The Effect of Uranium Siting in Two-Mica Granites on Uranium Concentrations and Radon Activity in Ground Water

Two-mica granites have been identified as a lithology with the potential for yielding ground water containing elevated levels of Rn-222. Wide variations between individual wells and between average activities for wells drilled into apparently lithologically similar rock bodies suggest complex controls on factors predisposing high radon concentrations in ground water.

Research into factors affecting levels of Rn-222 was conducted on wells drilled into four two-mica plutons in Maine and New Hampshire. The study included the analysis of the well water samples for uranium concentration as well as autoradiographic studies of rock samples from outcrops spatially associated with the specific wells. Photomicrographs of the rocks and the exposed autoradiographs were compared. In all the two-mica rock samples studied, uranium was found to be sited along grain boundaries, in microcracks, and on alteration sites in the rock rather than in discrete mineral phases.

Ground water with the higher concentrations of uranium was found to be associated with the siting of the uranium on altered plagioclase feldspar grains in the granite. Lower uranium concentrations and lower and less variable activities of Rn-222 was associated with the siting of uranium on altered biotite grains. The distinction between these two modes of siting may be attributable to the relative strength with which the uranium is retained in the two types of sites. The uranium is retained on the ferric hydroxide alteration sites on the biotite approximately 10⁶ more strongly than on the altered plagioclase.

The relatively loose retention of uranium on altered plagioclase grains provides the basis for mechanisms for the leaching, transport, and reconcentration of uranium. One implication of the existence of accumulations of uranium is that ground water in contact with secondary uranium mineralization will be in contact with the decay products of the uranium. This includes radon, which of all the isotopes in the U-238 decay chain, is the most soluble in ground water.