Migration of As, and H/He ages, in groundwater from West Bengal: Implications for monitoring

From 2002 to 2010 inclusive we monitored concentrations of arsenic (As) and major ions (Ca, Mg, Sr, Na, K, Fe, Mn, Cl, and SO₄) in groundwater from 14 domestic wells and three piezometer nests in a shallow aquifer (<60 m depth), and 3 wells in a deep aquifer (>70 m depth), in southern West Bengal, India. In the deep aquifer, concentrations of As did not change over time despite increases in the concentration of Fe in two wells. The shallow aquifer occurs in two sedimentological settings: palaeo-channel and palaeo-interfluve. At the top of the shallow aquifer of the palaeo-channel, decreases in all constituent concentrations with time, and an ³H/³He age of 1.4 years, proves that the aquifer is beginning to be flushed of pollutants. In As-polluted groundwater (>50 μg/L As) tapped from deeper grey sands of the shallow, palaeo-channel, aquifer, concentrations of As were mostly stable over time, but both increases and decreases occurred with time in response to downward migration of the chemically-stratified water column. In groundwater tapped from Pleistocene brown sands, the concentration of As remained either low and stable (<2 μg/L As), or increased at rates up to 34 μg/L per year. The increases were caused by the flow of As-rich groundwater either downward into brown sand at the base of palaeo-channels, or laterally into a confined, unpolluted, palaeo-interfluvial, aquifer of brown sand that lies regionally beneath a palaeosol.Under the present pumping regime, the prognosis for As-pollution in the shallow aquifer is complex. Wells in brown sand may become polluted over timescales of as little as 2 years, whilst some wells tapping As-polluted groundwater from grey sand will become fit for potable use (<50 μg/L) within a few decades. The evidence of flushing, and of declining As in some of the groundwater from palaeo-channels, which are conduits for recharge of the confined, As-free, palaeo-interfluve aquifer, and probably also the deeper aquifer, offers hopes that the spread of As-pollution will be limited.     

Hydrogeochemical study of the clastic aquifer in the Umm Gudair field,Kuwait

The Kuwait Group aquifer is a major source of brackish groundwater in the State of Kuwait. The aquifer ranges in thickness from 150–400 m. In the Umm Gudair well field, the water has total dissolved solids ranging from 3,130 to 4,790 mg/l. According to the Sulin classification, all the Kuwait Group aquifer samples are characterized as Cl–Mg or Cl–Ca genetic water types. The chemical data indicate that the aquifer is generally in equilibrium with respect to calcite, dolomite and gypsum but undersaturated with respect to halite and polygorskite. The study has indicated that de-dolomitization is the major geochemical process controlling the chemistry of this aquifer, increasing the major complexes of sulphate and carbonate due to the dissolution of gypsum. The ratio of Cl/Na indicates that the Na⁺ ions have been taken from the Kuwait Group aquifer by reverse ion exchange.      

Mobilization of arsenic and other naturally occurring contaminants in groundwater of the Main Ethiopian Rift aquifers

