Environmental influences upon mercury, radon and helium concentrations in soil gases at a site near Denver, Colorado

Variations in soil gas Hg, Rn and He concentrations and meteorological variables were monitored daily at one site over a period of 22 months. Air and soil temperature, humidity, barometric pressure, soil moisture, wind direction and velocity, soil freeze-thaw, water table elevation, crystal strain and gas emissions were determined simultaneously in order to assess the influence of the environmental variables on gas emission.Mercury concentrations were found to be higher in the summer while Rn and He concentrations were higher in the winter. It is hypothesized that adsorption-desorption controls the migration of Hg whereas Rn and He concentrations are controlled by diffusion and mass transport. Gas emissions respond to seasonal and shorter-term changes in environmental conditions. Stepwise multiple regression using gas emissions as the dependent variables suggests that environmental parameters account for 62% of the total Hg variance, 83% of the total Rn variance, and 33% of the total He variance. Temperature, barometric pressure and soil moisture exert the most influence on gas emissions with temperature effects dominating gas emissions throughout the year. Soil gas emissions display a predictable behavior during winter and summer when more stable meteorological conditions exist. During the transitional seasons of spring and fall, soil gas emissions become erratic and exhibit increased variability. Environmental variables are interrelated and appear to control the manner of gas migration.Increased adsorption by solids during the season of falling temperature decreases soil gas Hg<0.0001 ng L⁻¹, which is desorbed as the soil warms in the spring. Transport of Rn and He is primarily by convection. During the winter months, air temperatures are less than soil temperatures promoting upward movement of Rn and He by convection. During the summer, soil temperatures are less than air temperatures and an inversion layer below the level of sampling reduces upward flux and observed concentration.     

Geochemistry of unsaturated soil systems: Aqueous speciation and solubility of minerals and gases in capillary solutions

It has been shown that the capillary state of aqueous solutions in the unsaturated zone (UZ) modifies chemical equilibria due to the decreasing capillary pressure of the whole system (isobaric scenario) or of the aqueous phase only (anisobaric scenario). Meanwhile, the role of salinity in capillary solutions has not been explicitly taken into account up to now, at least not in a manner independent to capillarity. A consistent way to do so is considered in this paper by calculating activity coefficients as a function of capillary pressure through the extrapolation of the Davies model. The integrated approach thus defined is applied to the interpretation of some laboratory experiments (taken from the literature) carried out under different capillary conditions. Calculations and measurements of the boehmite ↔ bayerite hydration equilibrium in an atmosphere of varying humidity agree very satisfactorily if the anisobaric scenario is selected. The solubility of reactive gases O₂ and CO₂ is found to increase in the pore water when the relative humidity decreases. Consequently, and in agreement with experimental measurements, the extent of the pyrite oxidation depends on the relative humidity. The proposed model refines the manner with which chemical equilibria and mineralogical assemblies may be interpreted. In particular, the different scenarios that may be envisioned (isobaric, anisobaric and dual) are considered in the light of the possible modes of precipitation of the minerals (precipitation within the capillary solution, epitaxial type precipitation). Finally, it should be noted that the geochemical approach proposed here for the UZ fully ties in with and conforms to the methods used in the water-saturated zone (SZ).     

Hydrocarbon gases and helium isotopes in the Paleozoic alkaline-ultramafic massifs of the Kola Peninsula

Hydrocarbon gases (HCG) were studied in fluid inclusions from seven alkaline-ultramafic massifs of the Kola Peninsula. All the massifs (Sebljavr, Kovdor, Lesnaya Varaka, Ozernaya Varaka, Vuorijarvi, Turii Peninsula, and Salma) are central-type cratonic intrusions with ages of 360–410 Ma. Previous He isotopic investigations showed that the massifs have high ³He/⁴He ratios (up to 3.3 × 10^?5), which are usually higher than the upper mantle value. Similar to He, HCG were extracted by crushing. The HCG were analyzed for CH₄ (main component), C₂H₆, and C₃H₈. A comparison of HCG component contents with He isotope abundances and ratios showed that the HCG were probably not supplied by mantle-derived melts. Their formation during a postmagmatic stage at relatively low temperatures is our favored model.     

