Hydrogeochemical characteristics of a multi-layered coastal aquifer system in the Mekong Delta, Vietnam.

Groundwater is a primary freshwater source for various domestic, industrial and agricultural purposes, especially in coastal regions where there are lacking surface water supply. However, groundwater quality in coastal regions is often threatened by seawater intrusion and contamination due to both anthropogenic activities and natural processes. Therefore, insights into groundwater geochemistry and occurrences are necessary for sustainable groundwater management in coastal regions. The main aim of this study is to investigate the hydrogeochemical characteristics and their influencing factors in a coastal area of the Mekong Delta, Vietnam (MD). A total of 286 groundwater samples were taken from shallow and deep aquifers for analyzing major ions and stable isotopes. The results show that deep groundwater is dominated by Ca-HCO[Formula: see text], Ca-Na-HCO[Formula: see text], Ca-Mg-Cl, and Na-HCO[Formula: see text] while shallow groundwater is dominated by the Na-Cl water type. In this region, the main geochemical processes controlling groundwater chemistry are ion exchanges, mineralization and evaporation. Groundwater salinization in coastal aquifers of the Mekong Delta is caused by (1) paleo-seawater intrusion and evaporation occurring in the Holocene and Pleistocene aquifers, (2) dissolution of salt sediment/rock and leakage of saline from upper to lower aquifers due to excessive groundwater exploitation and hydraulic connection. High nitrate concentrations in both shallow and deep aquifers are related to human activities. These results imply that groundwater extraction may exacerbate groundwater quality-related problems and suitable solutions for sustainable groundwater management in the coastal area of the Mekong Delta are needed.

Quantification of Groundwater - Saline Surface Water Interaction in a Small Coastal Plain in North-East Tunisia using Multivariate Statistical Analysis and Geophysical Method.

Amultivariate statistical analysis used with geophysical investigation enabled the assessment of interaction between saline surface water and groundwater in the coastal plain of Wadi Al Ayn and Daroufa in CapBon peninsula, north-east Tunisia. The application of the Principal Component Analysis (PCA) identified the end members which contribute to groundwater recharge: the wastewater infiltrated from Wadi Al Ayn, the oilfield brine infiltrated to the aquifer through the sandy bed of Wadi Al Ayn, the intruded seawater near Wadi Daroufa in the downstream region and the fresh groundwater flowing from the upstream region. The contribution of wastewater in groundwater recharge varies from 1.4% in the upstream region of Wadi Al Ayn to 77% near its downstream part. The fraction of oilfield brine mixed with groundwater in the alluvial aquifer under Wadi Al Ayn varies from 1% to 13%; whereas the fraction of intruded seawater into the coastal part of the aquifer near Daroufa region varies from 2% to 21%.

Effects of metal pollution on sediments in a highly saline aquatic ecosystem: case of the Moknine continental Sebkha (eastern Tunisia).

Potential contamination of the sediments in the Sebkha of Moknine (Tunisia) ecosystem was assessed by means of enrichment factors (EFs) estimated against reference sediment located ≈70 km away from the Sebkha. The use of EFs is recommended as a reliable method for heavy metal contamination assessment, provided that (1) element contents are corrected following a careful normalization procedure, and (2) the reference sediment is unaffected by anthropogenic alterations. The degree of contamination was assessed by the modified contamination degree approach. The obtained results indicated that surface sediments were enriched up to 22.9, 13.2, 5.46 and 3.19 times with Pb, Cu, Cr and Zn, respectively. Lower enrichment factors for Ni and Co suggested that anthropogenic sources were less important for these metals. The modified degree of contamination showed that the sediments in the Sebkha of Moknine have suffered significant systematic heavy metal contamination following catchment urbanization and industrialization .

Time series analysis for the estimation of tidal fluctuation effect on different aquifers in a small coastal area of Saijo plain, Ehime prefecture, Japan.

Considering the current poor understanding of the seawater-freshwater (SW-FW) interaction pattern at dynamic hydro-geological boundary of coastal aquifers, this work strives to study tidal effect on groundwater quality using chemical tracers combined with environmental isotopes. In situ measurement data of electrical conductivity and groundwater level along with laboratory measurement data of hydro-chemical species were compared with tidal level data measured by Hydrographic and Oceanographic Department, Saijo City, Japan for time series analysis. Result shows that diurnal tides have significant effect on groundwater level as well as its chemical characteristics; however, the magnitude of effect is different in case of different aquifers. Various scatter diagrams were plotted in order to infer mechanisms responsible for water quality change with tidal phase, and results show that cations exchange, selective movement and local SW-FW mixing were likely to be the main processes responsible for water quality changes. It was also found that geological structure of the aquifers is the most important factor affecting the intensity of tidal effect on water quality.