This study investigates the mechanisms of arsenic (As) and other naturally occurring contaminants (F⁻, U, V, B, and Mo) mobilization from Quaternary sedimentary aquifers of the Main Ethiopian Rift (MER) and their enrichment in the local groundwater. The study is based on systematic measurements of major and trace elements as well as stable oxygen and hydrogen isotopes in groundwater, coupled with geochemical and mineralogical analyses of the aquifer rocks. The Rift Valley aquifer is composed of rhyolitic volcanics and Quaternary lacustrine sediments. X-ray fluorescence (XRF) results revealed that MER rhyolites (ash, tuff, pumice and ignimbrite) and sediments contain on average 72 wt. % and 65 wt. % SiO₂, respectively. Petrographic studies of the rhyolites indicate predominance of volcanic glass, sanidine, pyroxene, Fe-oxides and plagioclase. The As content in the lacustrine sediments (mean = 6.6 mg/kg) was higher than that of the rhyolites (mean: 2.5 mg/kg). The lacustrine aquifers of the Ziway-Shala basin in the northern part of MER were identified as high As risk zones, where mean As concentration in groundwater was 22.4 ± 33.5 (range of 0.60–190 μg/L) and 54% of samples had As above the WHO drinking water guideline value of 10 μg/L. Field As speciation measurements showed that most of the groundwater samples contain predominantly (∼80%) arsenate-As(V) over arsenite-As(III) species. The As speciation together with field data of redox potential (mean Eh = +73 ± 65 mV) and dissolved-O₂ (6.6 ± 2.2 mg/L) suggest that the aquifer is predominantly oxidative. Water-rock interactions, including the dissolution of volcanic glass produces groundwater with near-neutral to alkaline pH (range 6.9–8.9), predominance of Na–HCO₃ ions, and high concentration of SiO₂ (mean: 85.8 ± 11.3 mg/L). The groundwater data show high positive correlation of As with Na, HCO₃¯, U, B, V, and Mo (R² > 0.5; p < 0.001). Chemical modeling of the groundwater indicates that Fe-oxides and oxyhydroxides minerals were saturated in the groundwater, suggesting that the As reactivity is controlled by adsorption/desorption processes with these minerals. The data show that As and other oxyanion-forming elements such as U, B, Mo, and V had typically higher concentrations at pH > ∼8, reflecting the pH-dependence of their mobilization. Based on the geochemical and stable isotope variations we have established a conceptual model for the occurrence of naturally occurring contaminants in MER groundwater: 1) regional groundwater recharge from the Highland, along the Rift margins, followed by lateral flow and water–rock interactions with the aquifer rocks resulted in a gradual increase of the salinity and naturally occurring contaminants towards the center of the valley; and (2) local δ¹⁸O-rich lake water recharge into adjacent shallow aquifers, followed by additional mobilization of As and other oxyanion-forming elements from the aquifer rocks. We posit that the combined physical-chemical conditions of the aquifers such as oxidizing state, Na–HCO₃ composition, and pH>∼8 lead to enhanced mobilization of oxyanion-forming elements from Fe-oxides and consequently contamination of local groundwater. These geochemical conditions characterize groundwater resources along the Eastern African Rift and thus constitute a potential threat to the quality of groundwater in larger areas of Eastern Africa.     

CSM工法在软土地区深基坑承压水控制中的应用

以上海前滩33-01地块项目深基坑工程为背景,介绍了软土地区深厚承压水含水层的水文地质条件下,在上海地区首次采用50m深CSM工法等厚度水泥土搅拌墙作为落底式止水帷幕,隔断承压水含水层,保护基坑及周边环境的安全。通过CSM工法钻芯取样结果以及基坑监测数据的分析表明,基坑工程采取CSM工法等厚度水泥土搅拌墙,隔水效果良好,有效的控制了基坑降承压水对周边保护对象的影响,可以为类似工程提供一定的参考。     

软土的卸荷模量

软土的室内应力路径试验研究表明，软土的卸荷模量不仅取决于土的物理性质，而且还和所经历的应力路径密切相关。本文根据大量的应力路径试验，特别是模拟基坑开挖和软土隧道施工过程中的应力路径，研究了上海地区的几种典型软土的卸荷变形模量与应力路径的关系表达式。对基坑开挖及支护结构设计计算、软土隧道施工研究及其它需要准确确定软土的卸荷模量的岩土工程问题的研究均有较大的实用价值     

Evaluating the effect of soil structure on the ground response during shield tunnelling in Shanghai soft clay

When evaluating tunnel-induced ground response in Shanghai soft clay, the soil structure and its degradation behaviour of natural Shanghai soft clay during shield tunnelling should be properly considered. In this paper, a constitutive model that considers the initial soil structure and its destructuration is formulated within the framework of critical-state soil mechanics. The model is successfully calibrated and used to simulate the undrained behaviour of natural Shanghai soft clay. Based on the proposed model, finite-element analyses are conducted to simulate the short- and long-term ground responses induced by tunnelling at Shanghai metro line 2. The comparisons between numerical results and field measurements reported in literature indicate that the soil structure and the tunnel-induced destructuration significantly affects the magnitude and shape of the short-term surface settlement trough and horizontal displacement in Shanghai soft clay. The pore pressure variations around the tunnel are also affected by soil structure, which will significantly influence the long-term ground consolidation settlement in Shanghai soft clay.     

