Landscape heterogeneity drives contrasting concentration–discharge relationships in shale headwater catchments

Solute concentrations in stream water vary with discharge in patterns that record complex feedbacks between hydrologic and biogeochemical processes. In a comparison of three shale-underlain headwater catchments located in Pennsylvania, USA (the forested Shale Hills Critical Zone Observatory), and Wales, UK (the peatland-dominated Upper Hafren and forest-dominated Upper Hore catchments in the Plynlimon forest), dissimilar concentration–discharge (C–Q) behaviors are best explained by contrasting landscape distributions of soil solution chemistry – especially dissolved organic carbon (DOC) – that have been established by patterns of vegetation and soil organic matter (SOM). Specifically, elements that are concentrated in organic-rich soils due to biotic cycling (Mn, Ca, K) or that form strong complexes with DOC (Fe, Al) are spatially heterogeneous in pore waters because organic matter is heterogeneously distributed across the catchments. These solutes exhibit non-chemostatic behavior in the streams, and solute concentrations either decrease (Shale Hills) or increase (Plynlimon) with increasing discharge. In contrast, solutes that are concentrated in soil minerals and form only weak complexes with DOC (Na, Mg, Si) are spatially homogeneous in pore waters across each catchment. These solutes are chemostatic in that their stream concentrations vary little with stream discharge, likely because these solutes are released quickly from exchange sites in the soils during rainfall events. Furthermore, concentration–discharge relationships of non-chemostatic solutes changed following tree harvest in the Upper Hore catchment in Plynlimon, while no changes were observed for chemostatic solutes, underscoring the role of vegetation in regulating the concentrations of certain elements in the stream. These results indicate that differences in the hydrologic connectivity of organic-rich soils to the stream drive differences in concentration behavior between catchments. As such, in catchments where SOM is dominantly in lowlands (e.g., Shale Hills), we infer that non-chemostatic elements associated with organic matter are released to the stream early during rainfall events, whereas in catchments where SOM is dominantly in uplands (e.g., Plynlimon), these non-chemostatic elements are released later during rainfall events. The distribution of SOM across the landscape is thus a key component for predictive models of solute transport in headwater catchments

Time evolution of the mineralogical composition of Mississippi Valley loess over the last 10 kyr: Climate and geochemical modeling

International audienceAnthropogenic and natural climate change affect processes in the atmosphere, biosphere, hydrosphere, and pedosphere. The impact of climate on soil evolution has not been well-explored, largely due to slow rates and the complexity of coupled processes that must be observed and simulated. The rates of mineral weathering in loess deposited 23 kyr ago and experienc- ing soil formation for 13 kyr are explored here using the WITCH model for weathering and the GENESIS model for climate simulation. The WITCH model, which uses rigorous kinetic parameters and laws with provision for the effect on rates of devi- ation from equilibrium, can successfully simulate the depletion profiles in the soil for dolomite and albite if soil CO2 is assumed to rise over the last 10 kyr up to about 30-40ﰎ the present atmospheric pressure, and if the solubility product of the Ca-smectite is assumed equal to that of an Fe(III)-rich Ca-montmorillonite. Such simulations document that dissolution behavior for silicates and carbonates are strongly coupled through pH, and for Ca-smectite and feldspars through dissolved silica. Such coupling is not incorporated in simple geometric and analytical models describing mineral dissolution, and there- fore probably contributes to the long-standing observation rates

Evolution of porosity and diffusivity associated with chemical weathering of a basalt clast

Weathering of rocks as a result of exposure to water and the atmosphere can cause significant changes in their chemistry and porosity. In low-porosity rocks, such as basalts, changes in porosity, resulting from chemical weathering, are likely to modify the rock's effective diffusivity and permeability, affecting the rate of solute transport and thus potentially the rate of overall weathering to the extent that transport is the rate limiting step. Changes in total porosity as a result of mineral dissolution and precipitation have typically been used to calculate effective diffusion coefficients through Archie's law for reactive transport simulations of chemical weathering, but this approach fails to account for unconnected porosity that does not contribute to transport. In this study, we combine synchrotron X-ray microcomputed tomography ({mu}CT) and laboratory and numerical diffusion experiments to examine changes in both total and effective porosity and effective diffusion coefficients across a weathering interface in a weathered basalt clast from Costa Rica. The {mu}CT data indicate that below a critical value of {approx}9%, the porosity is largely unconnected in the basalt clast. The {mu}CT data were further used to construct a numerical pore network model to determine upscaled, effective diffusivities as a function of total porosity (ranging from 3 to 30%) for comparison with diffusivities determined in laboratory tracer experiments. By using effective porosity as the scaling parameter and accounting for critical porosity, a model is developed that accurately predicts continuum-scale effective diffusivities across the weathering interface of the basalt clast

