Sea surface temperature trend analysis by Mann-Kendall test and sen’s slope estimator: a study of the Hai Phong coastal area (Vietnam) for the period 1995-2020

Based on the Mann-Kendall test and Sen’s slope method, this study investigates the monthly, seasonal, and annual sea surface temperature (SST) trends in the coastal area of Hai Phong (West of Tonkin Gulf) based on the measurements at Hon Dau Station from 1995 to 2020. The results show a sea surface warming trend of 0.02°C/year for the period 1995-2020 (significant level α = 0.1) and of 0.093°C/year for the period 2008-2020 (significant level α = 0.05). The monthly SSTs in June and September increased by 0.027°C/year and 0.036°C/year, respectively, for the period 1995-2020, and by 0.080°C/year and 0.047°C/year, respectively, for the period 2008-2020. SST trends in winter, summer, and other months were either different for the two periods or not significant enough. This may be due to the impact of ENSO, which caused interannual SST variability in the Hai Phong coastal with two intrinsic mode functions (IMF) signals a period of ~2 (IMF3) and ~5.2 years cycle (IMF4). A combination of these signals had a maximum correlation of 0.22 with ONI (Oceanic Niño Index) delayed by 8 months. ENSO events took ~8 months to affect SST at Hai Phong coastal area for 1995-2020 and caused a variation of SST within 1.2°C.

Recharge mechanism and salinization processes in coastal aquifers in Nam Dinh province, Vietnam

In Nam Dinh province, in the Red River delta plain in Northern Vietnam, groundwater in the shallow Holocene aquifer shows elevated total dissolved solids up to 35 km from the coastline, indicating a saltwater intrusion from the Gulf of Tonkin. High groundwater salinities have been encountered below and adjacent to the Red River in the deep Pleistocene aquifer. Since 1996, large-scale groundwater abstraction was initiated from the deep aquifer, and the observed elevated salinities now raise concerns about whether the groundwater abstraction is undertaken sustainably. We have conducted a study to obtain a fundamental understanding of the recharge mechanisms and salinization processes in the Nam Dinh province. A holistic approach with multiple methods including transient electromagnetic sounding and borehole logging, exploratory drilling, sampling and analyzing primary ion and stable isotope compositions of water and pore water, groundwater head monitoring, hydraulic experiments laboratory of clay layers, and groundwater modeling by using the SEAWAT code. Results reveal that saline river water is leached from the Red River and its distributaries into the shallow aquifers. The distribution and occurrence of salty pore water in the Holocene aquitard clay shows that meteoric water has not been flowing through these low permeable clay layers. Marine pore water has, however, been leached out of the Pleistocene clay. When this layer is present, it offers protection of the lower aquifer against high salinity water from above. Salinity as high as 80 % of oceanic water is observed in interstitial pore water of the transgressive Holocene clay. Saltwater is transported into the Pleistocene aquifer, where the Holocene clay is directly overlying the aquifer.

Improvements of 210Po determination method in thermal water samples

Determination of naturally radionuclides have been known well as an important topic in environmental study in recently. One of the most toxic radioisotope in nature, a daughter product of 238U decay chain is 210Po (polonium). The improvement and optimizations methods for determination of this attractive isotope are still presenting so far. In this paper, a new improved method was elaborated for 210Po determination in thermal water sample. In the proposed method, analytical optimization of spontaneous/auto deposition does not use Teflon cup, magnetic stirring or any preparing equipment/item only normal glass and a side of square silver. In addition, the optimization was neglected with absent of purification of polonium (Liquid-liquid extraction methods/Ion exchange chromatography/Extraction chromatographic separations). The outcome of optimal procedure were simplify, less time consuming, great reduction of costs with chemical recovery >80% and could apply for any liquid environmental samples.

Geochemistry, zircon U-Pb geochronology and Sr-Nd-Hf isotopic composition of the Cha Val plutonic rocks in central Vietnam: Implications for Permian-Triassic Paleo-Tethys subduction-related magmatism