Aquifer characteristics and water quality of Miocene–Pleistocene sediments, Kuwait

Al-Atraf is one of the water well fields of Kuwait supplying Kuwait City with the brackish groundwater obtained from the Kuwait Group aquifer of Miocene–Pleistocene age. The study determined the hydrogeological and hydrochemical characteristics of the groundwater in order to identify the major chemical processes that influence the groundwater quality of the study area. The results of the aquifer test analyses indicate that the Kuwait Group is a confined to semi-confined aquifer, with a transmissivity ranging between 62 and 321 m²/day. The flow net analysis implied that the groundwater moves from southwest to northeast. The estimated transmissivity values agree well with those calculated from the aquifer test. The results of the chemical analyses data of the Al-Atraf field show that the groundwater is mainly brackish, of NaCl and Na₂SO₄ water types. The groundwater is undersaturated with respect to halite, gypsum and anhydrite and supersaturated with respect to quartz, dolomite and calcite in the direction of groundwater flow. The average Pco₂ of the groundwater is higher than the Pco₂ of the earths atmosphere indicating that the groundwater is supplied with CO₂ during the infiltration processes and dissolves the carbonate minerals under closed-system conditions.     

释水条件下地面沉降的滞后效应试验研究

 过度抽取地下水造成的地面沉降正逐渐成为一种严重的地质灾害。在现有成果的基础上,设计、研制地面沉降室内试验装置,并改进试样的饱和方法。使用该装置模拟释水引起的地面沉降,从地面沉降对水位变化的响应和地面沉降过程中的孔隙水压力变化两方面研究地面沉降的滞后性。分析结果得出：(1) 承压含水砂层和黏土层的变形均表现出滞后现象;(2) 相同试验条件下,黏土层变形的滞后时间随着土层厚度的增大而增加。地面沉降试验完成后,分别从黏土层的底部和顶部取出试样进行室内常规压缩固结试验,试验结果表明：抽水引起的地面沉降在垂直层面方向上是不均匀的,距离抽水含水层越近的部分其压缩程度越大。     

滨海相软土固结—渗透联合测定试验研究

将K0固结仪改装成高压固结—渗透联合测定试验仪,对上海地区滨海相原状软粘土试样、重塑软粘土试样和粉土试样进行了固结—渗透联合测定。对比分析了不同荷载和水头耦合作用下上海软土的变形及渗透特性,并对上海软土试样垂直向和水平向的e-k关系曲线进行了拟合,得到了孔隙比和渗透系数的关系表达式。试验结果表明：结构性对上海软土的固结...     

Nitrate reduction in a simulated free-water surface wetland system

The feasibility of using a constructed wetland for treatment of nitrate-contaminated groundwater resulting from the land application of biosolids was investigated for a site in the southeastern United States. Biosolids degradation led to the release of ammonia, which upon oxidation resulted in nitrate concentrations in the upper aquifer in the range of 65-400 mg N/L. A laboratory-scale system was constructed in support of a pilot-scale project to investigate the effect of temperature, hydraulic retention time (HRT) and nitrate and carbon loading on denitrification using soil and groundwater from the biosolids application site. The maximum specific reduction rates (MSRR), measured in batch assays conducted with an open to the atmosphere reactor at four initial nitrate concentrations from 70 to 400 mg N/L, showed that the nitrate reduction rate was not affected by the initial nitrate concentration. The MSRR values at 22 °C for nitrate and nitrite were 1.2 ± 0.2 and 0.7 ± 0.1 mg N/mg VSS_COD-day, respectively. MSRR values were also measured at 5, 10, 15 and 22 °C and the temperature coefficient for nitrate reduction was estimated at 1.13. Based on the performance of laboratory-scale continuous-flow reactors and model simulations, wetland performance can be maintained at high nitrogen removal efficiency (>90%) with an HRT of 3 days or higher and at temperature values as low as 5 °C, as long as there is sufficient biodegradable carbon available to achieve complete denitrification. The results of this study show that based on the climate in the southeastern United States, a constructed wetland can be used for the treatment of nitrate-contaminated groundwater to low, acceptable nitrate levels.     

