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
- 102 Downloads
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
- Altschuler, Z.S., Clarke, R.S. and Young, E.J., 1958, Geochemistry of uranium in apatite and phosphorite. USGS Professional Paper 314-D, 45–90.Google Scholar
- Ames, L.L., 1960, Some cation substitutions during the formation of phosphorite from calcite. Economical Geology, 55, 34–362.Google Scholar
- Barnes, V.E. and Bell, W.C., 1977, The Moore Hollow Group of Central Texas. University of Texas, Bureau of Economic Geology, Reports of Investigations 88, 169 p.Google Scholar
- Cech, I.M., Lemma, M., Prichard, H.M. and Kreither, C., 1987a, Radium-226 and radon-222 in domestic water of Houston-Harris County, Texas. In: Graves, B.J. (ed.), Radon, Radium and Other Radioactivity in Ground Water. Lewis Publishers, p. 377–402.Google Scholar
- Cothern, C.R., 1987, Estimating the health risks of the radon drinking water. Journal of American Water Works Association, 79, 153–158.Google Scholar
- Freed, R.L., 1986, Summary of the Precambrian and Paleozoic geology of south-central Texas. The Compass, 63, 130–144.Google Scholar
- Gilkeson, R.H. and Cowart, J.B., 1987, Radium, radon and uranium isotopes in groundwater from Cambrian-Ordovician sandstone aquifers in Illinois. In: Graves, B.J. (ed.), Radon, Radium and Other Radioactivity in Ground Water. Lewis Publishers p. 403–422.Google Scholar
- Ledger, E.B., 1981, Evaluation of the Catahoula Formation as a Source Rock for Uranium Mineralization with Emphasis on East Texas. Unpublished Ph.D. Dissertation, Texas A&M University, 249 p.Google Scholar
- Millard, H.T. Jr., 1976, Determination of uranium and thorium in USGS standard rocks by delayed neutron technique. In: Flanigan, F. (ed.), Descriptions and Analysis of Eight New USGS Rock Standards. USGS Professional Paper, 840, 61–66.Google Scholar
- Mills, W.A., 1990, Risk assessment and control management of radon in drinking water. In: Cothern, C.R. and Rebers, P. (eds.), Radon, Radium and Uranium in Drinking Water. Chelsea, Michigan, Lewis Publishers, 99, 27–37.Google Scholar
- Rich, R.A., Holland, H.D. and Petersen, U., 1977, Hydrothermal Uranium Deposits, Elsevier, 264 p.Google Scholar
- Rogers, J.J.W. and Adams, J.A.S., 1969, In: Wedepohl, K.H. (ed.), Handbook of Geochemistry, Springer, II/3, sections 90 and 92.Google Scholar
- Statens Stralevern, 1994, Action Plan for Further Work with Radon in Norway. Report to the Norwegian Ministry of Health, March 1994, 37 p.Google Scholar
- Statens Stralskyddsinstitut, 1993, Radon 1993—En Sapport over Laget [Radon 1993—a Status Report]. Report 93-10, Stockholm, 97 p.Google Scholar
- Wanty, R.B., Lawrence, E.P. and Gundersen, L.C.S., 1992, A theoretical model for the flux of radon from rock to ground water. Geological Society of America Special Paper, 271, 73–78.Google Scholar
- Williams, H., Turner, F.J. and Gilbert, C.M., 1982, Petrography: An Introduction to the Study of Rocks in Thin Sections. W.H. Freeman and Company, 626 p.Google Scholar
- Zikovsky, L. and Chah, B., 1990, The lognormal distribution of radon in groundwater. Groundwater, 28, 673–676.Google Scholar