Hydrogeochemical controls on uranium in aquifers of the Jacobsville Sandstone

The occurrence of elevated uranium (U) in sandstone aquifers was investigated in the Upper Peninsula of Michigan, focusing on aquifers of the Jacobsville Sandstone. The hydrogeochemical controls on groundwater U concentrations were characterized using a combination of water sampling and spectral gamma-ray logging of sandstone cliffs and residential water wells. 235U/238U isotope ratios were consistent with naturally occurring U. Approximately 25% of the 270 wells tested had U concentrations above the U.S. Environmental Protection Agency Maximum Contaminant Level (MCL) of 30 μg/L, with elevated U generally occurring in localized clusters. Water wells were logged to determine whether groundwater U anomalies could be explained by the heterogeneous distribution of U in the sandstone. Not all wells with relative U enrichment in the sandstone produced water with U above the MCL, indicating that the effect of U enrichment in the sandstone may be modified by other hydrogeochemical factors. Well water had high redox, indicating U is in its highly soluble (VI) valence. Equilibrium modeling indicated that aqueous U is complexed with carbonates. In general, wells with elevated U concentrations had low 235U/238U activity ratios. However, in some areas U concentrations and 235U/238U activity ratios were simultaneously high, possibly indicating differences in rock-water interactions. Limited groundwater age dating suggested that residence time may also help explain variations in well water U concentrations. Low levels of U enrichment (4 to 30 ppm) in the Jacobsville sandstone may make wells in its oxidized aquifers at risk for U concentrations above the MCL. On average, U concentrations were highest in heavy mineral and clay layers and rip up conglomerates. Uranium concentrations above 4 ppm also occurred in siltstones, sandstones and conglomerates. Uranium enrichment was likely controlled by deposition processes, sorption to clays, and groundwater flow, which was controlled by permeability variations in the sandstone. Low levels of U enrichment were found at deltaic, lacustrine and alluvial fan deposits and were not isolated to specific depositional environments.

Redox front geochemistry and weathering: theory with application to the Osamu Utsumi uranium mine, Poços de Caldas, Brazil

Numerical calculations describing weathering of the Poços de Caldas alkaline complex (Minas Gerais, Brazil) by infiltrating groundwater are carried out for time spans up to two million years in the absence of pyrite, and up to 500,000 years with pyrite present. Deposition of uranium resulting from infiltration of oxygenated, uranium bearing groundwater through the hydrothermally altered phonolitic host rock at the Osamu Utsumi uranium mine is also included in the latter calculation. The calculations are based on the quasi-stationary state approximation to mass conservation equations for pure advective transport. This approximation enables the prediction of solute concentrations, mineral abundances and porosity as functions of time and distance over geologic time spans. Mineral reactions are described by kinetic rate laws for both precipitation and dissolution. Homogeneous equilibrium is assumed to be maintained within the aqueous phase. No other constraints are imposed on the calculations other than the initial composition of the unaltered host rock and the composition of the inlet fluid, taken as rainwater modified by percolation through a soil zone. The results are in qualitative agreement with field observations at the Osamu Utsumi uranium mine. They predict a lateritic cover followed by a highly porous saprolitic zone, a zone of oxidized rock with pyrite replaced by iron-hydroxide, a sharp redox front at which uranium is deposited, and the reduced unweathered host rock. Uranium is deposited in a narrow zone located on the reduced side of the redox front in association with pyrite, in agreement with field observations. The calculations predict the formation of a broad dissolution front of primary kaolinite that penetrates deep into the host rock accompanied by the precipitation of secondary illite. Secondary kaolinite occurs in a saprolitic zone near the surface and in the vicinity of the redox front. Gibbsite forms a bi-modal distribution consisting of a maximum near the surface followed by a thin tongue extending downward into the weathered profile in agreement with field observations. The results are found to be insensitive to the kinetic rate constants used to describe mineral reactions.

Measurement of isotopic uranium in water for compliance monitoring by liquid scintillation counting with alpha/beta discrimination

A simple and inexpensive method is described for analysis of uranium (U) activity and mass in water by liquid scintillation counting using $\alpha$/$\beta$ discrimination. This method appears to offer a solution to the need for an inexpensive protocol for monitoring U activity and mass simultaneously and an alternative to the potential inaccuracy involved when depending on the mass-to-activity conversion factor or activity screen.^ U is extracted virtually quantitatively into 20 ml extractive scintillator from a 1-$\ell$ aliquot of water acidified to less than pH 2. After phase separation, the sample is counted for a 20-minute screening count with a minimum detection level of 0.27 pCi $\ell\sp{-1}$. $\alpha$-particle emissions from the extracted U are counted with close to 100% efficiency with a Beckman LS6000 LL liquid scintillation counter equipped with pulse-shape discrimination electronics. Samples with activities higher than 10 pCi $\ell\sp-1$ are recounted for 500-1000 minutes for isotopic analysis. Isotopic analysis uses events that are automatically stored in spectral files and transferred to a computer during assay. The data can be transferred to a commercially available spreadsheet and retrieved for examination or data manipulation. Values for three readily observable spectral features can be rapidly identified by data examination and substituted into a simple formula to obtain $\sp{234}$U/$\sp{238}$U ratio for most samples. U mass is calculated by substituting the isotopic ratio value into a simple equation.^ The utility of this method for the proposed compliance monitoring of U in public drinking water supplies was field tested with a survey of drinking water from Texas supplies that had previously been known to contain elevated levels of gross $\alpha$ activity. U concentrations in 32 samples from 27 drinking water supplies ranged from 0.26 to 65.5 pCi $\ell\sp{-1}$, with seven samples exceeding the proposed Maximum Contaminant Level of 20 $\mu$g $\ell\sp{-1}$. Four exceeded the proposed activity screening level of 30 pCi $\ell\sp{-1}$. Isotopic ratios ranged from 0.87 to 41.8, while one sample contained $\sp{234}$U activity of 34.6 pCi $\ell\sp{-1}$ in the complete absence of its parent, $\sp{238}$U. U mass in the samples with elevated activity ranged from 0.0 to 103 $\mu$g $\ell\sp{-1}$. A limited test of screening surface and groundwaters for contamination by U from waste sites and natural processes was also successful. ^

