The concentrations and distributions of U and Th in Paleozoic aquifers surrounding the Llano Uplift area, central Texas, U.S.A.: application to the sources of Ra and Rn in groundwater
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Interactions between groundwater and aquifer rock can result in anomalously high levels of radium (Ra) and radon (Rn) in groundwaters. An understanding of aquifer mineralogy and determining the sources of natural radionuclides are, therefore, essential to design possible means for improving groundwater quality. The Cambrian Hickory Sandstones and Cap Mountain Limestones constitute important aquifers in the area surrounding the Llano Uplift in central Texas. Groundwaters produced from these aquifers, however, have anomalous concentrations of Ra and Rn, much greater than the maximum contaminant levels (MCL) suggested by the USEPA. To determine the sources of Ra and Rn, the abundance, distribution, and nature of occurrence of U and Th, parent radionuclides of Ra and Rn, were examined in cored aquifer rock samples.
The Hickory, 136 m thick, consists of a coarse-grained sandstone lower submember, a calcareous sandstone middle, and a fossiliferous and hematitic sandstone upper, with thin shale laminae throughout. The Cap Mountain, 44 m thick, is a sandy limestone. Detrital materials are composed of 77% quartz, 19% feldspars, and 4% lithic fragments, and are classified as subarkose. Accessory minerals are less than 1% in average. Authigenic minerals, primarily clay, Fe-oxide minerals and carbonate, make up 18% of the bulk rock. Porosity is of secondary origin.
Analysis of U in a total of 128 sandstone and shale samples shows an average of 3.8 ppm, ranging from 1 to 8.5 ppm. Th concentrations in 21 bulk rock samples range from 8.4 to 18.6 ppm, averaging 13.7 ppm. The U and Th contents are similar to those of the underlying Precambrian granites, from which much of the sediments were derived. The primary modes of U occurrence are in: (1) biogenic materials such as phosphatic brachiopod fragments and intraclasts; (2) thin shaly laminae enriched in phyllosilicates; (3) authigenic minerals, particularly hydroxyl-oxides of iron and clay minerals; and (4) detrital accessory minerals.
Mobilization of U and its decay products by groundwaters can account for the Ra and Rn anomaly in the produced water, particularly interacting with intervals of high concentrations of shaly laminae, phosphatic materials, or iron-oxide cements.
Key wordsaquifer minerals water-rock interaction radionuclides modes of U occurrence Llano Uplift
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