上海结构性软黏土本构模型与试验验证

天然上海软黏土具有很强的结构性,传统的剑桥模型无法合理地对其进行描述,改进后的运动硬化结构模型不仅引入了描述土体结构性的状态变量,可以反映土体塑性应变发展过程中所产生的结构损伤对其力学行为的影响,还对原始运动硬化结构模型中的形状参数、塑性模量公式等进行了改进,能更好地描述上海软黏土的力学行为。基于改进之后的运动硬化结构模型,通过天然上海软黏土等压固结三轴不排水剪切试验对该模型进行参数标定,进而对偏压固结三轴不排水剪切试验与加-卸载等向压缩试验进行预测。结果表明:改进后的运动硬化结构模型可以更好地模拟上海软黏土在三轴试验中的结构损伤、应力路径以及应力应变关系,同时对加-卸载过程中土体的滞回特性也有较好的模拟效果。     

承压水降水引起周边环境变形的粘弹性分析

针对上海地区承压水降水引起的地面沉降问题,考虑上海地区软土的非线性渗流变形特征,建立了基坑降水与地面沉降三维粘弹性全耦合数学模型。采用3-D FEM,以上海M4号线董家渡修复工程基坑降水为例,利用抽水实验期间监测资料对计算参数进行了反演,进而对降水运行期间引起的地下水渗流场与地面沉降的变化趋势进行预测,并与实际情况进行了对比,其结果比较吻合。     

高温后上海软黏土的土-水特性及微观机理试验研究

土-水特征曲线(SWCC)是揭示非饱和土性状的主要本构关系,但对于100℃以上高温环境下软黏土的土-水特性研究很少。针对上海软黏土的常温(20℃)土样和105℃,150℃和200℃等高温加热4 h后的土样,采用气相法研究了高温作用后上海软黏土的土-水特征曲线随温度的变化,同时,通过扫描电镜(SEM)试验研究了高温后软黏土的土-水特性随温度变化的微观机理。试验结果表明100℃以上高温环境下上海软黏土的土-水特征曲线随温度升高呈逐渐下降趋势,说明随温度升高软黏土的持水能力下降,吸力降低,进气值减小。分析各温度下土样的SEM图像发现,土的总孔隙数量减少,但少量孔隙的孔径增大,从微观特征上说明了土-水特性随温度升高的变化趋势和变化机理。说明,孔隙数量和孔隙结构随温度升高发生的变化是导致软黏土的土-水特性随温度升高而变化的根本原因。     

地铁振动荷载下隧道周围饱和软黏土的孔压发展模型

以上海地铁2号线静安寺站-江苏路站区间隧道周围饱和软黏土为研究对象,以现场连续监测资料为基础,利用室内GDS(多功能循环三轴试验系统)试验仪器,探求地铁隧道周围不同深度饱和软黏土的孔隙水压力发展规律。通过室内GDS试验,提出了振动荷载作用下饱和软黏土中孔隙水压力发展模型,并利用现场实测资料对该模型进行修正,最终得到地铁行车荷载作用下饱和软黏土中孔隙水压力发展模型。     

锚拉式钢板桩坞壁原体观测研究

对外高桥造船基地二座特大型干船坞坞壁结构的受力状态和原位测试成果进行了分析研究,以便为上海地区软粘土中单锚钢板桩结构设计积累经验。通过对原位监测数据的筛选分析表明,上海地区软粘土所具有的流变特性对板式挡土围护结构的受力变形具有重要且共同的影响。对今后上海地区软粘土中单锚钢板桩结构设计提出了建议。     