together with abundant Permian-Triassic magmatic rocks. This magmatic complex provides important information to reconstruct the tectonic evolution of the Indochina block and surrounding areas. The Cha Val plutonic rocks mainly comprise diorite, quartz diorite, and granodiorite. Geochemically, they are metaluminous with low A/CNK (0.49 to 1.16 with an average of 0.85), medium to high K, low to medium SiO2, and Na2O/K2O>1. Trace and rare earth element compositions display enrichment in Cs, U, Pb, and Nd, but depletion in Ba, Nb, Ta, P, Eu, and Ti, similar to those of continental arc-related magmas. Rock-forming minerals of the Cha Val plutonic rocks are characterized by abundant hornblende. All observed petrographical and geochemical characteristics suggest that the Cha Val plutonic rocks are typical for I-type affinity generated from a subduction regime. LA-ICP-MS U-Pb zircon analyses of three representative samples yielded their crystallization ages between 258.0 Ma and 248.9 Ma, temporally coeval with Late Permian-Early Triassic magmatism previously reported in the Truong Son belt. The (87Sr/86Sr)i ratios (0.7081 to 0.7244), negative whole-rock εNd(t) values (-4.5 to -2.9), zircon εHf(t) values (-1.04 to 2.71), and whole-rock Nd and zircon Hf model ages (TDM2) (1394 Ma to 1111 Ma) indicate that the Cha Val plutonic rocks are derived from melting of Mesoproterozoic crustal materials with a minor contribution of mantle-derived melt. Together with other Permian-Triassic magmatic complexes along the Song Ma suture zone and the Truong Son Belt, the Cha Val plutonic rocks are a representative of magmatism associated with the subduction-collision that amalgamated the South China and Indochina blocks after the closure of a branch of Paleo-Tethys along the Song Ma suture zone during the Late Permian-Early Triassic Indosinian orogeny.

Pore water pressure responses of saturated sand and clay under undrained cyclic shearing

In this study, changes in the pore water pressure were observed for saturated specimens of a loose fined-grain sand (Nam O sand) and a soft silty clay (Hue clay) subjected to undrained cyclic shearing with different testing conditions. The cyclic shear tests were run for relatively wide range of shear strain amplitude (g = 0.05%-2%), different cycle numbers (n = 10, 50, 150 and 200) and various shear directions (uni-direction and two-direction with phase difference of q = 0o, 45o and 90o). It is indicated from the experimental results that under the same cyclic shearing condition, the pore water pressure accumulation in Hue clay is at a slower rate, suggesting a higher cyclic shear resistance of Hue clay than that of Nam O sand. Liquefaction is reached easily in nominally 50% relative density specimens of Nam O sand when g ³ 0.4%, meanwhile soft specimen of Hue clay is not liquefied regardless of the cyclic shearing conditions used in this study. The threshold number of cycles for the pore water pressure generation generally decreases with g meanwhile, the threshold cumulative shear strain for such a property mostly approaches 0.1%. In addition, by using this new strain path parameter, it becomes more advantageous when evaluating the pore water pressure accumulation in Nam O sand and Hue clay subjected to undrained uni-directional and two-directional cyclic shears.

Determination of actual characteristic curve of main ventilation fan at Quang Ninh underground coal mines using field measurement method

Determining a proper operation mode of the main ventilation fan at an underground coal mine primarily uses the theoretical characteristic curves of the fan’s manufacturer. Because these curves are developed in laboratory-standard conditions, the characteristic curves under different conditions in practice significantly change, seriously impacting the ventilation efficiency and environmental safety of mine. This paper presents a determination of the main fan's actual characteristic curve using a field measurement method. The method involves the (i) simultaneous measurement of airflow and air pressure at designated locations in fan drift and ventilation crosscut and (ii) statistical analysis and interpolation of the measured data. The results show that the fan actual pressure curve is permanently displaced to the left and steeper than the corresponding theoretical pressure curve in an on-site operating mode. The finding points out that on-site fans operate in overload mode that can quickly damage their mechanical components. This method provides mining engineers with an easy-to-apply tool for proper adjustment of the operation mode. This improves ventilation efficiency, increases environmental safety, and reduces the underground coal mine operational costs.

Flood susceptibility modeling using Radial Basis Function Classifier and Fisher’s linear discriminant function

Floods are among the most frequent highly disastrous hazards affecting life, property, and the environment worldwide. While various models are available to predict flood susceptibility, no model is accurate enough to be used for all flood-prone areas. Model development using different algorithms is a continuous process to improve the prediction accuracy of flood susceptibility. In the study, we used the Radial Basis Function and Fisher’s linear discriminant function to develop a flood susceptibility map for a case study of Quang Binh Province. The model development used ten variables (elevation, slope, curvature, river density, distance from river, geomorphology, land use, flow accumulation, flow direction, and rainfall). For model training and validation, input data was split into a 70:30 ratio according to flood locations. Statistical indexes were used to evaluate model performance such as Receiver Operating Characteristic, the Area Under the ROC Curve, Root Mean Square Error, Accuracy, Sensitivity, Specificity, and Kappa index. Results indicated that the radial basis function classifier model had better performance in predicting flood susceptible areas based on the statistical measures (PPV = 92.00%, NPV = 87.00%, SST = 87.62%, SPF = 91.58%, ACC = 89.50%, Kappa = 0.790, MAE = 0.204, RMSE = 0.292 and AUC = 0.957. Therefore, the radial basis function classifier algorithm model is appropriate for predicting flood susceptibility in Quang Binh Province.

