Surface evolution of dissolving minerals investigated with a kinetic Ising model

In natural weathering systems, both the chemistry and the topography of mineral surfaces change as rocks and minerals equilibrate to surface conditions. Most geochemical research has focused on changes in solution chemistry over time; however, temporal changes in surface topography may also yield information about rates and mechanisms of dissolution. We use stochastic dissolution simulations of a regular 2-D lattice with reaction mechanisms defined in terms of nearest neighbor interactions to elucidate how the surface area and reactivity of a crystal evolve during dissolution. Despite the simplicity of the model, it reproduces key features observed or inferred for mineral dissolution. Our model results indicate that: (i) dissolving surfaces reach a steady-state conformation after sufficient dissolution time, (ii) linear defects cause surface area and dissolution rate to vary in concert with one another, (iii) sigmoidal and non-sigmoidal rate vs. free-energy of reaction (ΔG_rxn) behavior can be rationalized in terms of the multiple steps occurring during dissolution, and (iv) surface roughness as a function of ΔG_rxn is highly sensitive to the reaction mechanism. When simulated times to reach steady-state are compared to published time series rate data using suitable scaling, good agreement is found for silicate minerals while the model significantly over-predicts the duration of the transient for Fe and Al oxides. The implication of our simple model is that many aspects of mineral dissolution behavior, including approach to steady-state, sigmoidal vs. non-sigmoidal rate vs. ΔG_rxn behavior, and development of rougher surfaces in conditions further from equilibrium can be explained by nearest neighbor interactions and simple Kossel-type models where reactivity of a surface is defined in terms of perfect surface, step, and kink sites.     

Comparison of arsenic concentrations in simultaneously-collected groundwater and aquifer particles from Bangladesh,India, Vietnam,and Nepal

One of the reasons the processes resulting in As release to groundwater in southern Asia remain poorly understood is the high degree of spatial variability of physical and chemical properties in shallow aquifers. In an attempt to overcome this difficulty, a simple device that collects groundwater and sediment as a slurry from precisely the same interval was developed in Bangladesh. Recently published results from Bangladesh and India relying on the needle-sampler are augmented here with new data from 37 intervals of grey aquifer material of likely Holocene age in Vietnam and Nepal. A total of 145 samples of filtered groundwater ranging in depth from 3 to 36 m that were analyzed for As (1–1000 μg/L), Fe (0.01–40 mg/L), Mn (0.2–4 mg/L) and S (0.04–14 mg/L) are compared. The P-extractable (0.01–36 mg/kg) and HCl-extractable As (0.04–36 mg/kg) content of the particulate phase was determined in the same suite of samples, in addition to Fe(II)/Fe ratios (0.2–1.0) in the acid-leachable fraction of the particulate phase. Needle-sampler data from Bangladesh indicated a relationship between dissolved As in groundwater and P-extractable As in the particulate phase that was interpreted as an indication of adsorptive equilibrium, under sufficiently reducing conditions, across 3 orders of magnitude in concentrations according to a distribution coefficient of 4 mL/g. The more recent observations from India, Vietnam and Nepal show groundwater As concentrations that are often an order of magnitude lower at a given level of P-extractable As compared to Bangladesh, even if only the subset of particularly reducing intervals characterized by leachable Fe(II)/Fe >0.5 and dissolved Fe >0.2 mg/L are considered. Without attempting to explain why As appears to be particularly mobile in reducing aquifers of Bangladesh compared to the other regions, the consequences of increasing the distribution coefficient for As between the particulate and dissolved phase to 40 mL/g for the flushing of shallow aquifers of their initial As content are explored.     

Geochronology of Pliocene volcanism in the Dzhavakheti Highland (the Lesser Caucasus). Part 1: Western part of the Dzhavakheti Highland

Isotopic-geochronological study of the Pliocene magmatic activity in western part of the Dzhavakheti Highland (northwestern region of the Lesser Caucasus) is carried out. The results obtained imply that the Pliocene magmatic activity lasted in this part of the highland approximately 2 million years from 3.75 to 1.75–1.55 Ma. As is established, the studied volcanic rocks correspond in composition mostly to K-Na subalkaline and more abundant normal basalts. Time constraints of main phases in development of basic volcanism within the study region are figured out. We assume that individual pulses of silicic to moderately silicic volcanism presumably took place in the Dzhavakheti Highland about 3.2 and 2.5 Ma ago.     

Quantitative sinkhole hazard assessment. A case study from the Ebro Valley evaporite alluvial karst (NE Spain)

Quantitative sinkhole hazard assessments in karst areas allow calculation of the potential sinkhole risk and the performance of cost-benefit analyses. These estimations are of practical interest for planning, engineering, and insurance purposes. The sinkhole hazard assessments should include two components: the probability of occurrence of sinkholes (sinkholes/km² year) and the severity of the sinkholes, which mainly refers to the subsidence mechanisms (progressive passive bending or catastrophic collapse) and the size of the sinkholes at the time of formation; a critical engineering design parameter. This requires the compilation of an exhaustive database on recent sinkholes, including information on the: (1) location, (2) chronology (precise date or age range), (3) size, and (4) subsidence mechanisms and rate. This work presents a hazard assessment from an alluvial evaporite karst area (0.81 km²) located in the periphery of the city of Zaragoza (Ebro River valley, NE Spain). Five sinkholes and four locations with features attributable to karstic subsidence where identified in an initial investigation phase providing a preliminary probability of occurrence of 0.14 sinkholes/km² year (11.34% in annual probability). A trenching program conducted in a subsequent investigation phase allowed us to rule out the four probable sinkholes, reducing the probability of occurrence to 0.079 sinkholes/km² year (6.4% in annual probability). The information on the severity indicates that collapse sinkholes 10–15 m in diameter may occur in the area. A detailed study of the deposits and deformational structures exposed by trenching in one of the sinkholes allowed us to infer a modern collapse sinkhole approximately 12 m in diameter and with a vertical throw of 8 m. This collapse structure is superimposed on a subsidence sinkhole around 80 m across that records at least 1.7 m of synsedimentary subsidence. Trenching, in combination with dating techniques, is proposed as a useful methodology to elucidate the origin of depressions with uncertain diagnosis and to gather practical information with predictive utility about particular sinkholes in alluvial karst settings: precise location, subsidence mechanisms and magnitude, and timing and rate of the subsidence episodes.     