Probabilistic analyses of soil consolidation by prefabricated vertical drains for single‐drain and multi‐drain systems

Natural soils are one of the most inherently variables in the ground. Although the significance of inherent soil variability in relation to reliable predictions of consolidation rates of soil deposits has long been realized, there have been few studies that addressed the issue of soil variability for the problem of ground improvement by prefabricated vertical drains. Despite showing valuable insights into the impact of soil spatial variability on soil consolidation by prefabricated vertical drains, available stochastic works on this subject are based on a single‐drain (or unit cell) analyses. However, how the idealized unit cell solution can be a supplement to the complex multi‐drain systems for spatially variable soils has never been addressed in the literature. In this study, a rigorous stochastic finite elements modeling approach that allows the true nature of soil spatial variability to be considered in a reliable and quantifiable manner, both for the single‐drain and multi‐drain systems, is presented. The feasibility of performing an analysis based on the unit cell concept as compared with the multi‐drain analysis is assessed in a probabilistic context. It is shown that with proper input statistics representative of a particular domain of interest, both the single‐drain and multi‐drain analyses yield almost identical results. Copyright © 2016 John Wiley & Sons, Ltd.     

Elasto-plastic analyses of deep foundations in cohesive soil

The bearing capacity of deep foundations has been analysed using finite elements in conjunction with elasto-viscoplasticity. The influence of elastic parameters, mesh size and strain conditions on computed collapse loads was observed. Axisymmetric problems in particular were of interst, because it was found that good numerical solutions could be obtained through to collapse provided reduced integration was used.     

Estimation of soil compaction parameters by using statistical analyses and artificial neural networks

This study presents the application of different methods (simple–multiple analysis and artificial neural networks) for the estimation of the compaction parameters (maximum dry unit weight and optimum moisture content) from classification properties of the soils. Compaction parameters can only be defined experimentally by Proctor tests. The data collected from the dams in some areas of Nigde (Turkey) were used for the estimation of soil compaction parameters. Regression analysis and artificial neural network estimation indicated strong correlations (r ² = 0.70–0.95) between the compaction parameters and soil classification properties. It has been shown that the correlation equations obtained as a result of regression analyses are in satisfactory agreement with the test results. It is recommended that the proposed correlations will be useful for a preliminary design of a project where there is a financial limitation and limited time.     

Source-rock identification by isotope analyses of natural gases from fields in the Val Verde and Delaware basins,west Texas

Methane, ethane and propane carbon-isotope ratios are controlled by the maturity of the organic material in their source rocks. Isotope data of natural gases can be used in some instances to identify their source rocks. Migration seems to cause no essential changes in the carbon-isotopic composition of the gas components. Under certain circumstances the isotope-maturity relationship can be used in approximating displacement along fault systems or in determining the direction of gas migration.     

土壤非饱和导水率模型中参数的敏感性分析

针对邵明安根据再分布过程得出的非饱和导水率模型,对其中参数的敏感性进行分析,通过实测资料计算非饱和导水率对不同参数的敏感度,对比参数对非饱和导水率的影响程度。研究表明采用线性关系表示土壤湿润剖面的平均湿度和湿润锋处湿度之间的关系时,非饱和导水率对参数的敏感性比其他参数高得多。     

Overestimation of the importance of phytate in NaOH-EDTA soil extracts as assessed by P NMR analyses

Solution ³¹P NMR analysis of NaOH-EDTA extracts is the most widely used method for determining the speciation of organic P in soils. In this paper, we bring together results from a number of our recent studies that highlight some of the difficulties in obtaining accurate quantitative results and propose that these difficulties may have resulted in the systematic overestimation of soil phytate concentrations. The first problem is peak identification, which is complicated by variations in chemical shift with pH and ionic strength, and the impracticality of the primary ³¹P chemical shift reference (85% H₃PO₄). Unambiguous peak assignment can be achieved by spiking suspected compounds directly into soil extracts prepared for NMR analysis; this ensures native and added compounds experience exactly the same chemical environment. The second problem is quantifying NMR signal for individual species in the crowded and overlapping phosphate monoester region. Spectral deconvolution has been employed for this purpose, but very different results are obtained depending on whether or not the fitting procedure includes a broad signal that probably comes from P in large, “humic” compounds. In particular, failing to consider this broad peak in the deconvolution procedure results in overestimation of small organic P compounds, including phytate. The ready decomposition of phytate is demonstrated with an incubation experiment in which phytate added at various concentrations to a calcareous soil is shown to be decomposed to produce orthophosphate. The decomposition follows a first order exponential decay with a half-life of 4-8 weeks.     

