Increase In Uranium Concentration Research Articles

Simultaneous ion-exchange sorption of uranium with concomitant impurities

The aim of the study is to elucidate the effect of simultaneous sorption of uranium with different concomitant impurities that are contained in underground leaching solutions (sulfates, chlorides, nitrates, phosphates, silicates, iron, aluminum, and manganese) at pH = 1.2. Different sorption isotherms are studied for the uranium sorption by the strongly basic anion-exchange resin (BO020): (i) uranium and a number of concomitant impurities; (ii) uranium in the presence of the studied impurities. The negative effect of impurities is manifested at low uranium concentrations. With an increase in uranium concentration, the depressing effect of impurities (Al, Fe, Mn) becomes less. The most negative effect is exerted by nitrates in the amount of 0.4 g/L, silicates - 0.3 and 1 g/L, chloride -1 g/L. The phosphorus contributes to better sorption of uranium in the low concentration range of 0.02 to 0.200 g/L but becomes a depressant at higher concentrations.

Diné citizen science: Phytoremediation of uranium and arsenic in the Navajo Nation

Mid-20th century mining in Naabeehó Bináhásdzo (Navajo Nation) polluted soil and groundwater with uranium and arsenic. The Diné and other indigenous residents of this region use groundwater for drinking, livestock, and irrigation, creating a serious environmental health risk. Currently, many individuals and communities on the Navajo Nation must purchase and transport treated water from hours away. Sunflowers (Helianthus annuus) preferentially take up uranium and arsenic, potentially representing a tool to remove these contaminants through on-site, low-cost phytoremediation. This study reports the results of a collaboration among researchers, high school students, teachers, and tribal leaders to analyze water chemistry and perform a phytoremediation experiment. In 2018 and 2019, we compiled existing data from the Navajo Nation Environmental Protection Agency (NNEPA) and collected samples from surface and groundwater. We then used sunflower seedlings grown in local soil to assess whether phytoremediation could be effective at removing arsenic and uranium. For the NNEPA-sampled wells, 9.5% exceeded the maximum contaminant level for uranium (30 μg per liter) and 16% for arsenic (10 μg per liter). For the new samples, uranium was highest in surface pools, suggesting leaching from local soil. Unlike studies from humid regions, sunflowers did not decrease uranium and arsenic in soil water. Instead, there was no change in arsenic concentration and an increase in uranium concentration in both planted and control treatments, attributable to weathering of uranium-bearing minerals in the desert soil. Because much of global uranium mining occurs in arid and semiarid regions, the ineffectiveness of phytoremediation on the Navajo Nation emphasizes the importance of prevention and conventional remediation. More generally, the participatory science approach created meaningful relationships and an important collaboration between a tribal chapter and a university, providing both cultural and scientific experiential learning opportunities for Diné high school students, undergraduate researchers, and senior personnel.

Uranium biosorption by immobilized active yeast cells entrapped in calcium-alginate-PVA- GO-crosslinked gel beads

Abstract A novel biosorbent, i. e. Saccharomyces cerevisiae entrapped in graphene oxide (GO), polyvinyl alcohol (PVA) and alginate and cross-linked in CaCl2- boric acid solution, was prepared, characterized and applied for U (VI) biosorption. The performance of U sorption and cations release (Na, K, Ca and Mg ions) was investigated under different contact time, initial uranium concentration and initial pH. Uranium sorption equilibrium basically achieved after 360 min. The kinetic data of U biosorption and Ca release were best described by the pseudo first-order equation. Both Langmuir and Freundlich models could fit the U sorption isotherm data. With increase of initial uranium (3.7 ~ 472.2 μmol/L) and sodium concentration (78.8 ~ 3911.7 μmol/L), the cations release ((Na + K)/2 + (Ca + Mg)) decreased from 116.9 to 30.1 μmol/g when the corresponding U sorption increased from 0.6 to 77.3 μmol/g. Initial solution pH at 3 was favorable for U sorption when pH ranged from 3 to 7. With increase of uranium concentration, ion exchange played a less role in U removal. The maximum U sorption capacity reached 142.1 μmol/g, calculated from the Langmuir model at initial pH 5. The O-containing functional group, such as carboxyl on the gel bead played an important role in U adsorption according to FTIR and XPS analysis. XPS analysis showed the existence of U (VI) and U (IV) on the surface of gel bead. Ion exchange, complexation and uranium reduction involved in uranium adsorption by the immobilized active dry yeast gel beads.