Intensified salinity intrusion in coastal aquifers due to groundwater overextraction: a case study in the Mekong Delta, Vietnam.

Groundwater salinization is one of the most severe environmental problems in coastal aquifers worldwide, causing exceeding salinity in groundwater supply systems for many purposes. High salinity concentration in groundwater can be detected several kilometers inland and may result in an increased risk for coastal water supply systems and human health problems. This study investigates the impacts of groundwater pumping practices and regional groundwater flow dynamics on groundwater flow and salinity intrusion in the coastal aquifers of the Vietnamese Mekong Delta using the SEAWAT model-a variable-density groundwater flow and solute transport model. The model was constructed in three dimensions (3D) and accounted for multi-aquifers, variation of groundwater levels in neighboring areas, pumping, and paleo-salinity. Model calibration was carried for 13 years (2000 to 2012), and validation was conducted for 4 years (2013 to 2016). The best-calibrated model was used to develop prediction models for the next 14 years (2017 to 2030). Six future scenarios were introduced based on pumping rates and regional groundwater levels. Modeling results revealed that groundwater pumping activities and variation of regional groundwater flow systems strongly influence groundwater level depletion and saline movement from upper layers to lower layers. High salinity (>2.0 g/L) was expected to expand downward up to 150 m in depth and 2000 m toward surrounding areas in the next 14 years under increasing groundwater pumping capacity. A slight recovery in water level was also observed with decreasing groundwater exploitation. The reduction in the pumping rate from both local and regional scales will be necessary to recover groundwater levels and protect fresh aquifers from expanding paleo-saline in groundwater.

Radiocesium leaching from litter during rainstorms in the Fukushima broadleaf forest.

Forests are important sources of dissolved radiocesium (137Cs) discharge downstream. To improve understanding of dissolved 137Cs discharge processes during rainstorms, we investigated the relationship between rainfall-runoff hydrological processes and the discharge of 137Cs leached from litter. Leaching tests were conducted with broadleaf litter collected in the area where saturated overland flow was generated during rainstorms in a broadleaf-tree-dominated forest. According to the leaching test results, the 137Cs leaching rate was higher in the early stage of the test and decreased afterward. There was no significant difference in the overall results between the agitation and non-agitation cases. The 137Cs leaching rate from litter after the 24-h test was up to 33.7%. A large proportion of the original 137Cs activity was present even after the tests, as leaching from litter during rainstorms in the headwater area could be an additional source of dissolved 137Cs in the stream water. If mixing of 137Cs originating from groundwater, soil water, and rainfall with the hydrological processes is assumed, differences between the observed and estimated 137Cs in the surface runoff water became larger under high flow conditions. This analysis indicates additional 137Cs loading on surface runoff water during rainstorms, where saturated surface area can expand as the surface runoff rate increases. Contact area between surface runoff and litter accumulated on the forest floor should increase and accelerate 137Cs leaching from the litter. Therefore, 137Cs leaching in the saturated surface area that is temporarily formed during rainstorms can play a principal role in dissolved 137Cs discharge during rainfall-runoff events. Contaminated litter in the temporally saturated region of forested headwaters is an important factor contributing to elevated levels of dissolved 137Cs during rainstorms in the Fukushima area.

Tracing the factors responsible for arsenic enrichment in groundwater of the middle Gangetic Plain, India: a source identification perspective.

