Here new data from field bioremediation experiments and geochemical modeling are reported to illustrate the principal geochemical behavior of As in anaerobic groundwaters. In the field bioremediation experiments, groundwater in Holocene alluvial aquifers in Bangladesh was amended with labile water-soluble organic C (molasses) and MgSO4 to stimulate metabolism of indigenous SO4-reducing bacteria (SRB). In the USA, the groundwater was contaminated by Zn, Cd and SO4, and contained <10 μg/L As under oxidized conditions, and a mixture of sucrose and methanol were injected to stimulate SRB metabolism. In Bangladesh, groundwater was under moderately reducing conditions and contained ∼10 mg/L Fe and ∼100 μg/L As. In the USA experiment, groundwater rapidly became anaerobic, and dissolved Fe and As increased dramatically (As > 1000 μg/L) under geochemical conditions consistent with bacterial Fe-reducing conditions. With time, groundwater became more reducing and biogenic SO4 reduction began, and Cd and Zn were virtually completely removed due to precipitation of sphalerite (ZnS) and other metal sulfide mineral(s). Following precipitation of chalcophile elements Zn and Cd, the concentrations of Fe and As both began to decrease in groundwater, presumably due to formation of As-bearing FeS/FeS2. By the end of the six-month experiment, dissolved As had returned to below background levels. In the initial Bangladesh experiment, As decreased to virtually zero once biogenic SO4 reduction commenced but increased to pre-experiment level once SO4 reduction ended. In the ongoing experiment, both SO4 and Fe(II) were amended to groundwater to evaluate if FeS/FeS2 formation causes longer-lived As removal. Because As-bearing pyrite is the common product of SRB metabolism in Holocene alluvial aquifers in both the USA and Southeast Asia, it was endeavored to derive thermodynamic data for arsenian pyrite to better predict geochemical processes in naturally reducing groundwaters. Including the new data for arsenian pyrite into Geochemist’s Workbench, its stability field completely dominates in reducing Eh–pH space and “displaces” other As-sulfides (orpiment, realgar) that have been implied to be important in previous modeling exercises and reported in rare field conditions. 相似文献
In recent decades, landslide disasters in the Himalayas, as in other mountain regions, are widely reported to have increased. While some studies have suggested a link to increasing heavy rainfall under a warmer climate, others pointed to anthropogenic influences on slope stability, and increasing exposure of people and assets located in harm’s way. A lack of sufficiently high-resolution regional landslide inventories, both spatially and temporally, has prevented any robust consensus so far. Focusing on Far-Western Nepal, we draw on remote sensing techniques to create a regional inventory of 26,350 single landslide events, of which 8778 date to the period 1992–2018. These events serve as a basis for the analyses of landslide frequency relationships and trends in relation to precipitation and temperature datasets. Results show a strong correlation between the annual number of shallow landslides and the accumulated monsoon precipitation (r = 0.74). Furthermore, warm and dry monsoons followed by especially rainy monsoons produce the highest incidence of shallow landslides (r = 0.77). However, we find strong spatial variability in the strength of these relationships, which is linked to recent demographic development in the region. This highlights the role of anthropogenic drivers, and in particular road cutting and land-use change, in amplifying the seasonal monsoon influence on slope stability. In parallel, the absence of any long-term trends in landslide activity, despite widely reported increase in landslide disasters, points strongly to increasing exposure of people and infrastructure as the main driver of landslide disasters in this region of Nepal. By contrast, no climate change signal is evident from the data.
