Since radium 226 can be found in low concentrations in almost all rock and soil, it is not a question of “if”, but “how much” radon you really have. Radon is generated in rock and soil and it escapes into the atmosphere through cracks and spaces between the rocks and soil. This results in an average outdoor radon concentration of about 0.4 pCi/L (picocuries per liter of air). At these concentrations there is no real danger. However, if radon is allowed to seep into homes and buildings through cracks and holes in the foundations and walls, the concentration can build up to much higher levels, at which point research suggests there is a reason for concern.
The average indoor radon concentration is about 1.3 pCi/l. Radon concentrations in a home can very depending on several factors including; house design, soil conditions, local geology, and the weather, such as high or low atmospheric pressures, and warm or cold climate. For example, indoor radon levels increase substantially during the winter months. As indoor temperatures increase relative to the outside temperatures, a thermal effect occurs. The rising warm air within a building is displaced by cold denser outside air, some of which seeps in through the foundation cracks, vents, and holes from the underlying soils.
Furthermore, exhaust fans inside the house can create a lower (negative) pressure inside the home relative to the surrounding soil and air, and radon can actually be drawn into the building.
Sealing the cracks in the foundation and other openings, such as drainage and sump pump sinks, is a basic part of most approaches to radon reduction. This technique reduces the flow of radon into your home thereby making other radon reduction techniques more effective and cost-efficient, and also reduces the loss of conditioned air. This technique, however, has its limitations. It is difficult to identify and permanently seal all the places where radon can enter, and if you have a finished basement, this is almost impossible without extensive and possibly expensive work. Furthermore, even if you were able to do so in an unfinished basement, normal settling of your house will eventually open new entry routes and reopen old ones.
You can try a special floor drain adapter for your drainage holes in your basement. I myself have purchased couple of these Dranjer D-R2 floor drains. They allow for water to flow down the drain but prevent any gas/air from coming up through it into the living space. According to the website I purchased these from it does the following: “Dranjer seals permit unrestricted flow of water into floor drains or sump pits while sealing out the entry of mold spores, insects and radon gas from the sub-slabe floor area.” To be completely honest, I am not sure these worked very well, as my radon detector did not register any significant changes.
As a side note: EPA does not recommend the use of sealing alone to reduce radon because, by itself, sealing has not been shown to lower radon levels significantly or consistently