Resupply mechanism to a contaminated aquifer: A laboratory study of U(VI) desorption from capillary fringe sediments

Contaminated capillary fringe sediments are believed to function as long-term source of U(VI) to Hanford’s 300 Area groundwater uranium plume that discharges to the Columbia River. The deep vadose zone at this site experiences seasonal water table elevation and water compositional changes in response to Columbia River stage. Batch and column desorption experiments of U(VI) were performed on two mildly contaminated sediments from this system that vary in hydrologic position to ascertain their U(VI) release behavior and factors controlling it. Solid phase characterization of the sediments was performed to identify mineralogic and chemical factors controlling U(VI) desorption. Low adsorbed U(VI) concentrations prevented spectroscopic analysis. The desorption behavior of U(VI) was different for the two sediments in spite of similar chemical and textural characteristics, and non-carbonate mineralogy. Adsorption strength and sorbed U(VI) lability was higher in the near-river sediment. The inland sediment displayed low sorbed U(VI) lability (∼10%) and measurable solid-phase carbonate content. Kinetic desorption was observed that was attributed to regeneration of labile U(VI) in the near river sediment, and carbonate mineral dissolution in the inland sediment. The desorption reaction was best described as an equilibrium surface complexation reaction. The noted differences in desorption behavior appear to result from U(VI) contamination and hydrologic history, as well as sediment carbonate content. Insights are provided on the dynamic adsorption/desorption behavior of contaminants in linked groundwater-river systems.     

Influence of basin characteristics on baseflow and stormflow chemistry in the Great Smoky Mountains National Park,USA

Relationships between stream chemistry and elevation, area, Anakeesta geology, soil properties, and dominant vegetation were evaluated to identify the influence of basin characteristics on baseflow and stormflow chemistry in eight streams of the Great Smoky Mountains National Park. Statistical analyses were employed to determine differences between baseflow and stormflow chemistry, and relate basin‐scale factors governing local chemical processes to stream chemistry. Following precipitation events, stream pH was reduced and aluminium concentrations increased, while the response of acid neutralizing capacity (ANC), nitrate, sulfate, and base cations varied. Several basin characteristics were highly correlated with each other, demonstrating the interrelatedness of topographical, geological, soil, and vegetative parameters. These interrelated basin factors uniquely influenced acidification response in these streams. Streams in higher‐elevation basins (>975 m) had significantly lower pH, ANC, sodium, and silicon and higher nitrate concentrations (p < 0.05). Streams in smaller basins (<10 km²) had significantly lower nitrate, sodium, magnesium, silicon, and base cation concentrations. In stormflow, streams in basins with Anakeesta geology (>10%) had significantly lower pH and sodium concentrations, and higher aluminium concentrations. Chemical and physical soil characteristics and dominant overstory vegetation in basins were more strongly correlated with baseflow and stormflow chemical constituents than topographical and geological basin factors. Saturated hydraulic conductivity, of all the soil parameters, was most related to concentrations of stormflow constituents. Basins with higher average hydraulic conductivities were associated with lower stream pH, ANC, and base cation concentrations, and higher nitrate and sulfate concentrations. These results emphasize the importance of soil and geological properties influencing stream chemistry and promote the prioritization of management strategies for aquatic resources. Copyright © 2012 John Wiley & Sons, Ltd.     