Structural investigation of the synthetic CaAn(PO4)2 (An = Th and Np) cheralite-like phosphates

The crystallographic structures of the synthetic cheralite, CaTh(PO₄)₂, and its homolog CaNp(PO₄)₂ have been investigated by X-ray diffraction at room temperature. Rietveld analyses showed that both compounds crystallize in the monoclinic system and are isostructural to monazite LnPO₄ (Ln = La to Gd). The space group is P2₁/n (I.T. = 14) with Z = 2. The refined lattice parameters of CaTh(PO₄)₂ are a = 6.7085(8) Å, b = 6.9160(6) Å, c = 6.4152(6) Å, and β = 103.71(1)° with best fit parameters R _wp = 4.87%, R _p = 3.69% and R _B = 3.99%. For CaNp(PO₄)₂, we obtained a = 6.6509(5) Å, b = 6.8390(3) Å, c = 6.3537(8) Å, and β = 104.12(6)° and R _wp = 6.74%, R _p = 5.23%, and R _B = 6.05%. The results indicate significant distortions of bond length and angles of the PO₄ tetrahedra in CaTh(PO₄)₂ and to a lesser extent in CaNp(PO₄)₂. The structural distortions were confirmed by Raman spectroscopy of CaTh(PO₄)₂. A comparison with the isostructural compounds LnPO₄ (Ln = Ce and Sm) confirmed that the substitution of the large rare earth trivalent cations with Ca²⁺ and Th⁴⁺ introduces a distortion of the PO₄ tetrahedra.     

Methodology of hydrogeological characterization of deep carbonate aquifers as potential reservoirs of groundwater. Case of study: the Jurassic aquifer of <Emphasis Type="Italic">El Maestrazgo</Emphasis> (Castellón,Spain)

A methodology for the characterization of deep carbonate aquifers has been developed and applied to El Maestrazgo Jurassic aquifer in Castellón, Spain. Characterization of these aquifer formations, located at more than 300 m deep, consisted of a previous phase of compilation, analysis and synthesis of the existing information about the area, followed by a coordinated combination of different speciality studies: geology, stratigraphy, structural analysis, hydrogeology, hydrochemistry, geophysics and remote sensing. Geological studies included geological mapping, definition of stratigraphical units and facies and structural analysis. The aim of the hydrogeology study was to define aquifer formations, recharge area, aquifer points inventory and groundwater flow directions for the establishment of piezometric and water quality observation nets. Special techniques were applied, like thermal infrared aerial images and the evaluation of submarine groundwater discharge by means of natural radium isotopes. Hydrochemical techniques, including majority elements characterization and stable isotopes (¹⁸O, ²H and ³H) determination, allowed classifying hydrochemical facies and establishing a renewal pattern for water within the system. Geophysics was useful in determining the aquifer geometry, the features of the basement and the petrophysical characteristics of the geological formations. Preliminary results show an important tectonic complexity and the possibilities for groundwater uses in the area of study.     

Flexural isostatic response of the Alps to increased Quaternary erosion recorded by foreland basin remnants,SE France

We test the hypothesis that flexural isostatic compensation of the mass removed by enhanced Quaternary erosion is responsible for uplift of the Western European Alps and their forelands. We use two well‐preserved and well‐dated (1.8 Ma) abandonment surfaces of foreland basin remnants in SE France (the Chambaran and Valensole plateaux) as passive benchmarks for tilting of the foreland. Estimating their initial slope from morphometric scaling relationships, we determine bulk post‐depositional tilting of 0.5–0.8% for these surfaces. The calculated isostatic response of the Alpine lithosphere to erosional unloading, using the method recently proposed by Champagnac et al. [Geology 35 (2007) 195–198] , yields a predicted tilting of 0.3–0.4% in the considered areas, explaining approximately half of the determined post‐depositional tilting. Such long‐term deformation being insensitive to cyclic loading/unloading because of glaciations, we suspect the other half to be related to as yet undetermined long‐wavelength and long‐lived tectonic process(es).     

Correlation of atmospheric occurences over the United States with some physical and chemical parameters. Part III: Frequencies of precipitation

Summary Correlation between some physical and chemical variables, measured at 28 stations of the United States Weather Bureau Network, and seasonal and annual frequencies of precipitation, has been attempted with the aim of gaining insight into the bearing of such variables, on occurrence of precipitation. — Concurrent trends of frequencies with local temperature functions, altitude parameters, precipitable water vapour increments, and some chemical species have been found.Contribution of the «Centro Nucleazione Aerosoli» of the National Research Council of Italy, Via Vettore 4 (Monte Sacro),Roma.