有机磷通过弱透水层迁移的实验研究

本文采用施压条件下有机磷溶液渗透通过粘性土层的试验,模拟有机磷越流通过弱透水层的过程,研究其迁移的衰减规律、降解转化的机理以及各种因子的影响规律。研究发现,进水的浓度越小,pH值越大,土层中的磷酸酶降解转化有机磷的效果越好;土层对有机磷的吸附量随着进水pH的增大而增大,进水浓度的减小而减小;对于酸性进水而言,粘性土层对有机磷的阻滞作用与进水浓度关系不大,较倚重于进水pH值的升高。     

Laboratory and field evidence in support of the electrogeochemically enhanced migration of ions through glaciolacustrine sediment

Ionic aureoles, overlying or contiguous to massive sulphide occurrences, are postulated to have been emplaced as a result of natural galvanic forces. The occurrences of these galvanic forces, most commonly referred to as the self-potential or spontaneous polarization phenomena, are well documented, but ionic migration in response to these electrical forces remained to be proven conclusively.The purpose of this study was to attempt to document ionic migration attributable solely to electrogeochemical transport. Pleistocene glaciolacustrine sediments rich in clay-size particles was the geological substrate chosen to minimize migration processes other than those of electrogeochemical transport.Laboratory experiments which simulated a field situation and utilized the radioisotope ⁶⁵Zn as a tracer, revealed a readily detectable diffusion of Zn²⁺ into glaciolacustrine clay; the distance of penetration of the Zn²⁺ being significantly increased by the application of a DC electrical potential, similar in magnitude to a natural self-potential. The rate of diffusion was greatest around the margins of the cathodic electrode located at the base of the clay layers; the areas shown to have the greatest current density. The resulting pattern of Zn penetration into the clay, when viewed in a vertical section, showed a double peaked increase in Zn concentration on either side, and a distinct decrease in Zn penetration directly above this cathode. Extrapolation of the average diffusion coefficient found for Zn²⁺ (D₀ = 6.3 × 10⁻⁹cm s⁻¹) to other ions commonly associated with base metal deposits, revealed that only H⁺ could be expected to move through appreciable thicknesses (>20 m) of clay in the 8000 years since sediment deposition.The hypothesis that H⁺ would diffuse through varved clay, and that the resulting pattern would be predictably modified by a self-potential field generated by an oxidizing sulphide body, was tested over the Magusi River volcanogenic massive sulphide deposit near Noranda, Quebec. The organic soil horizon (H) and top of the clay soil layer (C) were sampled and analyzed for conductivity, pH, and other major and minor elements. The mineral horizon revealed distinct changes in conductivity, pH, Ca, Mg, and Fe above the contacts of the hanging wall and footwall of the sulphide horizon, thus producing a double-peaked “rabbit-ear” anomalous pattern. The H soil horizon, when corrections for variable “total” element and carbon concentrations were applied, showed clear anomalies in Fe and other pH sensitive elements. A ratio of EDTA Fe/TOTAL Fe to total organic carbon revealed the best “rabbit-ear” anomaly above the sulphide horizoon. The use of organic horizon geochemistry as a method of evaluating EM conductors overlain by water-saturated lacustrine sediments is recommended.     

Heavy metal migration at a landfill site, Sarnia, Ontario, Canada—2: metal partitioning and geotechnical implications

Heavy metal profiles below a 15-year old sanitary landfill overlying a 30 m thick natural clay deposit are presented. Results indicate that unlike soluble species such as Cl⁻ and Na⁺ which have migrated distances up to 130 cm, Cu, Zn and Pb have migrated only up to 10 cm. The extent of Fe migration is estimated to be 20 cm. Highly reducing conditions at the interface (E_h= −130mV), coupled with the alkaline nature of the clay pore waters, have resulted in the precipitation of migrated heavy metals as carbonates. At the clay/waste interface, 88, 84 and 80% of the excess Fe, Zn and Pb, respectively, are present as secondary carbonates. This is confirmed by selective chemical dissolution analyses which also show that Fe, Zn, Pb and, to a greater extent, Cu are present in solid organic forms at the interface. Batch equilibrium studies clearly show that Cu and Pb removal from leachate is significantly increased by the presence of carbonates in the soil. For example, 75% more Pb is removed by the carbonate-rich bulk soil than the carbonate-free soil. The batch studies also show that when thepH> 5.2, removal of metal increases significantly due to precipitation as carbonates. From the results it is concluded that the presence of metal sludges in landfills lined naturally or artifically by a carbonate-rich clayey barrier reduces the rate of migration of numerous toxic transition metals and may also decrease the barrier porosity by precipitation. The decreases in porosity will be beneficial to the performance of the barrier due to reductions in both advection and diffusion.     

