The Effects of Uranium Distribution in Granitic Plutons on Groundwater Chemistry in Downeast Maine

Paleozoic granitic plutons in Downeast Maine intruded between 420 and 370 Ma that contain variable amounts of uranium and its decay products, some of which are in groundwater. A large fraction of drinking water is derived from groundwater from wells in these granites. Exposure to radionuclides in the uranium decay chain can lead to a number of adverse health effects. A key question is where these radionuclides are located in the granite and how easily they can enter groundwater. A critical issue is whether uranium is in minerals or along grain boundaries (or both). Uranium and other elements within groundwater and rock were examined in four granites: Gouldsboro (420 Ma), the S-type Corea granite (376 Ma) which is a rapakivi granite, Tunk Lake, (366 Ma) an s-type concentrically zoned granite, and the Lucerne (370 Ma), an s-type biotite granite. In situ gamma spectroscopy provides an overall radioactivity (and U, Th, and K) of 8.2 μR/hr for the Gouldsboro granite, 1.6 μR/hr for the Gouldsboro mafic dikes, 18.0 μR/hr for the Corea, 21.1 μR/hr for the Tunk Lake, and 28.7 μR/hr for the Lucerne. Radon emanation from crushed samples was measured using a RAD7 were found to be 5.7 for the Gouldsboro granite, 0.7 for the Gouldsboro mafic dikes, 12.0 for the Corea, 35.4 for the Tunk Lake, and 41.0 for the Lucerne. Groundwater samples from the Gouldsboro and Corea pluton were analyzed by ICPMS to determine their geochemistry. Uranium concentration increases with well depth, with shallow wells having 0.00 to 0.28 ppb, and deeper drilled wells having 0.03 to 7.11 ppb. Crushed samples from each pluton were leached using 2 M HNO₃ and 2 M HCl, and the leachate was analyzed on the ICP-MS. The ²³⁴U/²³⁸U ratio in the leachate can be used to evaluate whether the uranium is in mineral grains or along grain boundaries. The Corea, Tunk Lake, and Lucerne plutons had elevated ²³⁴U/²³⁸U ratios from the leaching that may be related to available uranium on grain boundaries. The Corea and Tunk Lake granites produce more radon than expected, which could be due to uranium on the grain boundaries, and this inference is supported by the leaching result of higher ²³⁴U/²³⁸U ratios. Thus, some of these granites may present a bigger challenge to public health because the uranium is more easily liberated.