basement depth below groundwater level.

To cope with this situation, you can follow a workaround solution in which you enter the groundwater concentration as a 'surrogate' soil concentration, Cartificial , using an artificial soil layer. A crucial step is the calculation of a surrogate soil concentration Cartificial corresponding to the groundwater concentration Cgw. To make the calculation easier,  we have made available an Excel tool that you can download here.

The steps in the workaround are as follows:

  • On the Soil tab, enter a groundwater level that is situated below the depth of the basement. Then, fill in the soil profile corresponding to the site information, but create an additional soil layer between the effective depth of the groundwater table (site information) and the value for groundwater table depth you entered in S-Risk.
    This is illustrated in step ‚ of the scheme. The properties of this artificial soil layer are not critical, but you need the information for the calculation tool. The properties of the artificial soil layer in S-Risk and in the calculation tool need to be exactly the same!
  • On the Water tab, choose the option of entering a groundwater concentration.
  • Use the calculation tool to calculate the surrogate soil concentration, Cartificial. The calculation tool contains detailed guidance on how to do so (see step ƒ in the scheme).
  • On the Concentration tab, enter the soil concentrations corresponding to the site information for the effective soil layers. Enter the surrogate soil concentration Cartificial from the calculation tool as the soil concentration for the artificial soil layer. Set the groundwater concentration Cgw equal to zero.
  • Run your simulation (see step „).

If you would need a remediation value for the groundwater layer, you can use the above type of simulation. It will give you a surrogate soil remedation value for the artificial soil layer. The groundwater remediation value then corresponds to the pore water concentration in equilibrium with the calculated soil concentration, which you can read from the extended report of application II).
last modified on 10/06/2015 - 16:44
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