Himalayan orogenic belt is the highest and largest continental collision and subduction zone on the Earth. The Himalayan orogenic belt has produced frequent large earthquakes and caused several geohazards due to landslides and housing collapse, having an impact on the safety of life and property along a length of over 2500 km. Here we took three earthquake clusters as examples, which occurred at Nepal Himalaya, eastern Himalayan syntaxis and western Himalayan syntaxis, respectively. Here we calculated the earthquake locations and fault plane solutions based on the waveform data recorded by seismic stations deployed in source areas by the Institute of Tibetan Plateau Research, Chinese Academy of Sciences. We found that at the Nepal Himalayan, the Main Himalayan Thrust is the major tectonic structure for large earthquakes to occur. At the eastern Himalayan syntaxis, most earthquakes are of the reverse or strike-slip faulting. The major tectonic feature is the combination of the NE-dipping thrust with the southeastern escape of the Tibetan plateau. At the western Himalayan syntaxis, intermediate-depth earthquakes are active. These observations reveal the geometry of the deep subduction of the continental plate with steep dipping angle. 相似文献
In the Himalaya, people live in widely spread settlements and suffer more from landslides than from any other type of natural
disaster. The intense summer monsoons are the main factor in triggering landslides. However, the relations between landslides
and slope hydrology have not been a focal topic in Himalayan landslide research. This paper deals with the contributing parameters
for the rainfall-triggered landslides which occurred during an extreme monsoon rainfall event on 23 July 2002, in the south-western
hills of Kathmandu valley, in the Lesser Himalaya, Nepal. Parameters such as bedrock geology, geomorphology, geotechnical
properties of soil, and clay mineralogy are described in this paper. Landslide modeling was performed in SEEP/W and SLOPE/W
to understand the relationship of pore water pressure variations in soil layers and to determine the spatial variation of
landslide occurrence. Soil characteristics, low angle of internal friction of fines in soil, medium range of soil permeability,
presence of clay minerals in soil, bedrock hydrogeology, and human intervention were found to be the main contributing parameters
for slope failures in the region. 相似文献
This paper describes the blind prediction carried out to simulate the response of a thin reinforced concrete wall tested under uni-directional (in-plane) quasi-static reverse cyclic loading. The specimen was a singly reinforced T-shaped wall panel with a shear-span ratio of 3.7. The response of the test specimen was simulated prior to the release of test results using a finite element model which had already been verified for its capabilities in capturing different failure patterns of rectangular walls, particularly out-of-plane instability. The numerical model predicted a flexural dominated response for the specimen accompanied by considerable out-of-plane deformations. The blind prediction report, submitted in advance to the principal investigator of the experimental campaign, included lateral load-top displacement response of the specimen, maximum out-of-plane deformation corresponding to each drift level, evolution of out-of-plane displacements throughout in-plane loading, response of the longitudinal reinforcement at the section exhibiting the maximum out-of-plane deformation, and von Mises as well as reinforcement stress distribution at some key points of the wall response. Furthermore, a parametric study was carried out addressing the effects of shear-span ratio, reinforcement eccentricity and axial load ratio on the wall response. Results of the numerical simulation that had been included in the blind prediction report have been compared with the experimental measurements indicating that the evolution of the out-of-plane deformation was well captured by the model. 相似文献
This paper proposes an enhancement to the current strength and confinement‐based design of transverse reinforcement in rectangular and circular reinforced concrete members to ensure that the flexural strength of reinforced concrete sections does not degrade excessively due to buckling of longitudinal bars until the desired level of plastic deformation is achieved. Antibuckling design criteria are developed based on a popular bar buckling model that uses a bar buckling parameter (combining the bar diameter, yield strength, and buckling length) to solely describe the bar buckling behavior. The value of buckling parameter that limits the buckling‐induced stress loss to 15% in compression bars at the strain corresponding to the design ductility is determined. For a bar of known diameter and yield strength, the maximum allowable buckling length can then be determined, which serves as the maximum limit for the tie/stirrup/hoop spacing. Lateral stiffness required to restrain the buckling tendency of main bars at the locations of the ties/stirrups/hoops depends on the flexural rigidity of the main bars and the buckling length (equal to or multiple of tie/hoop/stirrup spacing), whereas the antibuckling stiffness (ie, resistance) provided by the ties/stirrups/hoops depends on their size, number, and arrangement. Using the above concept, design recommendations for the amount, arrangement, and spacing of rectangular and circular ties/stirrups/hoops are then established to ensure that the antibuckling stiffness of the provided transverse reinforcement is greater than the stiffness required to restrain the buckling‐prone main bars. Key aspects of the developed method are verified using experimental tests from literature. 相似文献