Interpretaciones espectrométricas Gamma en diferentes tipos de condiciones de suelo – vuelo geofísico

El siguiente es un estudio comparativo en la calidad de la información geofísica radiométrica obtenida en condiciones húmedas y secas en terreno semiplano. En este trabajo se intentó resaltar la humedad del suelo como una de los factores primarios que influyen directamente las estadísticas de adquisición gamma y las condiciones de adquisición de vuelo para cada caso. También resaltamos las condiciones de altimetría topográfica que afectan directamente los resultados estadísticos de la espectrometría gamma. Fue posible verificar los niveles de elevación en el conteo de concentración de uranio que emanaba del radio, tal como lo sugieren Charbonneau y Darnley (1970) y Grasty (1997).Los valores encontrados en los ambientes húmedos muestran un incremento del 50 % en las concentraciones de uranio en la atmósfera y un incremento del 50 % en las concentraciones calculadas por la relación U/K, revelada por zonas hidrotérmicas.

Application of bimodal distribution to the detection of changes in uranium concentration in drinking water collected by random daytime sampling method from a large water supply zone

The random daytime (RDT) sampling method was used for the first time in the assessment of average weekly exposure to uranium through drinking water in a large water supply zone. Data set of uranium concentrations determined in 106 RDT samples collected in three runs from the water supply zone in Wroclaw (Poland), cannot be simply described by normal or log-normal distributions. Therefore, a numerical method designed for the detection and calculation of bimodal distribution was applied. The extracted two distributions containing data from the summer season of 2011 and the winter season of 2012 (nI=72) and from the summer season of 2013 (nII=34) allowed to estimate means of U concentrations in drinking water: 0.947μg/L and 1.23μg/L, respectively. As the removal efficiency of uranium during applied treatment process is negligible, the effect of increase in uranium concentration can be explained by higher U concentration in the surface-infiltration water used for the production of drinking water. During the summer season of 2013, heavy rains were observed in Lower Silesia region, causing floods over the territory of the entire region. Fluctuations in uranium concentrations in surface-infiltration water can be attributed to releases of uranium from specific sources – migration from phosphate fertilizers and leaching from mineral deposits. Thus, exposure to uranium through drinking water may increase during extreme rainfall events. The average chronic weekly intakes of uranium through drinking water, estimated on the basis of central values of the extracted normal distributions, accounted for 3.2% and 4.1% of tolerable weekly intake.

Yield and uranium concentration in two lettuce (Lactuca sativa L.) varieties influenced by soil and irrigation water composition, and season growth

The use of contaminated land or irrigation water for agriculture in the neighborhood of mines as well as the transfer of metals from soils to vegetables and to humans through the food chain is a worldwide concern. This study was conducted to compare the effects on lettuce (Lactuca sativa L. varieties Marady and Romana) grown in different seasons (autumn and summer) when exposed to high and low uranium concentrations, both in irrigation water and soils (total and available fraction), near a former uranium mine. Irrigation with uranium contaminated water (0.94 to 1.14mg/L) together with high concentrations of uranium in the soil available fraction (9.2 to 14.5mg/kg) had negative effects on both lettuce varieties, namely on leaf yield reduction (up to 53% and 87% in autumn and summer experiments, respectively) and increase of uranium concentration in the leaves (up to 2.13 and 5.37mg/kgdryweight) and in the roots (up to 4.64 and 28.2mg/kgdryweight) for Marady and Romana varieties, respectively. The effect on lettuce yield was mainly due to the soil salinity (electrical conductivity up to 6.31mS/cm) as a consequence of water irrigation with high electrical conductivity (1.67 to 1.82mS/cm) and low tolerance of lettuce to the soil salinity. The highest concentration of uranium in the Romana variety was also the result of its growth in a dry season when the frequency and amount of water irrigation was great. The consumption of uranium by an adult, teenager and child, from lettuce that concentrate more this element (234μgU/kg fresh weight) was, respectively, 16.7, 17.7 and 24.4% of the tolerable daily limit intake set by the World Health Organization (0.6mgU/kg body weight daily; WHO, 2005). These values suggest that lettuce had low impact on human uranium intake and should not present a concern for Cunha Baixa inhabitant's health assuming that this vegetable is the only food consumed with high uranium concentrations.