Experimental study of migration of potassium ion through a two-layer soil system

A barrier system based on the hydraulic trap design concept for a landfill was proposed. To study the field scenario in which a clay liner is underlain by a granular layer functioning as a secondary leachate drain layer, a laboratory advection–diffusion test was performed to investigate factors controlling the transport of contaminants in a two-layer soil system. The soils used for this study were Ariake clay and, the underlying layer, Shirasu soil from the Kyushu region of Japan. Potassium (K⁺) was selected as the target chemical species with an initial concentration of 905 mg L⁻¹. The effective diffusion coefficients (D _e) of K⁺ for Ariake clay and Shirasu soil were back-calculated using an available computer program, Pollute V 6.3. Values of D _e derived from this experiment are consistent with previously published ones. The Ariake clay has lower D _e than the Shirasu soil. The hypothesis that mechanical dispersion can be considered negligible is reasonable based on both the observation that the predicted values well fit the experimental data and the analyses of two dimensionless parameters. Parametric analyses show that transport of K⁺ through soils is controlled by advection–diffusion rather than diffusion only, whereas at low Darcy velocity (i.e., ≤10⁻⁹ m s⁻¹), transport of K⁺ will be controlled by diffusion. Applications of the test results and parametric analysis results in practical situations were reviewed.     

Fluoride hydrogeochemistry and bioavailability in groundwater and soil of an endemic fluorosis belt,central Iran

Presence of fluoride in groundwater is a public health problem in the so-called endemic fluorosis belt of the central Iran, where the groundwater is the major source of drinking water in most urban and rural areas. Therefore, an attempt has been made to determine the hydrogeochemical factors controlling fluoride enrichment in the groundwater resources at this belt. Fluoride concentrations ranged from 0.20 to 1.99 mg/L (1.02 ± 0.47) in groundwater samples. The presence of different F-bearing minerals and also clay minerals in the soils and aquifer materials was confirmed using XRD analysis. To identify probable sources of dissolved F^? and investigate groundwater quality, multivariate statistical analyses were carried out. Geochemical modeling indicated that all samples were undersaturated with respect to fluorite, halite, gypsum and anhydrite and mostly oversaturated with respect to calcite and dolomite. Contrary to most high-fluoride regions in the World, the high F^? content was dominated by Na–Cl- and Ca–SO₄-type groundwater in the study area. Besides, fluoride showed negative relationship with pH and HCO₃ ^? in groundwater. In order to assess the bioavailability of fluoride in soils, a two-step chemical fractionation method was applied. The results showed that fluoride in soils mostly accompanied with the residual and water-soluble fractions and was poorly associated with soil’s bonding sites. Calculated aqueous migration coefficient demonstrated that fluoride in the studied soils was mobile to easily leachable to the groundwater. Finally, the results demonstrated that combination of water–rock interaction and influence of clay minerals is geochemical mechanism responsible for controlling fluoride enrichment in groundwater.     

pH 值对有机磷在弱透水层中迁移转化的试验模拟

为研究渗透水pH 值对有机磷通过弱透水黏土层迁移转化的影响规律,采用黏性土层充压渗透装置模拟污染潜水中有机磷渗透通过弱透水层的迁移转化过程。研究发现,有机磷组分通过弱透水层的迁移能力总体较弱,不同pH 值条件下有机磷的矿化率为93. 5% ～ 99. 95%; 在进水pH 值为5. 5 ～ 8. 5 时,进水pH 值越高,土层中磷酸酶的活性也越高,降解转化有机磷的能力也越强; 黏土对有机磷的阻滞能力也增强。进水pH 值为5. 5 时,迁移率为6. 5%; 当pH 达到8. 5 时,矿化率明显增加,迁移率＜ 0. 05%,使其不易渗入深层地下水引起污染。     

