Effect of alkaline solutions on bentonite properties

The influence of alkaline aqueous solutions on the properties of bentonite was investigated to evaluate the performance of bentonitic engineered barriers when contacted with alkaline groundwater. Batch and hydraulic conductivity tests were conducted on Na-bentonite using six different alkaline aqueous solutions. For the batch tests, almost no change in the montmorillonite fraction of the bentonite was observed after reacting with alkaline solutions (pH = 8.4–13.1), regardless of the solution type. On the other hand, aluminosilicate minerals (e.g., albite) were dissolved and secondary minerals (e.g., anorthite) were formed in alkaline NaOH solutions (pH > 13). The cation (Ca or Na) concentration primarily affected the swelling properties of bentonite rather than the pH of the solution, which was comparable to the results of the hydraulic conductivity tests. For the Ca solutions, the hydraulic conductivity of the bentonite specimen to the 0.02 mol/L Ca(OH)₂ solution (6.5 × 10^?9 cm/s) was approximately an order of magnitude lower than that of the bentonite specimen to the 0.02 mol/L Ca(OH)₂ + 1 mol/L CaCl₂ solution (5.0 × 10^?8 cm/s), whereas the hydraulic conductivity to the 0.02 mol/L Ca(OH)₂ + 1 mol/L CaCl₂ solution (pH = 11.3) (5.0 × 10^?8 cm/s) was slightly higher than that to the 1 mol/L CaCl₂ solution (pH_i = 8.4) (4.4 × 10^?8 cm/s). For the NaOH solutions with pH > 13, the hydraulic conductivity of the bentonite specimen decreased with increasing Na concentration, suggesting that the effect of Na concentration was more dominant than that of permeant pH.     

Seismic Network,Seismic Activity and Seismological Research of Vietnam

In this paper,a historical review of the seismic network of Vietnam and the ability of the network to monitor earthquakes is presented.The seismic activity of Vietnam as a country of high seismictiy in the last century and since the beginning of the new century is described.The current state of seismological research in Vietnam and cooperation with institutions outside of Vietnam is outlined.     

A review of methods of parameter estimation for the extreme value type-1 distribution

Four important methods of estimating the parameters of the extreme value type-1 (Gumbel) distribution, namely: (1) moments (MMM); (2) maximum likelihood (MML); (3) maximum entropy (MME); and (4) probability weighted moments (PWM), are considered, suitable solution procedures are recommended and related problems discussed. Approximate formulae for computing the variances-covariances of the estimators by each method are given in terms of the population value of the scale parameter, and these are shown to indicate very well the performance of the related method.

A simulation study to evaluate the performance of these methods in terms of commonly used criteria, i.e. the bias, root mean square error and goodness-of-fit statistic is described. It was found that the MMM is not as good as the remaining three methods, of which the PWM is best in terms of the bias, and the MML is best in terms of the root mean square error and efficiency. The MME, by all the criteria, ranks second best and follows the MML more closely than the PWM.     

Canal-side highway in Ho Chi Minh City (HCMC), Vietnam – Issues of urban cultural conservation and tourism development

Since the city’s founding back in 1698 on the old Khmer city of Prei Nokor, rivers and canals have always been Ho Chi Minh City’s (HCMC’s) primary means of transportation. Despite the decline of the utilization of the city’s abundant waterways due to the increasing development of modern aerial services and more convenient roads, the HCMC’s canal-based urban form, with its water-based socio-economic structures that include port-boat-market activities and canal handcraft villages, still processes a rich cultural and religious diversity that dates back to the history of the city itself. Currently, developments in the city’s transportation systems are having a damaging impact on the canals’ cultural background. Rather than being recognized as a significant cultural asset that identifies the city, the canal system has been physically reorganized as the city’s drainage system and valued as only a supplementary landscape element; canal communities have been evicted, and canal built fabric has been removed due to recent canal-side transportation developments. To revitalize the urban cultural identity that is possessed by the canal system and to enrich it as a contemporary valuable layer in the city’s dynamic regional development, it is critical to implement cultural conservation and tourism in the development of canal-side transportation and supplementary projects in the canal vicinity.     

Occurrence of phthalate esters around the major plastic industrial area in southern Taiwan

The increasing usage and disposal of plastic products could cause the wide distribution of phthalate esters (PAEs) in various environmental media. In this study, six PAE compounds, namely dimethyl phthalate, diethyl phthalate, di-n-butyl phthalate, benzyl butyl phthalate, di(2-ethylhexyl) phthalate (DEHP), di-n-octyl phthalate, were analyzed in various samples collected from the major plastic industrial area of southern Taiwan, including soil, fertilizer and plastic products, for the purposes of identifying of the possible sources of PAEs and assessing the related health risk. The results show that PAEs in soil samples was dominated by DEHP, with the total concentrations in the range of 0.7?±?0.5, 0.2?±?0.1, and 0.3?±?0.2 mg kg^?1 for soil samples from farmland, household back gardens and the roadside, respectively. Contents of PAEs in chemical fertilizer (ND—0 0.87 mg kg^?1) were higher than that in organic fertilizer (ND—0.08 mg kg^?1), and PAEs concentrations (ND—316 mg kg^?1) in plastic mulching films were much less than those in the other types of plastic products (ND—1719 mg kg^?1), implying that major sources of PAEs in agricultural soil could be the use of chemical fertilizer and plastic products other than plastic mulching films. Health risk assessment suggested that, via the exposure to PAEs in soil, the potential non-cancer and carcinogenic risks for adults and children are minimal in most cases, except that a “moderate” carcinogenic risk for children exposure to DEHP. The results of this study can serve as a reference for further pollution prevention and environmental protection plans in relation to the industrial operation and discharge as well as the farming practices.     