高温后上海软黏土的物理性能试验研究

土的温度效应是岩土工程领域的热点问题,但对于100℃以上高温环境下土的物理性能的研究并不多。详细介绍了自行研制的高温加热试验装置,并利用该设备测定了上海软黏土样在105℃,120℃,150℃和200℃等高温加热2.5h和4.0 h后的体积变化、干密度和饱和度的变化,测定了105℃,150℃和200℃下恒温4.0 h土的导热情况。试验结果表明土样的体积和饱和度均随温度增加而减小,土样的干密度随温度的增加而增加,且随加热时间增加变化趋缓;湿土和干土的导热系数均随温度升高而增大,且干土的导热系数小于湿土的导热系数。说明软黏性土在100℃以上高温环境作用下会出现塑性变形和加热硬化现象,高温对土的导热系数影响显著。     

上海软土的非饱和三轴强度

由于吸力的影响,非饱和土的力学性状与饱和土有着很大的差异,饱和土的有关强度理论无法简单地运用于解决非饱和土问题。本文通过室内吸力控制的非饱和土三轴剪切试验,研究上海地区软土的强度特征。结果表明:上海软土(④层灰色淤泥质粘土)非饱和强度在低吸力范围内,与吸力基本呈线性增长关系,验证了非饱和土的双变量强度理论的适用性;上海地区第④层灰色淤泥质粘土的非饱和土抗剪强度参数?b约为10.6°。随着非饱和土问题的研究的逐步深入,非饱和土强度理论在上海地区软土地层中的应用,还有待进一步研究和完善。     

考虑地层结构时地面沉降滞后性的模拟分析

含水层的地层结构在一定条件下有可能是影响地面沉降的重要因素。本文以某地实测地面沉降观测资料和地下水位数据为依据,考虑高压缩性和低渗透性的粘土在合水层中形成厚夹层时,应用模拟技术分析研究其对地面沉降的滞后效应。     

上海某枢纽基坑工程浅层承压水回灌试验分析

随着上海城市建设进程的不断推进,深大基坑大量涌现,同时因城市立体空间设施的密集化,使得基坑周边环境趋于复杂,在此背景下因浅层承压水降水而引起的工程性地面沉降事故不断发生,给社会带来了巨大的经济损失。地下水人工回灌是控制因降水引起的工程性地面沉降的方法之一,目前地下水人工回灌系统主要应用于深层承压水的回灌。本文通过上海某地第一承压含水层的回灌试验,研究分析了浅层承压含水层回灌井的结构、回灌过程中地下水的流态变化规律、水土应力变化及对控制地面沉降作用的效率,证明了浅层承压水回灌在基坑环境保护中的可行性。试验研究对基坑建设中的地下水浅层承压水回灌系统的设计和施工有重要的指导意义。     

Performance enhancement of encased stone column with conductive natural geotextile under k0 stress condition

In this study, the performance of the stone column encased with conductive jute geotextile in improving the characteristics of soft clay under k₀ stress condition was evaluated using a custom-designed large-scale consolidation test setup. The electrokinetic-encased stone column (e-ESC) was designed as a cathode, while mild steel rods were chosen as anodes, and by applying a voltage gradient of 0.1 V/mm between the electrodes, electrokinetic processes were initiated. The efficacy of using the e-ESC in improving the performance of soft clay was assessed by comparing the results with an ordinary stone column (OSC) and encased stone column (ESC) reinforced soft clay. The results from the study reveal that the inclusion of stiffer material like OSC and ESC in soft clay has significantly increased undrained shear strength and modulus of subgrade reaction of the composite ground with a reduction in settlement and compression index. Due to the application of the voltage gradient between the electrodes, the rate of consolidation settlement of composite ground has significantly increased when compared to the OSC/ESC case. Further, it was confirmed from the chemical and mineralogical analysis that the pH, chemical composition, mineral phases, and microfabric of soft clay were altered with the voltage gradient.