Numerical modeling of ionian volcanic plumes with entrained particulates

Volcanic plumes on Jupiter's moon Io are modeled using the direct simulation Monte Carlo (DSMC) method. The modeled volcanic vent is interpreted as a “virtual” vent. A parametric study of the “virtual” vent gas temperature and velocity is performed to constrain the gas properties at the vent by observables, particularly the plume height and the surrounding condensate deposition ring radius. Also, the flow of refractory nano-size particulates entrained in the gas is modeled with “overlay” techniques which assume that the background gas flow is not altered by the particulates. The column density along the tangential line-of-sight and the shadow cast by the plume are calculated and compared with Voyager and Galileo images. The parametric study indicates that it is possible to obtain a unique solution for the vent temperature and velocity for a large plume like Pele. However, for a small Prometheus-type plume, several different possible combinations of vent temperature and velocity result in both the same shock height and peak deposition ring radius. Pele and Prometheus plume particulates are examined in detail. Encouraging matches with observations are obtained for each plume by varying both the gas and particle parameters. The calculated tangential gas column density of Pele agrees with that obtained from HST observations. An upper limit on the size of particles that track the gas flow well is found to be ∼10 nm, consistent with Voyager observations of Loki. While it is certainly possible for the plumes to contain refractory dust or pyroclastic particles, especially in the vent vicinity, we find that the conditions are favorable for SO₂ condensation into particles away from the vent vicinity for Prometheus. The shadow cast by Prometheus as seen in Galileo images is also reproduced by our simulation. A time averaged frost deposition profile is calculated for Prometheus in an effort to explain the multiple ring structure observed around the source region. However, this multiple ring structure may be better explained by the calculated deposition of entrained particles. The possibility of forming a dust cloud on Io is examined and, based on a lack of any such observed clouds, a subsolar frost temperature of less than 118 K is suggested.     

Numerical modeling of the development of small-scale magmatic emulsions by Korteweg stress driven flow

The occurrence of micron to millimeter size globular heterogeneities in igneous rocks is frequently explained by processes of liquid immiscibility. However, such textures have also been documented in miscible magmatic pairs. In this study, the ability of miscible magmas to develop transient surface tensions and mimic the behavior of immiscible liquids is tested for the whole spectrum of magmatic compositions. We implemented a numerical model that includes the effect of gradient stresses (namely Korteweg stress) in order to investigate the role of such stresses in the evolution of diffusive interfaces. The results show that an initially elongated heterogeneity surrounded by a miscible and compositionally diverse magma will tend to minimize its contact surface by relaxing to a spherical shape, advected by a Korteweg stress driven flow. If the initial aspect ratio of the heterogeneity exceeds a critical value, surface minimization may be achieved by drop breakup. In addition, it is shown that two neighboring heterogeneities may coalesce to a single spherical drop. These results imply that even for fully miscible magmas, rheological barriers may prevent efficient mechanical intermingling and induce the formation of small-scale globular textures, analogous to those commonly observed in immiscible liquids. A better understanding of the role of Korteweg stress may be of the utmost importance for deciphering the textures generated by the interaction of compositionally diverse magmas.     

The nature of coarse-grained crystalline hematite and its implications for the early environment of Mars

The Thermal Emission Spectrometer (TES) on the Mars Global Surveyor spacecraft has detected deposits of coarse-grained, gray crystalline hematite in Sinus Meridiani, Aram Chaos, and Vallis Marineris. We argue that the key to the origin of gray hematite is that it requires crystallization at temperatures in excess of about 100 °C. We discuss thermal crystallization (1) as diagenesis at a depth of a few kilometers of sediments originally formed in low-temperature waters, or (2) as precipitation from hydrothermal solution. In Aram Chaos, a combination of TES data, Mars Orbiter Camera images, and Mars Orbiter Laser Altimeter (MOLA) topography suggests that high concentrations of hematite were formed in planar strata and have since been exposed by erosion of an overlying light-toned, caprock. Lesser concentrations of hematite are found adjacent to these strata at lower elevations, which we interpret as perhaps due to accumulation from physical weathering. The topography and the collapsed nature of the chaotic terrain favor a hydrothermally charged aquifer as the original setting where the hematite formed. Concentration of iron into such an ore-like body would be chemically favored by saline, Cl-rich hydrothermal fluids. An alternative sedimentary origin requires post-depositional burial to a depth of ∼3-5 km to induce thermally driven recrystallization of fine-grained iron oxides to coarse-grained hematite. This depth of burial and re-exposure is difficult to reconcile with commonly inferred martian geological processes. However, shallow burial accompanied by post-burial hydrothermal activity remains plausible. When the hematite regions originally formed, redox balance requires that much hydrogen must have been evolved to complement the extensive oxidation. Finally, we suggest that the coexistence of several factors required to form the gray hematite deposits would have produced a favorable environment for primitive life on early Mars, if it ever existed. These factors include liquid water, abundant electron donors in the form of H₂, and abundant electron acceptors in the form of Fe³⁺.