Topography and landslides in weathered granitic rock areas—Hai Van landslide in central Vietnam

Many landslides occur every year during heavy rains at the Hai Van Pass and surrounding area in central Vietnam, where granitic rocks are distributed. As is common in granite areas, these landslides often occur as small-scale flow-type and slump-type landslides. However, several horseshoe-shaped loose slopes of widths and lengths of 500 to 800 m, which incorporate these landslides, are observed on slopes across the area. These topographies resemble those formed by past and present large-scale landslides. The presence of such a topography and the repeated occurrences of landslides within this topography are rare in granite areas, where shallow flow-type landslides are generally frequent. To understand the mechanism causing the landslides in the Hai Van region, and as a support for future risk assessment, the factors and processes leading to the formation of such a topography and their relationship with these landslides must be identified and assessed. This study investigated the history of past landslide movement in the Hai Van Pass and surrounding area through observations of drill cores and outcrops, and analysis of the direction of remanent magnetism in the granitic rocks. Mineral compositions, cracks, degrees of weathering, and topographic shapes of the granitic rocks and their relationship to the landslides occurring today were also investigated. The results of the study reveal no variation in the direction of remanent magnetism in the granitic rocks in the region that would indicate disturbance of the ground due to a past large-scale landslide. No evidence of such an event could also be found both in the drill cores and the rock outcrops. Further, results of the analysis of cracks and weathering pattern confirm that the topography of the region is affected by the weathering of the granitic rocks that progresses in concentric circles of various sizes. Thus, it can be concluded that these topographies were not formed by a singular large-scale landslide of the past, but rather by a composite of relatively shallow landslides occurring on the slope of dome structures unique to granite areas, which are formed by differential weathering and denudation regulated by cracks.     

Methodology for evaluation of potential sites for large-scale riverbank filtration

Despite being a simple and inexpensive pretreatment technology, the cost-effectiveness of riverbank filtration (RBF) depends on complex hydrogeological and hydrogeochemical variables. One of the most important issues for decision makers regarding RBF is optimal site selection. Therefore, a methodology for multicriteria site evaluation for large-scale RBF schemes is offered. The methodology is primarily designed as a prescreening method, applied over a wide area, but can also serve as a guide for evaluating individual RBF sites. To facilitate further discussion about improvements on the methodology, the reasoning behind each relevant factor and its weight in the evaluation is presented. The methodology is divided into three sequential steps through which a site can be assessed. The first step is to establish the existence of connectivity between the river and aquifer. This is termed the essential criterion, and is a binary determination of site suitability. If the site is determined to be suitable, it is then assessed via a set of quantity criteria, which measure the aquifer capacity and amount of bank filtrate that can be effectively abstracted. Lastly, water quality criteria are assessed by means of surface-water and groundwater quality. The quantity and quality criteria form a result expressed as the site suitability index (SSI), which ranges from 0 to 1, where higher scores represent increased suitability. Finally, the methodology is applied to evaluate existing sites of large-scale RBF application as a demonstration of its applicability. The success of these existing sites is compared to the calculated SSI value and discussed.

     

Effect of fines content on soil freezing characteristic curve of sandy soils

Acta Geotechnica - Soil freezing characteristic curve (SFCC) represents the relationship between soil temperature and unfrozen water content of soil during freezing and thawing processes. In this...     

Regional variations in trace element concentrations in tissues of black tiger shrimp Penaeus monodon (Decapoda: Penaeidae) from South Vietnam

The goal of the present study was to examine the specific bioaccumulation of 22 trace elements in muscle, exoskeleton and hepatopancreas of black tiger shrimp Penaeus monodon from the Mekong River Delta (MRD), and the South Key Economic Zone (SKEZ), South Vietnam. The general tendency in most trace element concentrations among different tissues were hepatopancreas>exoskeleton>muscle. Comparisons of trace element levels in tissues between the two regions showed that concentrations of Se in muscle and As in all three tissues were higher in SKEZ; whereas in MRD, the higher concentrations of most elements such as Mn, Cu, Cd, Ba, Hg, were observed in tissues. These geographical variations in trace element levels may reflect the differences in human activities between the two regions of South Vietnam. The target hazard quotient (THQ) values for trace elements (<1) indicate that local residents are not exposed to potential health risks via the consumption of shrimp.     

Regional variations in trace element concentrations in tissues of black tiger shrimp Penaeus monodon (Decapoda: Penaeidae) from South Vietnam

The goal of the present study was to examine the specific bioaccumulation of 22 trace elements in muscle, exoskeleton and hepatopancreas of black tiger shrimp Penaeus monodon from the Mekong River Delta (MRD), and the South Key Economic Zone (SKEZ), South Vietnam. The general tendency in most trace element concentrations among different tissues were hepatopancreas > exoskeleton > muscle. Comparisons of trace element levels in tissues between the two regions showed that concentrations of Se in muscle and As in all three tissues were higher in SKEZ; whereas in MRD, the higher concentrations of most elements such as Mn, Cu, Cd, Ba, Hg, were observed in tissues. These geographical variations in trace element levels may reflect the differences in human activities between the two regions of South Vietnam. The target hazard quotient (THQ) values for trace elements (<1) indicate that local residents are not exposed to potential health risks via the consumption of shrimp.