Radon is measured in picocuries per liter or air (pQ/L). The U.S. Environmental Protection Agency and the Centers for Disease Control and Prevention recommend that homes with radon levels or about 4 pG/L be fixed. The "action" levels in water are set much higher at 300 pG/L.
The water levels are considerably higher because it takes high levels of radon in water to produce significant levels of radon in air. The rule of thumb is that it takes 10,000 pG/L or radon in water to raise the level of radon in air 1 pG/L. Using this rule, it would take 40,000 pG/L in water to raise the indoor air radon level to 4 pG/L.