Fixed-ammonium in clays associated with crude oils

The association of ammonium (NH₄⁺) silicates with organic-rich sedimentary environments has stimulated interest in the chemical cycle of N, and its possible application as an indicator of in situ organic maturation reactions or crude oil migration. Fixed-NH₄ in clay minerals was determined from three hydrocarbon occurrences of similar depositional environment but different ages, depth and thermal maturity, to determine whether anomalously high NH₄-substitution occurs near mature hydrocarbons. Results show higher fixed-NH₄ concentrations in marginally mature mudstones than in immature sediments. The highest fixed-NH₄ concentrations were found in clays from sandstone reservoirs containing migrated crude oil.Fixed-NH₄ in clays from Holocene oil seep sediments in the Gulf of Mexico continental slope, offshore Louisiana, averages 0.08 wt % and increases with depth in shallow cores (420 cm), reflecting an early diagenetic trend that is apparently not influenced by migrating crude oil. Programmed pyrolysis shows that the sediments are thermally immature (av.T_max = 419°C). High Hydrogen Index values (av.= 359mg/g) are the result of biodegraded crude oil, and a high Oxygen Index (av.= 182mg/g) reflects the presence of authigenic carbonate.Fixed-NH₄ averages 0.16 wt % in Wilcox Group (Eocene) mudstones enclosing two sandstone reservoirs at Fordoche Field, onshore Louisiana. In comparison to these mudstones, anomalously high NH₄-fixation appears to occur in reservoir clay minerals. Pyrolysis shows that the sediments are marginally mature for crude oil generation (av.T_max = 432°C). Average Hydrogen Index (187 mg/g) and Oxygen Index values (75 mg/g) are consistent with oil-prone Type II and Type III kerogen. Increased pyrolysis Production Index values and solvent extraction shows the presence of migrated crude oil. This suggests that a reaction which releases NH₃ during crude oil generation or migration is recorded byNH₄⁺ substitution in clays.Fixed-NH₄ and total organic carbon (TOC) at Fordoche Field show no statistically significant correlation, suggesting that NH₄⁺ substitution in clay minerals is not simply related to the amount of organic matter in the section, but is also influenced by the presence of crude oil. Once NH₄⁺ has been fixed in clays, it is a more stable hydrocarbon proximity indicator than pore fluid tracers, because it is less influenced by later chemical or geological changes.     

Soil quality indicator of oxisols grown with sugarcane and native forest in northeastern São Paulo state,Brazil

The evaluation of soil quality is an important tool for degradation monitoring and sustainable management implementation. The objective of this study was to measure physical and chemical soil properties to set soil quality and validate a model of soil quality indicator in latosols (oxisols) under sugarcane cropping and native forest. The study was carried out in the cities of Araras, Santa Ernestina, and Guariba in São Paulo State, Brazil. We collected 24 samples of disturbed and undisturbed soil at 0.0–0.10 m layer from three areas grown with sugarcane and neighboring locations under native woodland. We assessed the following soil properties: (a) chemical—pH in CaCl₂, organic matter (OM), phosphorus (P), potassium (K⁺), calcium (Ca²⁺), magnesium (Mg²⁺), potential acidity (H?+?Al), aluminum (Al³⁺), and sulfur (S); (b) physical—macro- and microporosity, soil bulk density (Ds), aggregate stability, mean weight diameter (MWD), rill (K_r) and interrill (K_i) global erodibility, shear stress (τc), and magnetic susceptibility (MS). Data underwent multivariate statistics to identify the properties that denote soil quality and to set their weights within the functions of soil quality indicator (SQI). This study showed that the multivariate analysis was efficient in determining which physical and chemical properties were most sensitive, of which we can mention total sand, MS, clay, microporosity, Mg, Ca, pH, and OM. We can therefore conclude that the quality indicators of soils grown with sugarcane were lower than those under forest were, showing the need for adoption of conservation management practices.     

