GEOLOGIC OCCURRENCE OF NATURAL RADIUM226 IN GROUND WATER IN ILLINOIS / DISTRIBUTION GÉOLOGIQUE DU RADIUM226 NATUREL DANS LES EAUX TELLURIQUES DE L'ILLINOIS

The concentration of Ra 226 in 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. The Ra 226 content of the water was found to vary from a low of about 1 pC/1 in areas of recharge to a high of 20 to 100 pC/1 down-dip in the direction of water movement. Since the radium in the water is obtained from the decay of Uranium-238, the variation in the radium content may result from: (1) variation in the concentration of uranium in the sediments, possibly by translocation from areas of recharge down-dip in the direction of water movement; (2) the chemistry of the aquifer, which affects the solubility of the radium; (3) the physical conditions within the aquifer that limit the concentration and detemine the time required to attain the maximum value. The latter may be what we observe,-the short contact time in the recharge area prduces a lower concentration than that obtained after a longer contact time down-dip. However, a good correlation was found between the radium content and either the fluoride or the total solids content of the water. This may indicate that solubility effects determine the radium concentration.