The concentration of Ra226in water from the Cambrian, Ordovician and Silurian-Devonian aquifers has been determined in a large number of samples from the northern half of Illinois. From these, the iso-concentration lines have been established for each formation. In the Cambrian and Ordovician aquifers, the radium content of the water is correlated with the regional piezometric surface, being low at the area of recharge and slowly increasing down the hydraluic gradient. A smoothly curving profile is obtained in all areas except where extremely heavy pumpage has occured. In addition, a rapid increase in radium concentration is observed where these formations are overlain by the Pennsylvanian.