河北承德锗元素生态地球化学特征及其与道地药材黄芩适生关系

道地药材生长与生态地球化学研究对实现中医药科学化和标准化具有重要意义。从承德市滦河流域与金沟屯和五道岭典型研究区阐明区域尺度和不同地质建造区Ge元素地球化学背景特征,结合多元统计采用基于Nb元素的质量迁移系数、化学蚀变指数CIA和残积系数RF、生物富集系数论述Ge元素在基岩-风化壳-土壤-黄芩系统中的迁移聚集规律,探讨Ge元素生态地球化学特征与道地药材黄芩的适生关系。结果表明：滦河流域表层土壤Ge元素平均含量为1.336 mg·kg^-1,43.54%土壤样品Ge元素含量属丰富—较丰富水平;金沟屯和五道岭区表层土壤Ge元素平均含量分别为1.352 mg·kg^-1和1.268 mg·kg^-1。不同地质建造和表层土壤Ge元素含量与TFe₂O₃含量显著相关,土壤含铁矿物对Ge元素具有吸附作用。Ge元素含量随岩土风化程度升高而增大,金沟屯区土壤风化程度高于五道岭区,土壤成熟度相对较高,Ge元素富集程度相对较高。岩石风化过程中Ge元素与TFe₂O₃、V、Ti、Co、P、Pb、Cu、Zn、Al₂O₃、SiO₂、K₂O、Na₂O质量迁移系数值相近,风化土壤与新鲜基岩Si和Ge含量发生明显分异,Ge元素主要来源于硅酸盐矿物风化过程中晶格破裂和金属硫化物矿物风化释放。金沟屯和五道岭黄芩Ge元素BCF平均值分别为0.014和0.020,黄芩根部对土壤Fe与Ge的吸收表现出明显的协同作用,土壤pH影响着Ge元素形态和生物有效性。区域土壤丰富的Fe、P和Sr元素含量为优质黄芩生长提供了有利条件;Fe族元素含量丰富,pH呈微碱性的沙壤质土壤为道地药材黄芩适宜生长和定向栽培种植区。     

四川省沐川县西部地区土壤硒含量特征及影响因素

硒是人体和动物必需的微量有益元素,摄入适量硒是提高人群生活质量的重要标志。食用富硒农产品是缺硒地区人体获取和补充硒元素的重要途径。调查土壤硒的含量特征、圈定富硒土壤资源分布区、查明土壤硒迁移富集的影响因素,是高效利用富硒土壤资源和科学开发富硒农产品的重要依据。本文选择四川省沐川县西部地区采集土壤样品,采用原子荧光光谱法、X射线荧光光谱法、电感耦合等离子体质谱/发射光谱法等方法测定土壤中Se、Al₂O₃、TFe₂O₃、OrgC、Cd等元素含量和pH值,利用统计学及相关分析等方法研究了土壤硒等元素含量和分布特征,并对土壤硒含量的主要影响因素进行探讨。结果表明：①研究区表层土壤硒含量范围为0.08~1.30mg/kg,平均含量为0.39±0.15mg/kg,满足富硒土壤条件的土壤面积为112km²,占研究区总面积的52%,其中无公害富硒土壤面积为35km²。土壤Cd含量是造成富硒土壤和无公害富硒土壤面积具有较大差异性的主要因素。②地质背景与土壤硒含量密切相关,富硒土壤主要受含碳酸盐岩及夹碳质地层的砂岩、粉砂岩等地层控制,土壤硒来源稳定;③土壤硒含量随pH值、风化程度的降低而升高,随有机质、TFe₂O₃、S含量的升高而升高,其中有机质对土壤硒的影响显著大于TFe₂O₃,表明该区域铁氧化物对硒的吸附能力低于有机质。综上所述,研究区富硒土壤资源丰富,建议当地充分利用相关资源开发富硒农产品,同时应注重有机肥的平衡施肥,并进一步加强土壤-作物系统中硒迁移富集影响因素的协同分析。     

Geothermochronology based on noble gases: II. Stability of the (U-Th)/He isotope system in zircon