Spatial distribution of orofacial cleft birth defects and uranium-radium concentrations in tap water in Atascosa, Bee, Brooks, Calhoun, Duval, Goliad, Hidalgo, Jim Hogg, Jim Wells, Karnes, Kleberg, Live Oak, McMullen, Nueces, San Patricio, Refugio, Starr, Victoria, Webb, and Zavala Counties, Texas

Background Past and recent evidence shows that radionuclides in drinking water may be a public health concern. Developmental thresholds for birth defects with respect to chronic low level domestic radiation exposures, such as through drinking water, have not been definitely recognized, and there is a strong need to address this deficiency in information. In this study we examined the geographic distribution of orofacial cleft birth defects in and around uranium mining district Counties in South Texas (Atascosa, Bee, Brooks, Calhoun, Duval, Goliad, Hidalgo, Jim Hogg, Jim Wells, Karnes, Kleberg, Live Oak, McMullen, Nueces, San Patricio, Refugio, Starr, Victoria, Webb, and Zavala), from 1999 to 2007. The probable association of cleft birth defect rates by ZIP codes classified according to uranium and radium concentrations in drinking water supplies was evaluated. Similar associations between orofacial cleft birth defects and radium/radon in drinking water were reported earlier by Cech and co-investigators in another of the Gulf Coast region (Harris County, Texas).50, 55 Since substantial uranium mining activity existed and still exists in South Texas, contamination of drinking water sources with radiation and its relation to birth defects is a ground for concern. ^ Methods Residential addresses of orofacial cleft birth defect cases, as well as live births within the twenty Counties during 1999-2007 were geocoded and mapped. Prevalence rates were calculated by ZIP codes and were mapped accordingly. Locations of drinking water supplies were also geocoded and mapped. ZIP codes were stratified as having high combined uranium (≥30μg/L) vs. low combined uranium (<30μg/L). Likewise, ZIP codes having the uranium isotope, Ra-226 in drinking water, were also stratified as having elevated radium (≥3 pCi/L) vs. low radium (<3 pCi/L). A linear regression was performed using STATA® generalized linear model (GLM) program to evaluate the probable association between cleft birth defect rates by ZIP codes and concentration of uranium and radium via domestic water supply. These rates were further adjusted for potentially confounding variables such as maternal age, education, occupation, and ethnicity. ^ Results This study showed higher rates of cleft births in ZIP codes classified as having high combined uranium versus ZIP codes having low combined uranium. The model was further improved by adding radium stratified as explained above. Adjustment for maternal age and ethnicity did not substantially affect the statistical significance of uranium or radium concentrations in household water supplies. ^ Conclusion Although this study lacks individual exposure levels, the findings suggest a significant association between elevated uranium and radium concentrations in tap water and high orofacial birth defect rates by ZIP codes. Future case-control studies that can measure individual exposure levels and adjust for contending risk factors could result in a better understanding of the exposure-disease association.^

Uranium in marine basalts

The uranium content of glass from chilled margins of oceanic tholeiitic basalt flows is generally <0.1 ppm, even for old samples with highly altered crystalline interiors. Such low values represent the original whole rock concentrations, although subsequent to eruption low-temperature weathering has added uranium, and other elements, to the crystalline portions of these basalts. Consideration of the K/U ratios of altered samples suggests that basalt weathering may provide the major oceanic sink for these two elements.

(Table 1) Uranium and Thorium concentrations, activity ratios and calcium carbonate content of DSDP Site 54-424

The geochemical behaviour of uranium and thorium in metalliferous sediments and hydrothermal deposits has been widely studied and the main results have been summarised by Boström and Rydell. These isotopes may be used to clarify how the metal-rich solutions are introduced into sediment cover and seawater. Using radiochemistry followed by alpha spectrometry, we have measured uranium concentrations as high as several hundred p.p.m., which must clearly be associated with ocean ridge thermal activity, in sediments interbedded between the basaltic basement and the green hydrothermal mud at DSDP Site 424. These high uranium concentrations indicate the path followed by the hydrothermal fluid which, debouching at the sediment-water interface, formed the green mud.

(Table 2) Uranium concentrations and isotopic ratios in surface waters of the Atlantic Ocean in April 1977

Uranium concentrations and isotope ratios were measured in thirteen surface-water samples collected across the entire Atlantic Ocean. The mean isotope ratio was 1.15+/-0.01, and the mean concentration 3.1+/-0.2 µg/l.