Arsenic contamination in groundwater is of increasing concern because of its high toxicity and widespread occurrence. This study is an effort to trace the factors responsible for arsenic enrichment in groundwater of the middle Gangetic Plain of India through major ion chemistry, arsenic speciation, sediment grain-size analyses, and multivariate statistical techniques. The study focuses on the distinction between the contributions of natural weathering and anthropogenic inputs of arsenic with its spatial distribution and seasonal variations in the plain of the state Bihar of India. Thirty-six groundwater and one sediment core samples were collected in the pre-monsoon and post-monsoon seasons. Various graphical plots and statistical analysis were carried out using chemical data to enable hydrochemical evaluation of the aquifer system based on the ionic constituents, water types, hydrochemical facies, and factors controlling groundwater quality. Results suggest that the groundwater is characterized by slightly alkaline pH with moderate to strong reducing nature. The general trend of various ions was found to be Ca(2+) > Na(+) > Mg(2+) > K(+) > NH(4) (+); and HCO(3) (-) > Cl(-) > SO(4) (2-) > NO(3) (-) > PO(4) (3-) > F(-) in both seasons. Spatial and temporal variations showed a slightly higher arsenic concentration in the pre-monsoon period (118 microg/L) than in the post-monsoon period (114 microg/L). Results of correlation analyses indicate that arsenic contamination is strongly associated with high concentrations of Fe, PO(4) (3-), and NH(4) (+) but relatively low Mn concentrations. Further, the enrichment of arsenic is more prevalent in the proximity of the Ganges River, indicating that fluvial input is the main source of arsenic. Grain size analyses of sediment core samples revealed clay (fine-grained) strata between 4.5 and 7.5 m deep that govern the vertical distribution of arsenic. The weathering of carbonate and silicate minerals along with surface-groundwater interactions, ion exchange, and anthropogenic activities seem to be the processes governing groundwater contamination, including with arsenic. Although the percentage of wells exceeding the permissible limit (50 microg/L) was less (47%) than that reported in Bangladesh and West Bengal, the percentage contribution of toxic As(III) to total arsenic concentration is quite high (66%). This study is vital considering that groundwater is the exclusive source of drinking water in the region and not only makes situation alarming but also calls for immediate attention.

Groundwater age and mixing process for evaluation of radionuclide impact on water resources following the Fukushima Dai-ichi nuclear power plant accident.

Radionuclide contamination of groundwater causes critical impacts on water resources, human lives, and ecosystems. The intrusion of radionuclides into the groundwater flow system in Fukushima, Japan, could be illuminated by determining groundwater age and mixing processes. To do this, periodical field surveys were conducted in catchments contaminated by the Fukushima Dai-ichi Nuclear Power Plant accident. Sampling began in May 2011, which was 2 months after the disaster, and continued through June 2012. Chlorofluorocarbon (CFCs), tritium, and oxygen and hydrogen stable isotopes were used as environmental tracers. The observed tritium concentrations suggested that the water contained accident-derived radionuclides that exceeded the natural background baseline. Groundwater ages in the selected two headwater catchments were estimated to be between 10 and 26 years by combined use of multiple CFCs concentrations. In addition, the governing groundwater flow system was mostly approximated by a piston flow model; however, modern water fraction was also suggested based on the relationship between CFC-11 and CFC-12. The estimated water age and isotopic signals among stream water, spring water, and groundwater revealed that the intrusion of radionuclides into the groundwater was caused by the mixing between groundwater and modern water sources such as soil water and precipitation with relatively high radionuclide concentrations. This mixing was facilitated by a weathered and fractured granite bedrock and a thin unsaturated subsurface layer in the study area. Continued long-term monitoring of radionuclides in the groundwater will be necessary for water resources management in the future. CAPSULE: Radionuclide intrusion into the groundwater is related to the mixing between radionuclide-poor groundwater and modern water with relatively high radionuclide concentration.

Stable isotope characteristics of water resources in the coastal area of the Vietnamese Mekong Delta.

The stable isotopes of oxygen and hydrogen can provide useful insights into water origin and hydrological processes. The present study aims to investigate the characteristics of stable H/O isotopes of groundwater and surface water in a coastal area of the Vietnamese Mekong Delta. Isotopes and chloride concentrations of surface water show a highly seasonal and linearly spatial variability, depending on the distance to the sea. The seasonal variation of upstream discharge and rainfall plays an important role in changes of the isotopic compositions and chloride concentrations. Tide also influences on chloride concentrations of surface water while it does not change the isotopic compositions. Evaporation plays a crucial role in changes of isotopic compositions, while the influence of freshwater/seawater mixing on isotopic variabilities is negligible. Groundwater has a spatial heterogeneity in isotopic compositions and chloride concentrations, reflecting different recharge sources and seawater intrusion processes. Groundwater in shallow aquifers originates from rainfall and surface water with small evaporative losses, and it experienced different magnitudes of mixing with seawater. Groundwater in deep aquifers might be recharged by open-surface water evaporation in the last glacial age with minor impacts of seawater intrusion on these aquifers.

Six-year monitoring study of 137Cs discharge from headwater catchments after the Fukushima Dai-ichi Nuclear Power Plant accident.