The kinetics of He migration from zircon of variable degree of metamictization was investigated. The migration parameters of He were experimentally determined, the influence of radiation damage and the degree of metamictization on the stability of the (U-Th)/He isotope system was evaluated, the mechanisms of noble gas escape from zircon were investigated, new data on the kinetics of He migration were obtained and compared with previous results for the kinetics of Xe migration from zircon of the same geologic objects. It was shown that He occurs in two energy positions in the zircon lattice: the main position (more than 80% He) with an activation energy of ∼39 kcal/mol and k ₀ = 10¹¹ yr⁻¹ and the second position with an activation energy for migration of 5–10 kcal/mol and k ₀ ∼ 10⁶ yr⁻¹. It was concluded that He migration from the main energy position is better described by a single-jump mechanism. The migration of He from the second energy position is consistent with the diffusion mechanism. It was shown that deviations from the linear dependence in the lnln(He₀/He_t)-1/T coordinates are probably related to the destruction of volume defects containing He atoms at high temperatures (more than 1000°C on the experimental time scale) resulting in a change from the single-jump to diffusion mechanism and the presence of atoms migrating via the diffusion mechanism. It was shown that the peak width in the spectrum of radiogenic He release and the appearance of a second peak also depend on the fraction of atoms migrating in accordance with the diffusion mechanism. It was found that the low activation energy for He release from the second energy position indicates the existence of continuous He loss from the zircon lattice.     

Microscopic reversibility of Sm and Yb sorption onto smectite and kaolinite: Experimental evidence

The microscopic reversibility of the sorption of Sm and Yb onto kaolinite and smectite is investigated by introducing an isotopic disequilibrium between the clay and the solution. The experiments are performed at 25°C, in 0.025 or 0.5 M NaClO₄ and from pH 4 up to pH 7. The isotopic exchange is monitored as a function of time over a duration of 355 hours. The first stage of the experiment consists of equilibrating the clays with a natural or spiked lanthanide solution. The second stage consists of interchanging the solutions between twin phials (same clay, pH and ionic strength, but with different lanthanide isotopic compositions). The isotopic composition and concentration of aqueous lanthanides are analysed by ICP-MS. The results are as follows: (1) the lanthanide isotopic composition of the solution is rapidly modified and stabilised within 24 h; (2) the isotopic exchange between the solid and the solution is not always complete; (3) the degree of microscopic reversibility (isotopic exchange) decreases with increasing pH. These results are explained by the fact that exchange is easier for lanthanides linked to the surface as outer-sphere complexes (physical sorption), which predominate at low pH, than for atoms sorbed as inner-sphere complexes (chemical sorption) which predominate at high pH. The contrasted kinetics observed for the different kind of sites provide additional constraints for the modeling of migration processes in natural systems.     

Geology and engineering properties of laterites from Ilorin,Nigeria

Twelve samples of Nigerian laterites were obtained from Ilorin, a rapidly growing urban center, and capital of Kwara State, Nigeria. Three varieties of laterites (clay, gravel and crust) were identified and subjected to mineralogical, chemical and geotechnical analyses which included: identification of clay and non-clay minerals by X-ray diffraction (XRD) techniques; chemical composition by X-ray fluorescence spectrometer analysis; pH of soil in water; moisture contents and specific gravity determinations, grain size analysis; compaction test by Harvard Compaction Apparatus and unconfined compressive strength determination.The laterite soil samples are composed of kaolinite and illite clay minerals with some quartz and feldspar. They were found to be rich in SiO₂ (45%) Fe₂O₃, (16%) and Al₂0₃ (10%).These soils yielded maximum strength when compacted on the dry side of their optimum moisture content (OMC).The soils are not expected to perform very well as concrete aggregates since they contain high amounts of SiO₂ and Fe₂O₃. These oxides are known to have deleterious effects on construction materials, particularly concrete aggregates.     

Sulphate adsorption capacity and pH of upland podzolic soils in Scotland: Effects of parent material,texture and precipitation chemistry

A regional survey of podzol B horizons has been carried out to investigate the effects of parent material and soil texture on sulphate (SO₄²⁻) adsorption capacity and pH in Scottish soils. Sulphate adsorption was measured on equilibration of the soils with 10 and 100 mg 1⁻¹ SO₄²⁻ solutions. The results showed that soil texture and soil parent material had a significant effect on SO₄²⁻ adsorption. Significant correlations were found between sulphur (S) deposition loads and SO₄²⁻ adsorption, and between precipitation pH and soil pH, but not between total hydrogen ion (H⁺) load and soil pH, even on sensitive soils. Relationships between the chemical composition of atmospheric deposition and soil pH could be marginally improved if the possible amelioration of acidification by base cation inputs, especially on sensitive soils, was taken into account.     