Release of uranium from candidate wasteforms

AbstractLarge volumes of depleted natural and low-enriched uranium exist in the UK waste inventory. This work reports on initial investigations of the leaching performance of candidate glass and cement encapsulation matrices containing UO3 powder as well as that of uranium oxide powders. The surface areas of UO3 powder and the monolith samples of UO3 conditioned in the glass and cement matrices were very different making leaching comparisons difficult. The results showed that for both types of monolith conditioned samples a steady increase of uranium concentration in solution with time was generally not observed. The wt.% of uranium leached from UO3 conditioned in the lead borosilicate glass wasteform was approximately five orders of magnitude less than that leached from UO3 powder. Similarly, the quantities of uranium leached from UO3 conditioned in composite cement made with ordinary Portland cement, and from magnesium phosphate cement, were approximately four and three orders of magnitude, respectively, less than that leached from UO3 powder. The performance of a mixed oxide borosilicate glass wasteform was only slightly better than that of UO3 powder. This work shows that wasteforms based on encapsulation in lead borosilicate glass and cement matrices have the greatest potential for further development.

Leaching dynamics of uranium in a contaminated soil site

Samples from a potentially contaminated industrial area were analyzed for uranium using neutron activation analysis (NAA). Uranium concentration values had a typical uncertainty of 2 % and a detection limit of 1 Bq/kg. To investigate the potential leaching dynamics into ground water two techniques were employed. The US EPA Toxicity Characterization Leaching Procedure (TCLP) and the Sequential Extraction Procedure (SEP) were used to determine the concentration of uranium in the leachates. TCLP and SEP showed that very little of the uranium leached into solution under different chemical conditions. Values of uranium leachates ranged from 0.05 to 3.5 Bq/L; a concentration much lower than the results found in the soil concentrations which ranged from 29 to 155 Bq/kg. NAA showed an 8 % uncertainty for leachates with a detection limit of 0.13 Bq/L. To mimic environmental conditions and acid rain, pH 4.3 water was used as the extraction solvent instead of the acetic acid routinely used in TCLP. Results confirmed that very low amounts of uranium leached with values ranging from 0.0002 to 0.0122 Bq/L. These values represent 0.01–1 % of the uranium in the soil samples. The distribution of uranium in soil according to particle size was also investigated to evaluate its potential movement and possible contamination of the water table. Particles below 250 μm in diameter showed a linear increase in uranium concentration whereas those with a larger diameter had constant uranium content.

Multispectral processing of high‐resolution radiometric data for soil mapping

ABSTRACTSoil radioactivity is dominantly a product of the local geology and mineralization, physical and chemical weathering processes within the profile, and soil organic content. Recent studies have shown that local soil patterns can be confidently mapped at scales of 1:10 000 using new high spatial resolution airborne and ground radiometric data acquired with standard 256‐channel gamma‐ray spectrometry systems and sodium‐iodide (NaI) crystal packs. In this study, a new multispectral processing methodology was developed to assess the 256‐channel spectral soil response. The study identified seven distinct gamma‐ray energy peaks, all products of the potassium‐40, uranium‐238 or thorium‐232 decay series, whose spatial and spectral relationships could be used to interpret soil properties and/or soil conditions.In the case studies presented, interpretation of complementary thorium and uranium channels (isolated using the new multispectral processing methodology) reveal additional spatial patterns and spectral information that were unattainable using standard processing methods. In one example, soil conditions attributed to a local, developing clay‐soil unit were resolved by the increased 228Ac (actinium) to 208Tl (thorium) channel response. In a separate example, a change in ratio between 214Bi (uranium) energies 1120 keV and 1764 keV, produced during the same decay emission, was used to differentiate soil conditions in the top 40 cm of an expansive sandy loam on clay duplex soil unit. A decrease in the 1120 keV to 1764 keV ratio was interpreted as an increase in soil density as the lower gamma‐ray energy emissions are preferentially attenuated with increased density. Relative increases in the 1120 keV and 1764 keV channel responses were interpreted as an increase in uranium concentration in the soil profile. With the clay as the dominant uranium contributor, integrated interpretation of the 214Bi channels suggested that in these areas there was an increase in clay percentage within the surface sandy loam, which may result in local increased water‐logging susceptibility.This paper also theorizes that interpretation of the spatial displacement of thorium‐232 decay series 228Ac and 208Tl peaks, produced from decay events approximately 2 years (half‐life) apart, can be used to identify decay series disequilibrium conditions resulting from local soil–water processes or other recent soil activity. This information contributes to an understanding of the soil processes active in the area. In addition, identification of disequilibrium decay conditions may highlight locations where standard radiometric (radioelement) processing, which relies on equilibrium conditions, may be erroneous.

