ELEVATED URANIUM IN A FRACTURED SEDIMENTARY ROCK AQUIFER AFFECTED BY DISSOLUTION OF U-BEARING FLUOROAPATITE IN PHOSPHORITE

U-rich phosphorite breccias and layers in latest Cambrian dolostone are the source of elevated radionuclides in groundwater of a fractured rock aquifer in the Champlain Valley of NW Vermont. Of 131 private wells tested over a 12 km² area producing from or near fractured dolostone and phosphorite of the Clarendon Springs Formation, 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. ICPMS analysis indicates that the phosphorite layers and clasts contain an average of 185 mg/kg U (range = 80 to 420 mg/kg) and XRD indicates that they are dominated by fluoroapatite [Ca₅(PO₄)₃F]. SEM-EDS reveals that U occurs in a diffuse manner throughout the phosphorites where stoichiometry is consistent with occurrence of fluoroapatite, likely containing U(IV) substituted for Ca; also significant is the occurrence of isolated zones 5-10 microns in diameter that are very U-rich and exhibit stoichiometric ratios 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 reaction of U(IV)-bearing fluorapatite to U(VI)-bearing autunite. Reaction of phosphorite powders with dilute sulfuric acid (1 g in 25 mL of 0.3 M H₂SO₄) caused almost complete dissolution of the fluoroapatite, emphasizing the susceptibility of the likely U host to dissolution in the presence of relatively weak acid. Sequential chemical extraction (HOAc, HA-HCl, H₂O₂) paired with quantitative XRD of powders (pre- and post-extraction to measure mineral dissolution) and ICPMS of solutions (post-extraction) further aids in interpretation of U speciation and mobility; the majority of U (50 – 80%) was released by reaction with H₂O₂, and most of the remaining U was released by reaction with HOAc (12 -50 %). U released during these two stages (comprising 90 – 100 % of total U) occurs in conjunction with fluoroapatite dissolution. Accordingly, the combined results of sequential extraction, XRD, ICPMS and SEM-EDS indicate that U is released to groundwater by dissolution of fluoroapatite, likely driven by infiltration of meteoric water and organic acids from overlying soils augmented by pyrite oxidation, locally. The occurrence documented herein may indicate potential for similar U sources elsewhere.