[en] Analysis of the groundwaters of northern Illinois for radioactivity has been carried out for the past few years. Monitoring of gross alpha activity detects ²²⁶Ra and other alpha emitters. Some of the more common beta emitters that may be found in natural groundwaters are ¹⁴C, ⁴⁰K, and ⁸⁷Rb. The wells that most frequently show high levels of gross alpha activity are from 700 to over 2000 ft. deep. Of the wells tested, 72 had gross alpha levels greater than 15 pCi/l; 147 ranged from 5.0 to 14.9 pCi/l; and 115 ranged from 3.1 to 4.9 pCi/l. Standard treatment used for the water reduces the radioactivity content of the finished water to a point where it is well below acceptable levels. The presence of radioactivity in groundwater must be regarded as a hazard and reasonable precautions should be taken to protect the water consumer