Phosphate regulates uranium(VI) toxicity to Lemna gibba L. G3

The influence of phosphate on the toxicity of uranium to Lemna gibba G3 was tested in semicontinuous culture with synthetic mine water developed as an analogue of surface water of two abandoned uranium mining and ore processing sites in Saxony, Germany. Six concentrations of uranium were investigated under five different supply regimes of PO(4) (3-) at constant pH (7.0 +/- 0.5) and alkalinity (7.0 +/- 1.6 mg L(-1) total CO(3) (2-)). The results showed significant inhibition of specific growth rates in cultures exposed to the highest uranium concentrations (3500 and 7000 microg U L(-1)) at lowest PO(4) (3-) supply of 0.01 mg L(-1). An increase of phosphate concentration from 0.01 to 8.0 mg L(-1) resulted in an increase of EC(50) from 0.9 +/- 0.2 to 7.4 +/- 1.9 mg L(-1) (significant with Student's t test, P > 0.05). The accumulation of uranium in L. gibba increased exponentially with the increase in uranium concentration in cultures with 0.01 and 0.14 mg PO(4) (3-) L(-1). Accumulation also increased significantly when PO(4) (3-) supply was increased from 0.14 to 1.36 mg PO(4) (3-) L(-1) for all uranium concentrations. However, as the supply of PO(4) (3-) gradually increased from 1.36 to 8.0 mg PO(4) (3-) L(-1), uranium bioaccumulation increased slightly but insignificantly before leveling off. Uranium speciation modeling with PhreeqC geochemical code predicted increases in the proportions of uranyl phosphate species when PO(4) (3-) concentrations increase in the media. Most of these uranyl phosphate species have a high probability of precipitation [saturation indices (SI) > 0.93]. Therefore, the alleviation of uranium toxicity to L. gibba with phosphates is due to interactions among components of the media, mainly uranyl and phosphate which results in precipitation. Consequently, bioavailable fractions of uranium to L. gibba are reduced. This might explain lack of consistent EC(50) values for uranium to most aquatic organisms.

Biological and chemical interactions with U(VI) during anaerobic enrichment in the presence of iron oxide coated quartz

Microcosm experiments were performed to understand chemical and biological interactions with hexavalent uranium (U(VI)) in the presence of iron oxide bearing minerals and trichloroethylene (TCE) as a co-contaminant. Interactions of U(VI) and hydrous iron oxide moieties on the mineral oxide surfaces were studied during enrichments for dissimilatory iron reducing (DIRB) and sulfate reducing bacteria (SRB). Microbes enriched from groundwater taken from the Test Area North (TAN) site at the Idaho National Laboratory (INL) were able to reduce the U(VI) in the adsorption medium as well as the iron on quartz surfaces. Early in the experiment disappearance of U(VI) from solution was a function of chemical interactions since no microbial activity was evident. Abiotic removal of U(VI) was enhanced in the presence of carbonate. As the experiment proceeded, further removal of U(VI) from solution was associated with the fermentation of lactate to propionate and acetate. During later phases of the experiment when lactate was depleted from the growth medium in the microcosm containing the DIRB enrichments, U(VI) concentrations in the solution phase increased until additional lactate was added. When additional lactate was added and fermentation proceeded, U(VI) concentrations in the liquid phase again returned to near zero. Similar results were shown for the SRB enrichment but lower uranium concentrations were seen in the liquid phase, while in the enrichment with carbonate a similar increase in uranium concentration was not seen. Chemical and biological interactions appear to be important on the mobilization/immobilization of U(VI) in an iron oxide system when TCE is present as a co-contaminant. Interestingly, TCE present in the microcosm experiments was not dechlorinated which was probably an effect of redox conditions that were unsuitable for reductive dechlorination by the microbial culture tested.