Analysis of noble gases in water using a quadrupole mass spectrometer in static mode

A quadrupole-based mass spectrometer used in static mode has been employed to determine noble gas concentrations of water samples by isotope dilution. Water samples are degassed and spiked with minor isotopes of each noble gas. After separation of the various gas components, the isotopic ratios are measured in the mass spectrometer. Because a spike is added to each sample, the recovery of each gas component does not have to be quantitative. Equilibration temperatures are calculated from the gas concentrations, based on noble gas solubility data. An accuracy of ±1°C, from the air equilibration temperature, is attainable for air-saturated water samples prepared under controlled conditions in the laboratory. The method has been applied to groundwaters from the London Basin Chalk aquifer. The noble-gas-derived temperatures are consistent with climatic conditions prevailing at the periods of recharge.     

Numerical study of localization in soil systems

A numerical study of the mechanical behavior of heterogeneous soil systems, consisting of a bulk of sand with embedded stiff gravel inclusions or soft clay inclusions, is performed. A solution scheme using parallel computing is employed when analyzing two different categories of problems. First, a homogenization problem is studied, where use of a single representative volume element subjected to plane strain compression offers the possibility to investigate the coupling between the response at a local scale and at a global scale. Second, a plane strain footing problem with different heterogeneous soil systems is analyzed using a traditional finite element formulation. The material model utilized for the soil is a large deformation formulation of non-associated elasto-plasticity with an isotropic hardening law, able to represent dilation. It was found that the shape of the gravel or clay inclusions in the systems had no significant effect on the global responses, whereas the strain localizations in the two different soil systems, sand–gravel and sand–clay, were found to have different character. The effect of the initial density on the response was clearly observed in the localization patterns.     

含裂隙岩体单轴压缩裂纹扩展机制离散元分析

将由室内试验总结得到的岩石微观胶结模型嵌入离散元软件,对Lac du Bonnet花岗岩石进行预制单裂隙单轴压缩试验DEM数值模拟,分析了压缩过程中裂隙试样中应力的分布,并与理论计算结果进行对比分析,同时对各种断裂判据中裂纹起裂角的预测值进行了适用性的对比分析。结果表明,离散元模拟试样破坏形态与试验结果相近;离散元分析得到的应力分布与理论解在定性上相似;当预制角度较小时,侧向应力都处于拉压状态;由于裂隙左右两端压应变的集中造成了裂隙上下面拉应变的产生,造成了裂隙周围特殊的应力分布;当裂隙角度较大时,应力集中现象已不明显,因而,理论值与试验值有偏差;在断裂判据中最大周应力准则和最大能量释放率准则得到的裂纹扩展角与室内试验与DEM结果中的数值较为吻合。     

硐室片状危岩体病害机理分析与评价方法

长屿硐天遗址公园遗留了大量危岩体,造成较为严重的安全隐患。经现场勘查发现,长屿硐天景区存在12处片状危岩体,其结构特征和形成机理与常见的危岩体存在显著的不同。以W42危岩体为例,通过FLAC数值模拟计算,分析了片状危岩体的应力特征和卸荷作用,发现卸荷作用对开挖面3 m以内围岩影响较大;采用断裂力学方法,分析了应力强度因子KI与岩体内原生裂隙倾角α、原始裂隙开裂角θ0、原生裂隙长度c之间的关系和影响规律,得出了开裂角θ0在78°左右时,岩体原生裂隙更容易扩展开裂的规律。这些规律能够较为合理地解释片状危岩体的形成机理和结构特征,以及研究区域节理多为陡倾或直立的原因。     

Quantification of mixing processes in ore-forming hydrothermal systems by combination of stable isotope and fluid inclusion analyses