Evaluation of the Global Satellite Mapping of Precipitation (GSMaP) data on sub-daily rainfall patterns in Vietnam

This study aims to evaluate the performance of Global Satellite Mapping of Precipitation (GSMaP) data in observing the sub-daily rainfall patterns in Vietnam using synoptic gauge measurements considering seasonal variations in rainfall. Differences in the estimations of the three GSMaP products, including the standard version 7 (MVKv7) and two gauge-calibrated versions 6 and 7 (GAUv6 and GAUv7), were clarified based on rainfall characteristic parameters and statistical indices. The present study clarified that the contribution of sub-daily rainfall in Vietnam was higher during the afternoon than at other times, predominantly in the Central Highlands and Southern Plain, while it occurred most often during the evening to early morning in northern regions. Distinct regional features were also identified along the central coast. Most of the summer afternoon maximum fell in the western mountainous area, while the eastern coastal plain experienced an insignificant amount of rainfall. As rainfall characteristics varied with seasons and regions, the performance of GSMaP demonstrated this variation quite well compared to in-situ observations. However, GSMaP still exhibited high biases in rainy season and topographically heterogeneous areas, especially in the northern regions where sub-daily rainfall cycles had large variations. The standard GSMaP (MVK) product illustrated an afternoon peak better than the gauge-calibrated (GAU) product, which suggests that utilizing the MVK in the Central Highlands and Southern Plain and the GAU over central coastal regions would be appropriate when considering the characteristics of sub-daily precipitation.

Assessment and reduction of zenith path delay biases due to day boundary effect

The troposphere consists of dry air and water vapor, delaying the GNSS signal by about 2.4 m in the zenith direction. The water vapor only causes an error of about 0.2 m in distance measurement, but it is challenging to model and overcome. From 2003 the International GNSS Service (IGS) started to provide the new product of zenith path delay (ZPD) with an accuracy of 1.5-5 mm. However, we found an error in these products up to 30 mm at epochs between 2 days due to the day boundary effect and an average of 16mm RMS for nine days. Our research shows that for reducing the impact, the most critical factor is selecting the initial value for the ZPD, followed by satellite orbit/clock and, finally, the station coordinate values. By choosing an appropriate initial value for ZPD and employing a 3 days orbit/clock, the ZPD error due to the day boundary effect can be reduced to negligible. Meanwhile, the change in the station coordinate value in cm level does not impact the effect.

Geochemistry and mineralogy of the Truc Thon Clay, Hai Duong Province, North Vietnam: implication for paleoclimatic and provenance analysis

During the Miocene-Pleistocene, generally sub-tropical to tropical warm and humid paleoclimate prevailed in Southeast Asia with a gradual cooling trend. The Truc Thon clay (TTC) mine presents interesting outcrops for sedimentological and provenance analysis. The present study uses results of geological investigation in 16 outcrops and wells at the clay mine Truc Thon. The TTC has lens-shaped geometry, filled with two clay bodies, including grey-white clay and multicolor clay. Bulk mineralogy indicates the predominance of quartz and a relatively high amount of pyrophyllite. Clay mineralogy shows the presence of main kaolinite, followed by illite and mixed-layer illite-smectite. These may interpret a warm, humid paleoclimatic condition in the source areas. Illite may be inherited from basement rocks. The bulk rock geochemistry supports intense chemical weathering with the Chemical Index of Alteration (CIA) in the TTC ranged ca. 80.6-98 (average = 90.4). In combination with the geochemical proxies and the mineralogical composition of the TTC, the chemical weathering intensity indicated warm/hot, semi-humid/humid tropical paleoclimate in the source area. A combination of mineralogical and geochemical analyses with regional geological features show that the Hon Gai Triassic rocks are the main source for the TTC. Source materials are originally related to silicic rocks of granitic-granodioritic composition. Paleoclimatic conditions favored intense chemical weathering of the Hon Gai Triassic rocks and surrounding rocks, creating a ceramic mine of great industrial value.