Anomalias aerogamaespectrométricas (K, eU e eTh) da quadrícula de Araras (SP) e suas relações com processos pedogenéticos e fertilizantes fosfatos

Radionuclídeos estão presentes nas matérias-primas utilizadas para a fabricação de alguns fertilizantes e, conseqüentemente, a redistribuição de urânio, tório e potássio pela atividade agrícola pode levar a contaminação dos solos intensivamente fertilizados. Dados aerogamaespectrométricos de K, eU e eTh mostraram, na Quadrícula de Araras (SP), a coincidência entre valores radiométricos anômalos e sills de diabásio. Esta área está localizada no estado de São Paulo, situada entre as coordenadas 22º 00'/22º 30' de latitude sul e 47º 00' e 47º 30' de longitude oeste. O processamento e a integração dos dados de geologia, de solos, de formações superficiais, de drenagem, topográficos, de sensoriamento remoto e geofísicos, em ambiente de Sistemas de Informações Geográficas (SIG), através das geotecnologias, revelaram que: (i) as anomalias de eU e eTh ocorrem sobre Latossolos Roxos massivamente fertilizados com fosfatos, nas porções planas e elevadas do terreno; (ii) os níveis elevados de urânio estão associados a granulometria argilosa em regolitos sobre soleiras diabásicas; (iii) anomalias de potássio e tório acompanham os aluviões do Quaternário que percorrem as margens do Rio Moji, atribuindo-se às ocorrências de K anômalo a presença de feldspatos nos solos inseridos, e a ocorrência de eTh associada à fertilização local e ao transporte por argilas e óxidos de ferro provenientes das porções mais elevadas do terreno, onde situam-se os Latossolos Roxos sob a cultura de cana-de-açúcar. Das análises radioquímicas de U e Th em fertilizantes, observou-se a influência da origem geológica da fonte da matéria-prima utilizada para o seu fabrico, bem como o aumento nas concentrações de urânio com o teor de P2O5 do fertilizante. Medidas gamaespectrométricas terrestres mostraram boas relações com as detecções aerogamaespectrométricas. As informações gamaespectrométricas aliadas a detecções de susceptibilidade magnética (k) mostraram que o urânio e o tório tendem a se concentrar em solos oriundos do intemperismo de rochas básicas, mais desenvolvidos, argilosos e com k elevados. A retenção de urânio e tório em solos argilosos, aliada a informações de topografia, denotaram contagens gamaespectrométricas destes radionuclídeos maiores do que as encontradas na rocha fonte (sills de diabásio).

Stimulating the In Situ Activity of Geobacter Species To Remove Uranium from the Groundwater of a Uranium-Contaminated Aquifer

The potential for removing uranium from contaminated groundwater by stimulating the in situ activity of dissimilatory metal-reducing microorganisms was evaluated in a uranium-contaminated aquifer located in Rifle, Colo. Acetate (1 to 3 mM) was injected into the subsurface over a 3-month period via an injection gallery composed of 20 injection wells, which was installed upgradient from a series of 15 monitoring wells. U(VI) concentrations decreased in as little as 9 days after acetate injection was initiated, and within 50 days uranium had declined below the prescribed treatment level of 0.18 micro M in some of the monitoring wells. Analysis of 16S ribosomal DNA (rDNA) sequences and phospholipid fatty acid profiles demonstrated that the initial loss of uranium from the groundwater was associated with an enrichment of Geobacter species in the treatment zone. Fe(II) in the groundwater also increased during this period, suggesting that U(VI) reduction was coincident with Fe(III) reduction. As the acetate injection continued over 50 days there was a loss of sulfate from the groundwater and an accumulation of sulfide and the composition of the microbial community changed. Organisms with 16S rDNA sequences most closely related to those of sulfate reducers became predominant, and Geobacter species became a minor component of the community. This apparent switch from Fe(III) reduction to sulfate reduction as the terminal electron accepting process for the oxidation of the injected acetate was associated with an increase in uranium concentration in the groundwater. These results demonstrate that in situ bioremediation of uranium-contaminated groundwater is feasible but suggest that the strategy should be optimized to better maintain long-term activity of Geobacter species.