Since headwater catchments are the source areas of 137Cs for downstream river systems, 137Cs discharge from headwater areas needs to be evaluated. Dissolved form (Dissolved), coarse organic matter (Org), and suspended sediments (SS) were sampled and 137Cs concentrations were measured from June 2011 to November 2016 in four headwater catchments in Yamakiya District, located 35 km northwest of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP). The data up to September 2013 (2.5 y after the accident) have been already published (Iwagami et al., 2017a, b). The data up to November 2016 (5.7 y after the accident) are newly reported in the present paper together with data at a new sampling site. The whole data from June 2011 to November 2016 is discussed. The normalized 137Cs concentrations (137Cs concentrations normalized by the average deposition density of each catchment) in Dissolved, Org, and SS were in the order of 10-6 m2/L, 10-2 m2/kg, and 10-1 m2/kg, respectively, before 2013 and declined to around 10-8 m2/L, 10-4 m2/kg, and 10-2 m2/kg, respectively, in 2016. As a result of the decontamination program, the discharge of SS increased, whereas 137Cs concentrations in SS declined significantly and the total flux of 137Cs decreased. Although the clear effect of land use on decline trend in normalized 137Cs concentrations in Dissolved was not found, more data are necessary for elucidating the relation between them.

An overview of the rangelands atmosphere-hydrosphere-biosphere interaction study experiment in northeastern Asia (RAISE).

Intensive observations, analysis and modeling within the framework of the rangelands atmosphere-hydrosphere-biosphere interaction study experiment in northeastern Asia (RAISE) project, have allowed investigations into the hydrologic cycle in the ecotone of forest-steppe, and its relation to atmosphere and ecosystem in the eastern part of Mongolia. In this region, changes in the climate have been reported and a market oriented economy was introduced recently, but their impact on the natural environment is still not well understood. In this RAISE special issue, the outcome is presented of the studies carried out by six groups within RAISE, namely: (1) Land-atmosphere interaction analysis, (2) ecosystem analysis and modeling, (3) hydrologic cycle analysis, (4) climatic modeling, (5) hydrologic modeling, and (6) integration. The results are organized in five relevant categories comprising (i) hydrologic cycle including precipitation, groundwater, and surface water, (ii) hydrologic cycle and ecosystem, (iii) surface-atmosphere interaction, (iv) effect of grazing activities on soils, plant ecosystem and surface fluxes, and (v) future prediction. Comparison with studies on rangelands in other parts of the world, and some future directions of studies still needed in this region are also summarized.

Contribution of radioactive 137Cs discharge by suspended sediment, coarse organic matter, and dissolved fraction from a headwater catchment in Fukushima after the Fukushima Dai-ichi Nuclear Power Plant accident.

Radiocesium (137Cs) migration from headwaters in forested areas provides important information, as the output from forest streams subsequently enters various land-use areas and downstream rivers. Thus, it is important to determine the composition of 137Cs fluxes (dissolved fraction, suspended sediment, or coarse organic matter) that migrate through a headwater stream. In this study, the 137Cs discharge by suspended sediment and coarse organic matter from a forest headwater catchment was monitored. The 137Cs concentrations in suspended sediment and coarse organic matter, such as leaves and branches, and the amounts of suspended sediment and coarse organic matter were measured at stream sites in three headwater catchments in Yamakiya District, located ∼35 km northwest of Fukushima Dai-ichi Nuclear Power Plant (FDNPP) from August 2012 to September 2013, following the earthquake and tsunami disaster. Suspended sediment and coarse organic matter were sampled at intervals of approximately 1-2 months. The 137Cs concentrations of suspended sediment and coarse organic matter were 2.4-49 kBq/kg and 0.85-14 kBq/kg, respectively. The 137Cs concentrations of the suspended sediment were closely correlated with the average deposition density of the catchment. The annual proportions of contribution of 137Cs discharge by suspended sediment, coarse organic matter, and dissolved fraction were 96-99%, 0.0092-0.069%, and 0.73-3.7%, respectively. The total annual 137Cs discharge from the catchment was 0.02-0.3% of the deposition.

Vertical distribution and temporal dynamics of dissolved 137Cs concentrations in soil water after the Fukushima Dai-ichi Nuclear Power Plant accident.

Radiocesium (137Cs) migration from headwater forested areas to downstream rivers has been investigated in many studies since the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, which was triggered by a catastrophic earthquake and tsunami on 11 March 2011. The accident resulted in the release of a huge amount of radioactivity and its subsequent deposition in the environment. A large part of the radiocesium released has been shown to remain in the forest. The dissolved 137Cs concentration and its temporal dynamics in river water, stream water, and groundwater have been reported, but reports of dissolved 137Cs concentration in soil water remain sparse. In this study, soil water was sampled, and the dissolved 137Cs concentrations were measured at five locations with different land-use types (mature/young cedar forest, broadleaf forest, meadow land, and pasture land) in Yamakiya District, located 35 km northwest of FDNPP from July 2011 to October 2012. Soil water samples were collected by suction lysimeters installed at three different depths at each site. Dissolved 137Cs concentrations were analyzed using a germanium gamma ray detector. The dissolved 137Cs concentrations in soil water were high, with a maximum value of 2.5 Bq/L in July 2011, and declined to less than 0.32 Bq/L by 2012. The declining trend of dissolved 137Cs concentrations in soil water was fitted to a two-component exponential model. The rate of decline in dissolved 137Cs concentrations in soil water (k1) showed a good correlation with the radiocesium interception potential (RIP) of topsoil (0-5 cm) at the same site. Accounting for the difference of 137Cs deposition density, we found that normalized dissolved 137Cs concentrations of soil water in forest (mature/young cedar forest and broadleaf forest) were higher than those in grassland (meadow land and pasture land).

