SYNSEDIMENTARY PHOSPHORITE IN LATE CAMBRIAN DOLOSTONE OF THE NORTHERN APPALACHIANS AS THE SOURCE OF ELEVATED RADIONUCLIDES IN A FRACTURED ROCK AQUIFER

Groundwater with high levels of naturally-occurring radioactivity (alpha, U, and Ra) was discovered in numerous bedrock wells in northwestern Vermont in 2000; given concerns about chronic health risks, detailed geologic mapping and geophysical surveys were carried out, demonstrating that the radioactivity is confined to the Late Cambrian Clarendon Springs Formation (Ccs). Of 131 private wells tested within a 12 km² area producing from fractured dolostone of the Ccs, 30 contained alpha radiation in excess of the EPA MCL of 15 pCi/L; mean value was 33 pCi/L and the highest value was 1300 pCi/L. U is similarly elevated.

Ongoing instrumental analysis and sequential extraction indicate that U-rich synsedimentary phosphorite breccias and layers in dolostone are the source of the elevated radionuclides. U is distributed in a diffuse manner throughout black microcrystalline phosphorite layers and clasts, which are comprised dominantly by fluoroapatite [Ca₅(PO₄)₃F] and contain 80 to 420 ppm U. Mineral dissolution experiments indicate that U is mainly released when fluoroapatite dissolves, implying that U(IV) is substituted in 7-fold Ca2 sites. Also significant is the occurrence of isolated 5-10 micron diameter zones that exhibit stoichiometries consistent with autunite [Ca(UO₂)₂(PO₄)₂·nH₂O). Many occur adjacent to decomposing pyrite, indicating that sulfuric acid generated by pyrite oxidation may trigger fluoroapatite dissolution and formation of secondary autunite. Aqueous U and SO₄ are positively correlated in groundwater, consistent with pyrite oxidation as a cause of U release from fluoroapatite. Efforts to trace groundwater flow using multi-element analysis indicate that the U-rich groundwater generated from the Ccs phosphorites is largely contained within the Ccs, a phenomenon that is attributed to low transmissivity caused by limited connectedness of sparse, discontinuous fractures and bedding planes.

In summary, the results of this study indicate that U is released to groundwater (at levels 10-100x the 30 ppb EPA U MCL) by dissolution of U-rich fluoroapatite, a process driven by infiltration of meteoric water and organic acids from overlying soils augmented locally by pyrite oxidation. The occurrence documented herein may indicate potential for U sources in similar aquifers elsewhere.