Radon is a noble gas, and is always seeking to reach equilibrium pressure-wise. When pressure in a building is lower than soil pressure, the radon moves from the soil into the structure. Radon in well water is released into the air when water is heated, aerated, or agitated, such as with showers, dishwashers, and washing machines.
Common mitigation techniques include Active Soil Depressurization, where we seek to create a lower pressure under a slab or crawlspace barrier than above the floor; Ventilation, primarily through the installation of Energy Recovery Ventilators; and aeration systems for treating well water.
There are also a variety of lesser-used techniques such as block-wall depressurization, adjustments to HVAC systems, and pressurization for unusual situations.