Spatial variations in larch needle and soil δ15N at a forest-grassland boundary in northern Mongolia.

The spatial patterns of plant and soil δ15N and associated processes in the N cycle were investigated at a forest-grassland boundary in northern Mongolia. Needles of Larix sibirica Ledeb. and soils collected from two study areas were analysed to calculate the differences in δ15N between needle and soil (Δδ15N). Δδ15N showed a clear variation, ranging from -8 ‰ in the forest to -2 ‰ in the grassland boundary, and corresponded to the accumulation of organic layer. In the forest, the separation of available N produced in the soil with 15N-depleted N uptake by larch and 15N-enriched N immobilization by microorganisms was proposed to cause large Δδ15N, whereas in the grassland boundary, small Δδ15N was explained by the transport of the most available N into larch. The divergence of available N between larch and microorganisms in the soil, and the accumulation of diverged N in the organic layer control the variation in Δδ15N.

Temporal changes in dissolved 137Cs concentrations in groundwater and stream water in Fukushima after the Fukushima Dai-ichi Nuclear Power Plant accident.

The concentration of dissolved 137Cs in groundwater and stream water in the headwater catchments in Yamakiya district, located ∼35 km north west of Fukushima Dai-ichi Nuclear Power Plant (FDNPP), was monitored from June 2011 to July 2013, after the earthquake and tsunami disaster. Groundwater and stream water were sampled at intervals of approximately 2 months at each site. Intensive sampling was also conducted during rainstorm events. Compared with previous data from the Chernobyl NPP accident, the concentration of dissolved 137Cs in stream water was low. In the Iboishi-yama catchment, a trend was observed for the concentration of dissolved 137Cs in stream water to decline, which could be divided into two phases by October 2011 (a fast flush of activity as a result of rapid washoff and a slow decline as a result of soil fixation and redistribution processes). The highest 137Cs concentration recorded at Iboishi-yama was 1.2 Bq/L on August 6, 2011, which then declined to 0.021-0.049 Bq/L during 2013 (in stream water under normal water-flow conditions). During the rainfall events, the concentration of dissolved 137Cs in stream water increased temporarily. The concentration of dissolved 137Cs in groundwater at a depth of 30 m at Iboishi-yama displayed a decreasing trend from 2011 to 2013, with a range from 0.039 Bq/L to 0.0025 Bq/L. The effective half-lives of stream water in the initial fast flush and secondary phases were 0.10-0.21 and 0.69-1.5 y, respectively in the three catchments. The effective half-life of groundwater was 0.46-0.58 y at Koutaishi-yama and 0.50-3.3 y at Iboishi-yama. The trend for the concentration of dissolved 137Cs to decline in groundwater and stream water was similar throughout 2012-2013, and the concentrations recorded in deeper groundwater were closer to those in stream water. The declining trend of dissolved 137Cs concentrations in stream water was similar to that of the loss of canopy 137Cs by throughfall, as shown in other reports of forest sites in the Yamakiya district.

Natural recovery of steppe vegetation on vehicle tracks in central Mongolia.

Steppe desertification due to vehicle travel is a severe environmental issue in Mongolia. We studied natural vegetation recovery on abandoned vehicle tracks in the central Mongolia steppe through vegetation surveys and stable isotopic techniques. The following issues were addressed: (i) invasion of pioneering plant species, (ii) alteration of soil surface features, and (iii) contribution of revegetated plants to soil organic matter (SOM). The pioneering plant species that firstly invaded the abandoned tracks are those that could germinate, root and survive in the compacted track surface. Salsola collina is one of these candidate plants. Due to revegetation, soil surface hardness was reduced. With the improvement of surface microenvironmental conditions, other plants began to colonize and establish; con-comitantly species richness and species diversity increased. Carbon isotope ratios of SOM at the top surface layer indicated that C4 -derived carbon contributed more to SOM in the early phase of recovery and decreased with further recovery.