Dynamics of CO<Subscript>2</Subscript> fluxes and concentrations during a shallow subsurface CO<Subscript>2</Subscript> release

A field facility located in Bozeman, Montana provides the opportunity to test methods to detect, locate, and quantify potential CO₂ leakage from geologic storage sites. From 9 July to 7 August 2008, 0.3 t CO₂ day⁻¹ were injected from a 100-m long, ~2.5-m deep horizontal well. Repeated measurements of soil CO₂ fluxes on a grid characterized the spatio-temporal evolution of the surface leakage signal and quantified the surface leakage rate. Infrared CO₂ concentration sensors installed in the soil at 30 cm depth at 0–10 m from the well and at 4 cm above the ground at 0 and 5 m from the well recorded surface breakthrough of CO₂ leakage and migration of CO₂ leakage through the soil. Temporal variations in CO₂ concentrations were correlated with atmospheric and soil temperature, wind speed, atmospheric pressure, rainfall, and CO₂ injection rate.     

Heave Studies on Fly Ash-Stabilised Expansive Clay Liners

Clay liners or compacted earthen barriers are important barrier materials used for preventing contaminant transport through soils. A low hydraulic conductivity (k) is a significant parameter that governs the design and construction of clay liners. Compacted expansive clays, which are montmorillonite clays, also have a very low hydraulic conductivity (k). When expansive clays are blended with fly ash, an industrial waste, the hydraulic conductivity (k) further reduces as the ash-clay blends result in increased dry densities at increased fly ash contents. Hence, fly ash-stabilised expansive clay can also be proposed as a unique clay liner material. As expansive clays undergo heave when they come into contact with water, it is necessary to study the heave behaviour of fly ash-stabilised expansive clay liners. This paper presents heave studies on fly ash-stabilised expansive clay liners. Fly ash in different contents by dry weight of the expansive clay was added to the clay, and the ash-clay blend was compacted as a liner overlying a natural field soil layer. Compacted lateritic clay was used for simulating the natural field soil into which contaminants migrate. Calcium chloride (CaCl₂) solution of varying concentration (5, 10, 20, 50, 100 and 500 mM) was used as the permeating fluid in the heave studies. The rate of heave and the amount of heave of the fly ash-stabilised expansive clay liners were monitored. Deionised water (DIW) was also used as inundating fluid for comparative study. Heave (mm) decreased with increase in solute concentration for all fly ash contents. For a given solute concentration, heave decreased up to a fly ash content of 20 % and thereafter it increased when the fly ash content was increased to 30 %. Heave of the fly ash-stabilised expansive clay liners was correlated with their permeability, liquid limit (LL) and free swell index (FSI) pertaining to the respective fly ash content and CaCl₂ concentration.     

Tebuthiuron leaching in three Brazilian soils as affected by soil pH

Underground contamination water by herbicides depends on the interactions between their molecules with physical and chemical soil characteristics and climatic conditions. Studies with columns can estimate the leaching potential of herbicides in soils. This study aimed to determine the effect of soil pH on tebuthiuron leaching, and capacity of bioindicators to detect tebuthiuron residues in three Brazilian soils. Cucumber plants (Cucumis sativus) were more negatively affected when grown in soils with lower amounts of organic matter and clay, and in these soils, the tebuthiuron levels reached greater depths in the column. There was a positive correlation between tebuthiuron concentration and cucumber intoxication, and a negative correlation between tebuthiuron concentration and dry matter cucumber in all soils. The tebuthiuron leached up to 50 cm depth even in soils with higher organic matter and clay content. The increasing of soil pH can affect the leaching of nonionic herbicides, and liming practice may elevate the environmental contamination risk by tebuthiuron. The bioindicator method using Cucumis sativus is viable and can be recommended to detect tebuthiuron concentrations above 0.2 mg kg^?1.     

