Preliminary evidence of a link between surface soil properties and the arsenic content of shallow groundwater in Bangladesh

The extremely heterogeneous distribution of As in Bangladesh groundwater has hampered efforts to identify with certainty the mechanisms that lead to extensive mobilization of this metalloid in reducing aquifers. We show here on the basis of a high-resolution transect of soil and aquifer properties collected in Araihazar, Bangladesh, that revealing tractable associations between As concentrations in shallow (< 20 m) groundwater with other geological, hydrological, and geochemical features requires a lateral sampling resolution of 10–100 m. Variations in the electromagnetic conductivity of surface soils (5–40 mS/m) within a 500 m × 200 m area are documented with 560 EM31 measurements. The results are compared with a detailed section of groundwater As concentrations (5–150 μg/L) and other aquifer properties obtained with a simple sampling device, “the needle-sampler”, that builds on the local drilling technology. By invoking complementary observations obtained in the same area and in other regions of Bangladesh, we postulate that local groundwater recharge throughout permeable sandy soils plays a major role in regulating the As content of shallow aquifers by diluting the flux of As released from reducing sediments.     

Eruption processes and deposit characteristics at the monogenetic Mt. Gambier Volcanic Complex, SE Australia: implications for alternating magmatic and phreatomagmatic activity

The ～5 ka Mt. Gambier Volcanic Complex in the Newer Volcanics Province, Australia is an extremely complex monogenetic, volcanic system that preserves at least 14 eruption points aligned along a fissure system. The complex stratigraphy can be subdivided into six main facies that record alternations between magmatic and phreatomagmatic eruption styles in a random manner. The facies are (1) coherent to vesicular fragmental alkali basalt (effusive/Hawaiian spatter and lava flows); (2) massive scoriaceous fine lapilli with coarse ash (Strombolian fallout); (3) bedded scoriaceous fine lapilli tuff (violent Strombolian fallout); (4) thin–medium bedded, undulating very fine lapilli in coarse ash (dry phreatomagmatic surge-modified fallout); (5) palagonite-altered, cross-bedded, medium lapilli to fine ash (wet phreatomagmatic base surges); and (6) massive, palagonite-altered, very poorly sorted tuff breccia and lapilli tuff (phreato-Vulcanian pyroclastic flows). Since most deposits are lithified, to quantify the grain size distributions (GSDs), image analysis was performed. The facies are distinct based on their GSDs and the fine ash to coarse+fine ash ratios. These provide insights into the fragmentation intensities and water–magma interaction efficiencies for each facies. The eruption chronology indicates a random spatial and temporal sequence of occurrence of eruption styles, except for a “magmatic horizon” of effusive activity occurring at both ends of the volcanic complex simultaneously. The eruption foci are located along NW–SE trending lineaments, indicating that the complex was fed by multiple dykes following the subsurface structures related to the Tartwaup Fault System. Possible factors causing vent migration along these dykes and changes in eruption styles include differences in magma ascent rates, viscosity, crystallinity, degassing and magma discharge rate, as well as hydrological parameters.     

The evolution of continental roots in numerical thermo-chemical mantle convection models including differentiation by partial melting

Incorporating upper mantle differentiation through decompression melting in a numerical mantle convection model, we demonstrate that a compositionally distinct root consisting of depleted peridotite can grow and remain stable during a long period of secular cooling. Our modeling results show that in a hot convecting mantle partial melting will produce a compositional layering in a relatively short time of about 50 Ma. Due to secular cooling mantle differentiation finally stops before 1 Ga. The resulting continental root remains stable on a billion year time scale due to the combined effects of its intrinsically lower density and temperature-dependent rheology. Two different parameterizations of the melting phase-diagram are used in the models. The results indicate that during the Archaean melting occurred on a significant scale in the deep regions of the upper mantle, at pressures in excess of 15 GPa. The compositional depths of continental roots extend to 400 km depending on the potential temperature and the type of phase-diagram parameterization used in the model. The results reveal a strong correlation between lateral variations of temperature and the thickness of the continental root. This shows that cold regions in cratons are stabilized by a thick depleted root.     