Disequilibrium of uranium isotopes in some Syrian groundwater

Uranium concentration in groundwater samples from three areas of Syria was determined using α-spectrometry and INAA. It was in the range of 0–6.13 μg/l in the phosphate areas, and lower than 1 ppb in the volcanic areas. The activity ratio of 234U/ 238U was investigated, and disequilibrium of uranium isotopes was found to occur ( 234U/ 238U=[0.52−2.02]. The excess of 234U was calculated. This excess can be interpreted by higher mobility of 234U, which more readily forms the soluble (UO 2) 2+ ion in comparison with 238U, most of which remains in the insoluble 4 + state. This excess increases with increase in uranium concentration. Thorium concentration was measured using INAA, it was found to be in the rang 0–1.15 μg/l.

U 3+ new activator ion in various fluoride matrices for infrared laser: synthesis and optical properties

Recently, there has been considerable interest in infrared diode pumped solid state lasers. U 3+ in fluoride matrices is a good candidate, however the trivalent state is not the most stable valence state. We show that γ-irradiation can reduce 100% of U 4+ in matrices such as LiYF 4, LiYbF 4 and β-YF 3, but it fails mostly in the fluoride glasses where tetravalent sites are present. A dose rate of about 500 kGy is necessary for reducing 1000 ppm of uranium in the single crystals. Then the conversion rate decreases with the increase of uranium concentration. The large crystal-field in LiYF 4 and β-YF 3 induces a large ground multiplet splitting, allowing a four level scheme laser at room temperature for the 4 I 11 2 → 4 I 9 2 transitions terminating at the highest energy found multiplet. These transitions occur at 2.5, 2.83 and 2.97 μm for LiYF 4 and 2.29, 2.5 and 2.66 μm in β-YF 3. Moreover, the emission cross sections for these transitions are at least 10 times larger than the ones for 4f—4f transitions such as the emission at 2.7 μm of Er 3+ for example in LiF 4.

Structural Changes Around Uranium at the Surface of Waste Glasses Caused by Leaching

AbstractStructural changes occurring at the surface of leached borosilicate glasses containing 3%wt of uranium have been investigated using fluores- cence EXAFS spectroscopy at grazing incidence. X-ray penetration depths between 30 Å and above 1000 Å are possible by varying the incidence angle of the X-ray monochromatic radiation in the vicinity of the critical angle for total external reflection, øc. This enables the surface and bulk structure to be compared. It is found that a local increase in uranium concentration takes place near the surface of the glass during the first 30 minutes of leaching time. Uranium is coordinated with oxygen and other uranium atoms. The local distribution can be modelled by a square planar island structure comprised of uranyl-type units. This structure changes with corrosion time. After 30 minutes of leaching there is evidence of partial removal of the surface layer.

Postdepositional enrichment of uranium in sediment from the Bering Sea

Deep-sea sediments were collected from the northern North Pacific and the Bering Sea and separated into HCl soluble and HCl insoluble fractions. These sediments were analyzed for their uranium and thorium concentrations and 234 U 238 U activity ratios by alphaspectrometry. The mean concentration of total uranium in the oxic northern North Pacific sediment was relatively low (1.25 ppm) and showed little variation with sediment depth. The concentration of total uranium in the surface sediment of the Bering Sea Basin was also low (0.85 ppm), similar to those of the northern North Pacific sediment, but it was higher (2.36 ppm) in the strongly reducing sediment below 40 cm. This increase in uranium concentration with depth is ascribed to higher quantities of HCl soluble uranium. It is suggested that uranium diffuses through interstitial water and is postdepositionally precipitated during early diagenesis in the subsurface reducing environment. A postdepositional precipitation rate of uranium in the Bering Sea Basin sediment was estimated to be 28 μg cm −2 10 3 yr −1. In the reducing Bering Sea Basin sediment, the concentration of total uranium in the layers enriched in manganese carbonate was 1.68 ppm, which was slightly less than that of the other normal deep layer; this depletion of uranium may be due to dilution with volcanic material of coarser texture.

Increase in uranium concentration in streams draining Rocky Flats, Colorado

Abstract Uranium has been determined by fission track measurements of two suites of water samples taken at different times from a variety of surface waters in an area in central Colorado. Although this area surrounds the Rocky Flats nuclear plant at which parts for weapons are fabricated, the variations in U are inferred to be influenced most strongly by the distance of water flow across soil derived from the U-rich Laramie Formation. This flow produces progressive increases in U concentrations as flow proceeds eastward and hence is much more closely related to the local geochemistry and geology than to the presence of the Rocky Flats plant.