Carbon sources and signals through time in an Alpine groundwater Basin, Sagehen California

In 2003, chlorofluorocarbon (CFC) apparent ages, major ion chemistry and C isotopes were determined in nine springs from Sagehen Basin, a high elevation watershed in the eastern Sierra Nevada. Springs with similar apparent ages, which ranged between 15 and 45 a, had very similar chemistry despite being found in different areas of the watershed. In agreement with earlier studies, concentrations of rock-derived cations (Ca²⁺ and Na⁺), conductivity, temperature and pH increase with apparent age, documenting the chemical evolution of this groundwater system. In contrast with the cation data, δ¹³C and ΣCO₂ show no correlation with apparent age. δ¹³C displays a strong linear relationship with 1/ΣCO₂ (R² = 0.91). This is consistent with results from a previously developed soil respiration/diffusion model. Spring radiocarbon content ranged between 85 and 110 pmc and varied with apparent age, whereby the youngest groundwater has the highest radiocarbon values. The spring radiocarbon is set by the soil pCO₂ and its trend can be best described assuming the soil CO₂ is composed of a mixture of 50–66% fast- (15–25 a) and 33–50% slow- (4 ka) cycling components. These results are consistent with previous soil C studies. The C isotope data indicate that in Sagehen Basin the groundwater ΣCO₂ is inherited from the soil zone with little, if any, contribution from the dissolution of disseminated calcite.     

Geochemical and thermodynamic aspects of sorption of strontium on kaolinite dominated clay samples at Kalpakkam

The mobility of strontium in subsurface is largely influenced by sorption on to clay minerals. In the present study, kaolinite clay samples collected from the Kalpakkam nuclear plant site were employed to understand the sorption characteristics of strontium by batch method. The effect of several parameters such as time, strontium ion concentration, pH, temperature and ionic strength was investigated. The kinetic studies suggested pseudo-second-order mechanism. The experimental sorption data was fitted to Langmuir adsorption model for obtaining the sorption capacity of the sorbent. The maximum sorption capacity was 5.77 mg/g at 298 K and was found to increase with an increase in temperature. It was observed that the distribution coefficient (K _d) of strontium on clay increased as the pH of the solution increased. The distribution coefficient was found to decrease with an increase in concentration of Na⁺ and Ca²⁺ ions. This variation of K _d suggests that cation exchange is the predominant sorption process. It was also observed that sorption process is endothermic. The thermodynamic parameters such as ∆G ⁰, ∆H ⁰ and ∆S ⁰ were calculated. The negative values obtained for ∆G ⁰ indicated that the sorption of strontium on clay was spontaneous at all studied concentrations. ∆G ⁰ becomes more negative with an increase in temperature, suggests that the sorption process is more favorable at higher temperatures.     

Bearing capacity of strip foundations reinforced with geogrid sheets by using upper bound finite‐element limit analysis

By using the upper bound finite‐elements limit analysis, with an inclusion of single and two horizontal layers of reinforcements, the ultimate bearing capacity has been computed for a rigid strip footing placed over (i) fully granular, (ii) cohesive‐frictional, and (iii) fully cohesive soils. It is assumed that (i) the reinforcements are structurally strong so that no axial tension failure can occur, (ii) the reinforcement sheets have negligible resistance to bending, and (iii) the shear failure can take place between the reinforcement and soil mass. It is expected that the different approximations on which the analysis has been based would generally remain applicable for reinforcements in the form of geogrid sheets. A method has been proposed to incorporate the effect of the reinforcement in the analysis. The efficiency factors, η_c and η_γ, to be multiplied with N_c and N_γ , for finding the bearing capacity of reinforced foundations, have been established. The results have been obtained (i) for different values of ? in case of fully granular and cohesive‐frictional soils, and (ii) for different rates at which the cohesion increases with depth for a fully cohesive soil. The optimum positions of the reinforcements' layers have also been determined. The effect of the reinforcements' length on the results has also been analyzed. As compared to cohesive soils, the granular soils, especially with higher values of ?, cause a much greater increase in the bearing capacity. The results compare reasonably well with the available theoretical and experimental data from literature. Copyright © 2013 John Wiley & Sons, Ltd.