Influence of Drying on the Stiffness and Strength of Cement-Stabilized Soils

Nowadays, improving the strength and deformation properties of soft soils by deep soil mixing is a commonly used technique. There is also an increasing interest in the use of this technique for foundation/structural elements and excavation retaining walls applications. The compressive strength and elastic modulus of the soil mix material are key parameters in the design of these structures. However, there is very limited information available on the impact of exposure to air drying (in the case of retaining wall) on the strength and stiffness of cement stabilized soils. The aim of this study is to investigate the effects of different curing conditions (immersion in water, cycles of wetting and drying, continuous air drying) on the mechanical properties of soils treated with cement in the laboratory. Free–free resonance tests and unconfined compression tests were performed on specimens of silt and sand treated with blastfurnace slag cement. Strength increases more rapidly than stiffness between 7 and 30 days. The strength of stabilized soils submitted to cyclic wetting and drying before the cement hydration process is complete continues to increase. As long as the periods of drying do not induce microcracks, the stiffness of the treated soil specimens also increases with time. However, the stiffness is lower than for the specimens cured in water indicating a disruptive effect of the imposed wetting–drying cycles on stiffness. Continuous exposure to air drying inhibits strength development due to insufficient water for hydration. Significant stiffness decreases were observed on specimens of stabilized silt and are attributed to microcracking.     

Present and future Laurentian Great Lakes hydroclimatic conditions as simulated by regional climate models with an emphasis on Lake Michigan-Huron

Regional climate modelling represents an appealing approach to projecting Great Lakes water supplies under a changing climate. In this study, we investigate the response of the Great Lakes Basin to increasing greenhouse gas and aerosols emissions using an ensemble of sixteen climate change simulations generated by three different Regional Climate Models (RCMs): CRCM4, HadRM3 and WRFG. Annual and monthly means of simulated hydro-meteorological variables that affect Great Lakes levels are first compared to observation-based estimates. The climate change signal is then assessed by computing differences between simulated future (2041–2070) and present (1971–1999) climates. Finally, an analysis of the annual minima and maxima of the Net Basin Supply (NBS), derived from the simulated NBS components, is conducted using Generalized Extreme Value distribution. Results reveal notable model differences in simulated water budget components throughout the year, especially for the lake evaporation component. These differences are reflected in the resulting NBS. Although uncertainties in observation-based estimates are quite large, our analysis indicates that all three RCMs tend to underestimate NBS in late summer and fall, which is related to biases in simulated runoff, lake evaporation, and over-lake precipitation. The climate change signal derived from the total ensemble mean indicates no change in future mean annual NBS. However, our analysis suggests an amplification of the NBS annual cycle and an intensification of the annual NBS minima in future climate. This emphasizes the need for an adaptive management of water to minimize potential negative implications associated with more severe and frequent NBS minima.     

基于ENVISAT/ASAR资料的土壤湿度反演方法

提出了利用ENVISAT/ASAR/AP模式VV极化的低入射角数据和黄土高原陆面过程试验资料对黄土高原平凉试验区表层土壤湿度进行反演的方法。结果表明:在相对平坦的混合地表类型区域,反演的土壤湿度与地面实测值平均误差<0.02 cm3.cm-3,绝对误差均在0.04 cm3.cm-3范围内,该结果能较好地对黄土高原塬区土壤湿度进行干旱监测;但在陡峭山坡及塬边等地形起伏较大的区域,结果较差。     

Capillary hysteresis and gravity segregation in two phase flow through porous media

We study the gravity driven flow of two fluid phases in a one dimensional homogeneous porous column when history dependence of the pressure difference between the phases (capillary pressure) is taken into account. In the hyperbolic limit, solutions of such systems satisfy the Buckley-Leverett equation with a non-monotone flux function. However, solutions for the hysteretic case do not converge to the classical solutions in the hyperbolic limit in a wide range of situations. In particular, with Riemann data as initial condition, stationary shocks become possible in addition to classical components such as shocks, rarefaction waves and constant states. We derive an admissibility criterion for the stationary shocks and outline all admissible shocks. Depending on the capillary pressure functions, flux function and the Riemann data, two cases are identified a priori for which the solution consists of a stationary shock. In the first case, the shock remains at the point where the initial condition is discontinuous. In the second case, the solution is frozen in time in at least one semi-infinite half. The predictions are verified using numerical results.