Palaeo-hydrogeological control on groundwater As levels in Red River delta,Vietnam

To study the geological control on groundwater As concentrations in Red River delta, depth-specific groundwater sampling and geophysical logging in 11 monitoring wells was conducted along a 45 km transect across the southern and central part of the delta, and the literature on the Red River delta’s Quaternary geological development was reviewed. The water samples (n = 30) were analyzed for As, major ions, Fe²⁺, H₂S, NH₄, CH₄, δ¹⁸O and δD, and the geophysical log suite included natural gamma-ray, formation and fluid electrical conductivity. The SW part of the transect intersects deposits of grey estuarine clays and deltaic sands in a 15–20 km wide and 50–60 m deep Holocene incised valley. The NE part of the transect consists of 60–120 m of Pleistocene yellowish alluvial deposits underneath 10–30 m of estuarine clay overlain by a 10–20 m veneer of Holocene sediments. The distribution of δ¹⁸O-values (range −12.2‰ to −6.3‰) and hydraulic head in the sample wells indicate that the estuarine clay units divide the flow system into an upper Holocene aquifer and a lower Pleistocene aquifer. The groundwater samples were all anoxic, and contained Fe²⁺ (0.03–2.0 mM), Mn (0.7–320 μM), SO₄ (<2.1 μM–0.75 mM), H₂S (<0.1–7.0 μM), NH₄ (0.03–4.4 mM), and CH₄ (0.08–14.5 mM). Generally, higher concentrations of NH₄ and CH₄ and low concentrations of SO₄ were found in the SW part of the transect, dominated by Holocene deposits, while the opposite was the case for the NE part of the transect. The distribution of the groundwater As concentration (<0.013–11.7 μM; median 0.12 μM (9 μg/L)) is related to the distribution of NH₄, CH₄ and SO₄. Low concentrations of As (?0.32 μM) were found in the Pleistocene aquifer, while the highest As concentrations were found in the Holocene aquifer. PHREEQC-2 speciation calculations indicated that Fe²⁺ and H₂S concentrations are controlled by equilibrium for disordered mackinawite and precipitation of siderite. An elevated groundwater salinity (Cl range 0.19–65.1 mM) was observed in both aquifers, and dominated in the deep aquifer. A negative correlation between aqueous As and an estimate of reduced SO₄ was observed, indicating that Fe sulphide precipitation poses a secondary control on the groundwater As concentration.     

Probabilistic modelling of auto-correlation characteristics of heterogeneous slopes

Spatial variability of soil materials has long been recognised as an important factor influencing the reliability of geo-structures. This study stochastically investigates the influence of spatial variability of shear strength on the stability of heterogeneous slopes, focusing on the auto-correlation function, auto-correlation distance and cross-correlation between soil parameters. The finite element method is merged with the random field theory to probabilistically evaluate factor of safety and probability of failure via Monte-Carlo simulations. The simulation procedure is explained in detail with suggestions on improving efficiency of the Monte-Carlo process. A simple procedure to create cross-correlation between random variables, which allows direct comparison of the influence of each strength variable, is discussed. The results show that the auto-correlation distance and cross-correlation can significantly influence slope stability, while the choice of auto-correlation function only has a minor effect. An equation relating the probability of failure with the auto-correlation distance is suggested in light of the analyses performed in this work and other results from the literature.     

Predicting water flow in fully and partially saturated porous media: a new fractal-based permeability model

Hydrogeology Journal - Predicting the permeability of porous media in saturated and partially saturated conditions is of crucial importance in many geo-engineering areas, from water resources to...     

Tectonic implications of Mesozoic magmatism to initiation of Cenozoic basin development within the passive South China Sea margin

International Journal of Earth Sciences - The South China Sea (SCS) is one of the classical example of a non-volcanic passive margin situated within three tectonic plates of the Eurasian,...     

Flood vulnerability among rural households in the Red River Delta of Vietnam: implications for future climate change risk and adaptation

Natural Hazards - The Red River Delta (RRD) of Vietnam, one of the world’s most densely populated deltas, is already vulnerable to flooding events, and climate change forecasts project...     

Modeling of EM logging tools in arbitrary 3-D borehole geometries using PML-FDTD

We discuss the numerical modeling of logging-while-drilling (LWD) tools for hydrocarbon exploration in arbitrary three-dimensional geometries using a new finite-difference time-domain (FDTD) scheme in cylindrical coordinates. Two locally conformal FDTD (LC-FDTD) schemes are employed to simulate eccentric LWD tools in realistic logging environments. An anisotropic perfectly matched layer absorbing boundary condition extended to cylindrical coordinates is incorporated in the FDTD method to simulate unbounded geophysical formations. Frequency-domain data are obtained from the time-domain results using a ramp-modulated sinusoidal source and an efficient early-time extraction algorithm. The FDTD simulations are validated against both numerical mode matching and pseudoanalytical approaches and show very good agreement.     

Stochastic analysis of unsaturated seepage through randomly heterogeneous earth embankments

Spatial variability of material properties is inherent in both natural soil deposits and earth structures, yet it is often ignored during geotechnical design. With the objective of developing novel methods for assessing the effects of soil variability on groundwater flow, this study presents a stochastic finite element model of seepage through a flood defense embankment with randomly heterogeneous material properties. Stochastic modeling is undertaken by means of a Monte Carlo simulation which involves a large number of finite element analyses, each with randomly varied porosity at element level, which leads to a corresponding random variation of both permeability and water retention properties across the embankment domain. This provides a statistical distribution of responses, such as total flow rate and time to reach steady state, instead of a single deterministic result as in conventional studies of seepage through unsaturated heterogeneous soils. As the degree of heterogeneity increases, water tends to flow along the most permeable paths inside the soil mass, resulting in an irregular shape of the predicted wetting fronts and pore pressure contours. The mean and standard deviation of the computed quantities strongly depend on the statistics of the input porosity field. Simulations are also conducted to compare the statistical variation of flow rate with and without dependency of the water retention curve on porosity. With recent growth in computer speed, stochastic finite element models based on the Monte Carlo approach can become a powerful design tool, especially if a quantitative assessment of geotechnical risks is required. Copyright © 2011 John Wiley & Sons, Ltd.     

压密注浆工艺在处理浜填土地基中的应用

通过对压密注浆工艺的技术和经济分析比较,并介绍该工艺在处理浜填土地基中的成功经验,从而得出压密注浆工艺是一种造价低、施工操作简便、周期短、加固效果良好的处理地基方法。     

Soil redox conditions and plant–soil relationships in a micronesian mangrove forest

We examined characteristics of the soil environment related to redox in the rhizospheres of three mangrove species that commonly grow in similar hydrogeomorphic settings: Rhizophora apiculata BL, Bruguiera gymnorrhiza (L.) Lamk., and Sonneratia alba J. Redox potential (Eh) values and extractable P concentrations were significantly higher, whereas soluble reactive P (SRP) concentrations were significantly lower in Bruguiera plots than in either Rhizophora or Sonneratia plots. Correlations among Eh, SRP, pyrite and extractable P were significant only at plots with negative Eh values. Sonneratia and Bruguiera plots had significantly different soil redox conditions. Bruguiera plots were characterized by more oxidized soil conditions (ca. 80 mV), lower dissolved P concentrations in porewater (ca. 50%), and higher extractable P concentrations in soil (ca. 25%). Differences in Eh and P reflect real differences among species and appeared not to be associated with hydrology. Species-specific differences may be engendered by the trees themselves, or they may reflect microsite differences that have favored the germination, growth, and survival of particular mangrove species.