In the Schwarzwald area, southwest Germany, more than 400 hydrothermal veins hosting different gangue and ore mineral assemblages cross-cut the crystalline basement rocks. Many of the post-Variscan fluorite-barite-quartz veins are considered to have precipitated through mixing of a deep saline brine with meteoric, low salinity waters. This hypothesis was tested using carbon, sulfur, and oxygen isotope data of sulfides, sulfates and calcite, coupled with fluid inclusion studies. Primary hydrothermal calcites from the deposits show a positive correlation of their δ¹³C (V-PDB) and δ¹⁸O (V-SMOW) values, which range from −12 to −3‰ and from 12 to 18.5‰, respectively. Carbon and oxygen isotope compositions of paragenetically young, remobilized calcite types are shifted towards higher values and range from −12 to −1‰ and from 20 to 25‰, respectively. We developed an improved calculation procedure for modeling the covariation of carbon and oxygen isotopes in calcite resulting from mixing of two fluids with different isotopic compositions and total carbon concentrations. In our model, the carbon speciation in the two model fluid end-members and the fluid mixtures are calculated using a speciation and reaction path code. The carbon and oxygen isotope covariation of primary Schwarzwald calcites can effectively be modeled by a mixing trend of a deep saline brine and a meteoric, low salinity water. Sulfur isotope data of barites from 44 hydrothermal fluorite-barite-quartz veins vary from 9 to 18‰ (CDT), sulfide ore minerals show δ³⁴S values between −14.4 and 2.9‰. Calculated sulfide-sulfate equilibrium temperatures are in the range between 300 and 350 °C. These temperatures differ significantly from the formation temperatures of 150 to 200 °C of most of the deposits as estimated from fluid inclusions, and are interpreted as preserved paleotemperatures of the deep aquifer. This assumption has been carefully checked against possible contamination of an equilibrated sulfide-sulfate system from the deep aquifer with sulfate from surface-derived sources, considering also the kinetics of the sulfide-sulfate isotope exchange. A combination of the S isotopic results with microthermometric fluid inclusion data and constraints on the temperature of the meteoric water was used to calculate mixing ratios of the two fluid end-members. The results indicate that mass fractions of the deep saline brine in the mixed fluid were between 0.5 and 0.75. Considering all geologic, geochemical and isotopic information, we propose that the majority of the post-Variscan hydrothermal veins in the Schwarzwald area were precipitated by district-scale mixing of a homogeneous deep saline brine with meteoric waters.     

Cl,Br and I analyses of metamorphic and sedimentary rocks by isotope dilution mass spectrometry

Isotope dilution mass spectrometry with negative thermal ions was applied to determine Cl, Br and I in rocks which, in part, contained less than 100 ppb of Br and I. Two sets of samples have been investigated: a) a series of Al-rich metapelites of increasing metamorphic grade from the Damara Orogen, Namibia and b) fresh and hydrothermally altered greywackes from the Pb−Zn deposit Bad Grund in Germany. It was found that regional metamorphism of Al-rich metapelites causes no strong fractionation of the halides. The Br/Cl ratios in the metapelites are similar to those of sea water. The I/Cl and I/Br ratios, however, are 500 times higher. The I depleted in the ocean most probably is associated with organic matter stored in sediments. The unaltered greywackes from near the Pb−Zn vein have about the same concentrations of halides as the metamorphic Al-rich pelites. By contrast, the hydrothemally altered greywackes contain about twice as much Cl and I, and about 2–5 times as much Br as the unaltered samples. The element ratios, however, are similar for all three categories of rocks, thereby indicating that sea water played no role in the hydrothermal system.     

普硅水泥和矿渣水泥加固滨海软土效果对比分析

采用室内无侧限抗压强度试验,证明了矿渣硅酸盐水泥和普通硅酸盐水泥在加固滨海软土的效果方面存在较大的差异。通过X射线衍射分析,对两种水泥加固软土效果差异的内在机制和原因进行了探讨。结果显示,水泥土的强度与水泥水化产物的生成量之间存在一致性,矿渣水泥加固土可生成更多的水化产物,并能够限制软土中可溶盐对水泥土的影响。     

Estimation of parameter errors in inverse problems. Determining limb-darkening coefficients in classical eclipsing systems

The estimation of parameters and their errors is considered using the observed light curve of the eclipsing binary system YZ Cas as an example. The error intervals are calculated using the differential-correction and confidence-region methods. The error intervals and reliability of the methods are investigated, and the reliability of limb-darkening coefficients derived from the observed light curve analyzed. A new method for calculating parameter errors is proposed.     

天津港土性相关距离的计算研究和统计分析

相关距离是随机场理论应用于实际工程可靠度分析的一个重要参数。根据随机场理论,对土性参数相关距离的计算方法进行了研究和改进,提出求解相关距离的波动函数法和加权拟合相关函数法,使相关距离的计算更加简便易行,且精度有所提高。结合大量工程地质勘察资料,对天津港地区典型土层的垂直向相关距离及水平向相关距离进行了计算和统计,获得相关距离的地区性代表值,可作为此地区实际工程的参考。同时,对取样间距及土性指标对相关距离的影响进行了分析,提出了当取样间距变化时相关距离的确定原则,并指出相关距离是反映土的空间固有变异性的基本属性,由不同土性指标求得的相关距离值基本相同。