A mathematical model is used to investigate whether radon degassing from groundwater may contribute to indoor radon levels. Specifically, the transport of radon in the soil-gas phase from the groundwater-soil gas interface to under-pressurized dwellings is modelled. The question whether radon in groundwater may contribute to indoor radon levels arises from observed high radon concentrations in groundwater, and recent findings that advection in the gas phase may be an important transport mechanism for radon into slightly under-pressurized dwellings. Most previous radon transport investigations did not consider groundwater as a potential source for contributing to indoor radon. The mathematical model includes a method to directly calculate indoor radon concentrations and an equivalent continuum approach to represent cracks in concrete foundations. The results of the simulations indicate that radon, which partitions from groundwater to the soil gas, may be advectively transported by the gas phase to slightly under-pressurized dwellings in relatively permeable soils such that indoor radon concentrations may exceed 148 Bq/m3, which is the action limit imposed by EPA. ¿ American Geophysical Union 1996