Impact energy absorption of three mouthguard materials in an aqueous environment

PubMedID: 19208026High impact energy absorption is an essential property for mouthguard materials. The impact test performance of three popular mouthguard materials was evaluated, using the procedure in American Society for Testing and Materials (ASTM) Standard D3763. Conventional ethylene vinyl acetate (EVA; T&S Dental and Plastics, Myerstown, PA, USA) served as the control. Pro-form™ (Dental Resources Inc., Delano, MN, USA), another EVA material, and PolyShok™ (Sportsguard Laboratories, Kent, OH, USA), an EVA product containing polyurethane were also evaluated. Specimens having dimensions of 3 inch × 3 inch × 4 mm were prepared from each material. After processing that followed manufacturer recommendations, specimens were conditioned for 1 h in 37°C deionized water and loaded at 20 mph by a 0.5 inch diameter indenter containing a force transducer (Dynatup Model 9250 HV; Instron Corp., Canton, MA, USA). Both large-diameter (3 inches) and small-diameter (1.5 inch) support rings were used. For comparison, two specimens of each material were tested in the dry condition. Energy absorption was determined from the area under the force-time curve at 30 ms, and results for the water-conditioned specimens were compared using anova and the Kruskal-Wallis test. For the large-diameter support ring, energy absorption (mean ± SD in ft·lbf inch-1), normalized to specimen thickness, was: EVA (n = 5), 110.2 ± 48.4; Pro-form™ (n = 4), 110.0 ± 11.3; PolyShok™ (n = 5), 105.7 ± 16.5. For the small-diameter support ring, energy absorption was: EVA (n = 6), 140.5 ± 13.9; Pro-formTM (n = 5), 109.0 ± 26.0; PolyShok™ (n = 6), 124.4 ± 28.4 (1 ft·lbf inch -1 = 0.534 J cm-1). Because of substantial variation within some specimen groups, there was no significant difference in energy absorption for the three water-conditioned mouthguard materials and the two support ring sizes. The energy absorption for each material was much greater for other specimens tested in the dry condition. © 2009 John Wiley & Sons A/S

Uranium–thorium chronometry of weathering rinds: Rock alteration rate and paleo-isotopic record of weathering fluids.

International audienc

XAFS Determination of As(V) Associated with Fe(III) Oxyhydroxides in Weathered Mine Tailings and Contaminated Soil from California, U.S.A.

The speciation of Arsenic (As) in soils and natural waters is critical in determining its environmental fate. As(III) is the most mobile and toxic of the inorganic arsenic species, but is readily oxidized to As(V), which although still quite toxic is far less mobile. Adsorption to sediment particles may remove As(V) from contaminated water, or the precipitation of arsenic minerals such as scorodite (FeAsO4·2H2O) may control the equilibrium aqueous concentration. We have employed XAFS spectroscopy to examine the solid-phase speciation of arsenic in mine tailings samples and a contaminated soil from California. Quantitative speciation of As was determined using XANES fitting methods and EXAFS analysis ; in all samples, As(V) predominates. In an oxidized tailings (no residual sulfides), we find As(V) adsorbed/coprecipitated on Fe(III) oxyhydroxides. In less oxidized tailings containing residual sulfides, there is evidence for scorodite a a similar Fe(III) arsenate and a reduced arsenic phase (arsenopyrite or arseniferous pyrite). In a soil contaminated by smelter waste, we detect Mg3(AsO4)2·8 H2O (hoernesite), and suggest the presence of an additional weakly bound As(V) species. XAFS spectroscopy is one of the few techniques for direct determination of arsenic speciation in complex matrices such as soils and mine tailings

Measurement of specific fracture energy and surface tension of brittle materials in powder form

V článku uvedeno Libor M. Hlaváč.This article presents a method for the experimental measurement of specific fracture energy and surface tension of a brittle materials in a powder form. This work is focused on testing a method on the mineral, almandine. A hydraulic press was used in the experiment to crush powder particles, and statistical evaluation was used to analyze the change in the powder surface. The powder was subject to various conditions during crushing. The crushing was performed both in air and in water. It was done at three different compression speeds, namely 15.8 MPa/s, 3.95 MPa/s and 2.25 MPa/s. The experimental results showed measurable differences in the specific fracture energy values in the presented regimes